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A field guide to mosasaurs and other marine squamates from the Western Interior Sea, including Tylosaurus, Mosasaurus, Clidastes and Platecarpus Squamata Lizards, Snakes and Mosasaurs Squamates form a group of reptiles which includes all of today’s snakes, lizards and their closest relatives. During the Late Cretaceous, the Western Interior Sea was home to a variety of marine squamates, including mosasaurs and dolichosaurids. Mosasaurs were large, fully aquatic reptiles that evolved from lizard-like ancestors and became the dominant marine predators of the seaway. They had streamlined bodies, paddle-shaped limbs, powerful tails, and flexible jaws that allowed them to hunt a wide range of prey, including fish, sharks, turtles, seabirds, and other marine reptiles. By the Late Cretaceous, mosasaurs were the largest carnivorous animals found in the world’s seas and oceans Dolichosaurids were smaller elongated marine lizards that retained many terrestrial features, including functional limbs. They likely swam by undulating their bodies and tails and fed on small fish and invertebrates in shallow coastal waters. Both of these squamate groups seem to have been close relatives of today’s monitor lizards and snakes. Anatomy (ABOVE) A labeled diagram of the skeleton of the medium-sized mosasaur Platecarpus Skeletal Anatomy Although they originated from terrestrial lizard ancestors, mosasaurs underwent significant evolutionary changes in order to move into fully marine environmental niches and become effective swimmers. The backbones of these animals were heavily built yet fairly flexible which would have allowed their bodies to move in powerful lateral waves as they swam. In contrast to land-dwelling lizards, mosasaurs had limb bones modified to support broad flippers. The bones of the arms and legs became shortened and flattened over millions of years while the phalanges of the front and back feet grew longer in fully aquatic species who no longer had to support their own weight on land. Many species also developed more finger and toe bones than their terrestrial ancestors which increased the surface areas of their flippers. The skulls of mosasaurs were built similarly to those of living monitor lizards but with a few added features which helped them survive at sea. Their heads were long and thin and were lined with sharp, cone-shaped teeth ideal for seizing slippery prey. Much like many of today’s snakes, mosasaurs had an extra set of pterygoid teeth on the roofs of their mouths which helped them hold onto prey and guide it into the throat during swallowing. The tails of mosasaurs also became increasingly specialized in order to give these animals more propulsion when swimming. Their tail bones ended in a downward bend which may have supported a crescent-shaped fin which was somewhat similar to that of a shark or an ichthyosaur. The extent of this tail fluke in different mosasaur genera and their exact shapes remain uncertain however, since fossils preserving these tail flukes are very rare. Some mosasaurs may have had a more paddle-like tail instead of a full fluke. (BELOW) The bones of the skull of the mosasaur Platecarpus . The pterygoid bone which supports the extra set of teeth at the back of the mosasaur's mouth is somewhat obscured in this drawing but it extends all the way to the back of the skull. (BELOW) The bones of the forelimbs and hindlimbs of a mosasaur. Many bones in the skeletons of mosasaurs were connected with extensive amount of cartilage instead of bone, a feature seen in many types of marine tetrapods. External Anatomy Mosasaurs had a sleek, streamlined body shape that was well suited for movement through the water. Their bodies became more narrow toward the tail, while their necks remained relatively short. This would have given these animals a smooth, torpedo-like profile. Their limbs were transformed into sturdy flippers, which were used to help with steering and balance rather than propulsion. Their primary driving force for swimming came from their large tails. Rare soft-tissue fossils have provided valuable insights into the skin covering of mosasaurs. These remains tell us that the bodies of mosasaurs were covered in small, closely packed scales that created a smooth outer surface and likely helped reduce water resistance. Most scales were rhomboidal or diamond-shaped and overlapped one another in a pattern similar to those seen in many modern reptiles. In certain specimens, slightly enlarged scales are found along the back, creating a shallow ridge that may have improved stability while swimming. Fossilized skin impressions also indicate that scale size and shape varied across different regions of the body, with smaller scales found around joints and other areas that might have needed more flexibility (BELOW) Features of the external anatomy of the mosasaur Platecarpus . Fossils from this particular genus which preserve patches of skin tell us that different areas on the bodies of these animals had their own specific scale types and textures. (BELOW) A fossil on display at the KU Natural History Museum preserves a patch of scales from a Tylosaurus. Note the small ridges, or keels, running along the middle of each of these small scales, similar to those seen in some living snakes. (BELOW) Artistic reconstructions of a mosasaur with two types of tail structures. The evidence in favor of the approach in illustration A is a fossil of Prognathodon which shows what appears to be the outline of a soft tissue tail lobe situated above the caudal vertebrae at the end of the tail. However, it is not clear from this fossil alone just how common this feature would have been within the mosasaur family tree. Some mosasaurs may have had a more paddle like tail as in illustration B. Squamates Clade Mosasauria Clidastes Dallasaurus Ectenosaurus Eonatator Globidens Gnathomortis Jormungandr Latoplatecarpus Mosasaurus Platecarpus Plesioplatecarpus Plioplatecarpus Prognathodon Russellosaurus Selmasaurus Tylosaurus Family Dolichosauridae Coniasaurus Mosasaurs (Mosasauroidea) Giant Ocean Lizards Mosasaurs were a group of large marine reptiles that lived during the Late Cretaceous. These sea reptiles were descended from terrestrial animals related to living monitor lizards and they evolved a suite of adaptations for life in the water. Their bodies were long and streamlined, reducing drag as they swam and their limbs transformed over millions of years into broad, paddle-like flippers used for steering. A powerful tail, which may have ended in a crescent-shaped fluke would have helped push them through the water. Some of the best known features of mosasaurs have to do with characteristics found in their skulls. They had strong jaws lined with sharp, backward-curving teeth designed to grasp slippery prey. Flexible joints in the skull and lower jaw may have allowed them to swallow large animals whole, much like many of today's snakes. They had a second set of teeth behind their palates which helped them keep ahold of their food. Some mosasaur species only grew to be around 5 feet long while others were behemoths who could measure over 50 feet in length. Clidastes Temporal Range: 85 - 70 million years ago Geographic Range: Central and eastern North America, parts of Europe and Asia Species: C propython, C. iguanavus? Diet: Carnivorous, fish, squid, seabirds Clidastes was a mosasaur genus which lived in the Late Cretaceous seas that covered much of North America, including the Western Interior Sea. Although it belonged to the same broader lineage as later giants such as Mosasaurus, Clidastes was much lighter and more streamlined in build, generally reaching lengths of around 10-20 feet. Its slender body was ideally suited for speed and agility in the water. One of the defining characteristics of Clidastes was the specialized structure of its vertebral column. The genus' name translates roughly to “locked vertebrae” which refers to the way adjacent vertebrae interlocked with one another. This would have given this genus a backbone that was both strong and flexible. This arrangement gave Clidastes extra stability while swimming without hampering the side-to-side motion needed for effective propulsion through the water. Compared with many later mosasaurs, Clidastes had a more gracile physique. These mosasaurs had narrow skulls, elongated bodies, and powerful tails. The rear portion of the tail was reinforced by enlarged neural spines and lower chevrons that anchored strong swimming muscles. Its jaws were equipped with numerous sharp, recurved teeth designed to seize and hold slippery prey. Rather than crushing large animals, Clidastes likely specialized in catching fish, soft-bodied cephalopods, and other small marine organisms. (BELOW) A skull of Clidastes on display at the KU Natural History Museum. The skull is positioned upside-down with the mandibles on either side of the cranium. The shapes of the palate and the pterygoid teeth are visible in center frame (BELOW) A fossil Clidastes tooth. Original photo taken by Dylan Falkner/Prehistoric Alabama Dallasaurus Temporal Range: 93.9 - 89.8 million years ago Geographic Range: Northern Texas Species: D. turneri Diet: Carnivorous, fish, squid Dallasaurus is an important animal for those studying mosasaurs because it preserves a rare glimpse into the early stages of mosasaur evolution. Unlike the giant, fully aquatic mosasaurs that dominated the oceans later in the Cretaceous, Dallasaurus retained a mix of basal and derived features that reveal how these reptiles transitioned from life on land to life at sea. Fossils of this small predator were discovered in Turonian-aged rocks near Dallas County, Texas, giving the genus its name. One of the most distinctive characteristics of Dallasaurus was its limb structure. Rather than having fully developed flippers like later mosasaurs, it retained relatively unspecialized limbs with recognizable digits and joints. These limbs were better suited for movement on land than those of more derived mosasaurs, suggesting that Dallasaurus occupied a transitional stage between terrestrial lizards and fully marine reptiles. At the same time, Dallasaurus already displayed adaptations for life in the water. Its body and tail were already deeper than those of its terrestrial relatives, a feature which is sometimes seen in animals who are moving into an aquatic niche. This would suggest that aquatic adaptations evolved in mosasaurs before the complete transformation of the limbs into flippers. Dallasaurus is also notable for its relatively small size. At approximately 3 feet long, Dallasaurus was tiny compared with later mosasaur giants such as Mosasaurus and Tylosaurus . Its diminutive size likely allowed it to hunt in shallow coastal environments and pursue smaller prey. Ectenosaurus Temporal Range: 85.8 - 82 million years ago Geographic Range: Kansas, northern Texas, Alabama Species: E. clidastoides, E. everhartorum, E. tlemonectes, E. shannoni Diet: Carnivorous, fish, squid Ectenosaurus was an animal whose name means “drawn-out lizard,” a name which refers to its elongated snout and jaws. This long skull is one of the genus’s defining traits. Unlike the broader skulls of many mosasaurs, Ectenosaurus had a narrow, streamlined snout that likely allowed it to capture small, fast-moving prey with precision. This skull shape is one of the key features used to differentiate Ectenosaurus from close relatives such as Platecarpus. Perhaps the most important fossil discoveries associated with Ectenosaurus are the rare traces of its skin and associated soft-tissue impressions. These fossils revealed that these animals were covered in tightly anchored scales supported by an intricate network of fiber bundles within the skin. The arrangement of these tissues suggests that the front portion of the body was relatively rigid rather than highly flexible. This evidence somewhat changed our view of mosasaur locomotion. Early reconstructions often portrayed mosasaurs swimming like giant eels or sea snakes by undulating their entire bodies. The preserved soft tissues of Ectenosaurus indicate instead that much of the body remained stiff during swimming, with propulsion generated primarily by the tail. This swimming style, which was likely used by most types of mosasaurs, would have been more efficient and hydrodynamic than simple undulation and would have echoed the swimming styles of modern sharks and other fast marine predators. Eonatator Temporal Range: 84 - 72 million years ago Geographic Range: North America, parts of South America, northern Europe Species: E. sternbergii, E. coellensis Diet: Carnivorous, small fish, squid, sea turtles Eonatator was a relatively small mosasaur that lived in the oceans of the Late Cretaceous and belonged to the specialized halisaurine branch of the mosasaur family. Adults were only about 9 feet long, making them one of the smaller members of the mosasaur group. The name Eonatator , means “dawn swimmer,” which reflects its status as an early representative of the halisaurines. The femurs of these animas were unusually long and narrow, while the vertebral column contained fewer pygal vertebrae than is seen in many other mosasaurs. Their skulls also had a somewhat rounded process on its quadrate bone, a distinctive characteristic that separates Eonatator from closely related genera such as Halisaurus . These differences were significant enough that paleontologists ultimately classified Eonatator as a separate genus. Globidens Temporal Range: 85 - 66 million years ago Geographic Range: Parts of the US, Colombia, Angola, Brazil, Jordan Species: G. alabamensis, G. dakotensis, G. phosphaticus, G. simplex Diet: Durophagous: ammonites, turtles Globidens was among the most highly specialized mosasaurs of the Late Cretaceous and stood apart from most of its relatives because of its globe-shaped dentition. Whereas many mosasaurs had long, sharp teeth adapted for seizing fish and other soft-bodied prey, Globidens had distinctive rounded teeth shaped like small domes or spheres. This unusual dental arrangement was so characteristic that it inspired the genus name, which translates to “globe tooth.” Globidens ' teeth gave it the ability to feed on hard-shelled animals. Its rounded teeth were designed for crushing rather than piercing which would have made them less likely to break during feeding. Instead of impaling prey, the teeth acted much like heavy-duty crushing tools capable of breaking shells and armor. This specialized feeding strategy allowed Globidens to consume organisms such as ammonites, nautiloids, clams, and marine turtles that many other mosasaurs would have struggled to break open. Globidens also had a heavily constructed skull and powerful jaw apparatus. The skull was built to withstand repeated high-pressure bites, while the strongly articulated jaws generated the force necessary to crack open durable shells. (BELOW) A fossil Globidens tooth shown from above (A) and below (B). Original photo taken by Dylan Falkner Gnathomortis Temporal Range: 81 - 79 million years ago Geographic Range: Colorado Species: G. stadtmani Diet: Carnivorous, fish, sharks, marine reptiles Gnathomortis was a large mosasaur that lived during the Late Cretaceous in the ancient seaway that covered parts of Colorado. Although it was originally classified as a species of Prognathodon, in 1999 it was moved into its own new genus. Gnathomortis' n ame means “jaws of death.” The most impressive feature of Gnathomortis would have been its massive jaw apparatus. Its lower jaws measured roughly 4 feet long and were supported by extremely powerful muscles. Evidence for these muscles comes from a depression on the outer surface of the mandible, a feature also seen in some modern lizards. This feature suggests that Gnathomortis was capable of generating a particularly strong bite, enabling it to tackle very large prey. Jormungandr Temporal Range: 80 million years ago Geographic Range: North Dakota Species: J. walhallaensis Diet: Carnivorous, fish, sharks, marine reptiles Jormungandr is among the most recent mosasaurs to be described in the twenty-first century. This marine reptile was recovered from the Pierre Shale deposits of North Dakota and its remains date back to roughly 80 million years ago during the Late Cretaceous. It was described and named in 2023. The genus takes its name from Jörmungandr, the legendary sea serpent of Norse mythology. Because the fossil material includes an almost complete skull along with portions of the rest of the skeleton, scientists have been able to study its anatomy in a great deal of detail. The most distinctive aspect of Jormungandr is its combination of anatomical traits that link different stages of mosasaur evolution. Its skeleton has features associated with both early and more advanced mosasaurine species. For instance, it retained the relatively large number of teeth characteristic of more basal forms such as Clidastes , but it also would have had thickened skull elements that resemble those of later genera such as Mosasaurus. Because of these similarities, researchers initially suspected that the specimen belonged to an early member of the genus Mosasaurus before determining that it was distinct enough to represent an entirely new genus. Latoplatecarpus Temporal Range: 80 - 79 million years ago Geographic Range: Central North America, western Russia Species: L willistoni, L. nichollsae Diet: Carnivorous, fish, sharks, marine reptiles Latoplatecarpus was a plioplatecarpine mosasaur whose fossils have been discovered in several regions, including the Western Interior Seaway of North America, the Gulf Coast, and parts of Russia. It was also one of the larger plioplatecarpine mosasaurs, with some individuals exceeding 25 feet in length. The name Latoplatecarpus translates to “wide flat wrist” which highlights the animal’s unique broad foreflippers. These paddle-like limbs were not used for propulsion alone but also played an important role in steering and maintaining stability in the water. Their expanded shape suggests that Latoplatecarpus could make quick turns and precise movements while pursuing prey. Its body design suggests that it was a predator that relied on speed, agility, and quick reactions to capture its food. Latoplatecarpus fossils were originally assigned to the genus Plioplatecarpus before being later recognized as belonging to a separate genus, Mosasaurus Temporal Range: 82.7 - 66 million years ago Geographic Range: Central North America, parts of Europe and Africa Species: M. missouriensis, M. hoffmani, M. conodon, M. lemonnieri Diet: Carnivorous, fish, sharks, marine reptiles Mosasaurus missouriensis was one of the largest Interior Sea mosasaurs, likely hunting for sharks, sea turtles, and other marine reptiles living within the waters of its shallow ocean habitat. These animals were close relatives of the european species Mosasaurus hoffmani , one of the most massive known species of mosasaurs. The history of the discovery of Mosasaurus missouriensis is closely tied to the early development of American paleontology. Fossils attributed to this species were among the earliest mosasaur remains discovered in North America. As early as 1804, members of the Lewis and Clark Expedition reported finding the remains of a large aquatic animal along the Missouri River in what is now South Dakota. Additional fossils were collected during the 1830s from the same general region, attracting the attention of scientists and naturalists. The species was formally described in 1834 and initially named Ichthyosaurus missouriensis because its true identity was not yet understood. Later discoveries and comparisons with other marine reptiles revealed that the animal was actually a mosasaur. Subsequent studies reassigned the species to the genus Mosasaurus , where it remains to this day. Platecarpus Temporal Range: 84 - 81 million years ago Geographic Range: Worldwide distribution Species: P. tympaniticus, P. coryphaeus? Diet: Carnivorous, fish, sharks, marine reptiles Platecarpus was a medium-sized genus of mosasaur. Its fossils are particularly common in the Niobrara Chalk deposits of central and western Kansas, where it was one of the most successful mosasaurs of its time. Platecarpus grew to be about 19 feet long. Its skull was more lightly built than the skulls of giant mosasaurs such as Tylosaurus . It had a wider body than many other more gracile mosasaurs like Mosasaurus and Tylosaurus . Fossil evidence suggests that Platecarpus primarily fed on moderate-sized fish and may also have consumed squid. Some fossils of Platecarpus are found with the remains of fish bones still inside of their abdominal cavities. Exceptional fossil discoveries have alsp preserved rare soft-tissue details from these animals, most notably fossilized skin impressions, which show that they had different shapes of scales on different parts of their bodies. Platecarpus would later evolve into larger and more heavily built mosasaurs like Plioplatecarpus which show up in rock layers from later in the Cretaceous period. (BELOW) A skull of a Platecarpus on display at the KU Natural History Museum. In this particular mount the bones of the sclerotic ring are visible in the orbit. In life these rings of small bony plates would encircle the outside of the animal's eyes. (BELOW) A skeleton of Platecarpus on display at the Sternberg Museum of Natural History. The forelimb bones of this animal are shown at the bottom of the photo. Note the truncated humerus and forearm bones. Plesioplatecarpus Temporal Range: 88 - 84 million years ago Geographic Range: Central and western North America Species: P. planifrons Diet: Carnivorous, fish, sharks, marine reptiles Plesioplatecarpus was a mid-sized mosasaur that was found the seas of the Late Cretaceous. For many years, its fossils were assigned first to the genus Clidastes and later to Platecarpus , but subsequent research revealed enough anatomical differences to justify the creation of a separate genus. Its name, meaning “similar to Platecarpus ,” acknowledges its close relationship to that genus. It was most likely a slender and quick swimmer like many of the other plioplatecarpine mosasaurs. Plioplatecarpus Temporal Range: 82 - 66 million years ago Geographic Range: Central North America, parts of Europe and Africa Species: P. marshi, P. depressus, P. primaevus, P. peckensis Diet: Carnivorous, fish, sharks, marine reptiles Plioplatecarpus was a highly specialized mosasaur that lived during the late Cretaceous period. Although not as large as famous giants such as Mosasaurus or Tylosaurus , it was a successful marine predator in its own right that lived in shallow seas across North America and Europe. Most species reached lengths of about 19 feet, making them medium-sized members of the mosasaur family. One of the most distinctive features of Plioplatecarpus was its relatively short skull combined with fairly large eyes. The enlarged eyes may have been an adaptation for hunting in dimly lit environments, possibly allowing it to forage at greater depths than many other mosasaurs. Its broad geographic distribution suggests that it was well adapted to life in open waters rather than being limited to shallow coastal habitats. These mosasaurs were probably comparatively fast and maneuverable, with body proportions that differed from those of the larger predators of the Late Cretaceous. Some researchers have even compared their swimming style and agility to those of modern pinnipeds like seals and sea lions. Their tooth structure suggests that, like Platecarpus , they specialized in feeding on soft-bodied animals like fish and squid. Prognathodon Temporal Range: 83.6 - 66 million years ago Geographic Range: Found in North America, Europe, western Asia, Africa Species: P. overtoni, P. solvayi, P. giganteus, P. hashimi, P. hudae, P. lutugini, P. primus Diet: Carnivorous, fish, sharks, marine reptiles Prognathodon was one of the most heavily-built mosasaurs of the Late Cretaceous oceans. While many mosasaurs were built for catching fish and other soft-bodied prey, Prognathodon evolved a skull and dentition designed to deliver a much more powerful bite. Its name means “forejaw tooth” and refers to the massive jaws and prominent teeth that made it somewhat unique among mosasaurs. Fossils of Prognathodon have been discovered in Europe, North America, Africa, the Middle East, and even New Zealand. One of the characteristic features of the genus Prognathodon was its extremely deep skull. Its jaw muscles were heavily developed, allowing it to generate a powerful bite force compared to many other mosasaurs. Its teeth were large, thick, and strongly built, with heavily striated crowns and a shape suited for gripping, crushing, and tearing prey. Another unusual trait was the curvature and arrangement of the front teeth, which were more strongly curved than those of most other mosasaurs. Unlike the slender teeth of fish-eating mosasaurs, Prognathodon ’s teeth could withstand a lot of stress which suggests that it was capable of hunting and tearing apart very large prey. (BELOW) A Prognathodon tooth apex from the Campanian Ozan Formation of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo Russellosaurus Temporal Range: ~90 million years ago Geographic Range: Northern Texas Species: R. coheni Diet: Carnivorous, fish, squid Russellosaurus was a small and gracile mosasaur that lived during the Turonian stage of the Late Cretaceous. Although it lacked the enormous size of later mosasaurs such as Mosasaurus and Tylosaurus, it is one of the most scientifically important early members of the group. Fossils recovered from Texas represent some of the oldest well-preserved mosasaur remains known from North America and add a valuable insight into the early evolution of these marine reptiles. The skull of Russellosaurus was relatively narrow and slender in its construction. It had sixteen teeth in both the upper and lower jaws, a broad snout tip, and delicate cranial bones that created a streamlined appearance. Rather than being adapted for crushing large prey, its skull structure suggests it would have been an animal built for making quick turns while pursuing small animals. Russellosaurus is the namesake of the Russellosaurina, a group that includes several basal mosasaurs and helps bridge the evolutionary gap between early forms and later lineages such as the tylosaurines and plioplatecarpines. Selmasaurus Temporal Range: 86.3 - 84 million years ago Geographic Range: Kansas, Alabama Species: S. johnsoni, S. russelli Diet: Carnivorous, fish, sharks, marine reptiles Selmasaurus was a relatively small mosasaur whose fossils have been discovered in Alabama and Kansas. The genus is represented by two species, S. russelli and S. johnsoni. Selmasaurus had an unusualy well-fused skull. Unlike most mosasaurs who had flexible skulls capable of widening to accommodate large prey, many of the bones of the Selmasaurus' head were very strongly connected to one another and lacked the extensive cranial flexibility seen in other members of the group. As a result, it likely focused on smaller prey that could be swallowed whole. This is also supported by its relatively low tooth count. For a long time it was considered to be the mosasaur genus with the fewest teeth. Tylosaurus Temporal Range: 92.1 - 66 million years ago Geographic Range: Central North America, parts of Europe and Africa Species: T. proriger, T. rex, T. nepaeolicus, T. kansasensis, T. ivoensis, T. iembeensis Diet: Carnivorous, fish, sharks, marine reptiles Tylosaurus was among the largest and most powerful marine reptiles of the Late Cretaceous and could grow to be over 43 feet long. One of its most distinctive features was a blunt, toothless snout extension whose purpose remains mysterious. This cranial feature gives Tylosaurus its genus name, translating approximately to “knob reptile”. As a top predator in the seas of western North America, Tylosaurus hunted a wide variety of animals, including fish, sharks, sea turtles, seabirds, plesiosaurs, and even other mosasaurs. Tylosaurus is one of the best known animals from the Western Interior Seaway. It would have occupied the highest level of the food chain and played a role similar to that of modern large toothed whales. Tylosaurus has even been designated the state ocean fossil of Kansas (BELOW) A skeleton of Tylosaurus on display at the Sternberg Museum of Natural History. Despite the large size of this particular skeleton this animal had not reached maximum size at the time of its death (BELOW) A closeup view of the skull of Tylosaurus on the skeleton in Hays. In this mount the pterygoid teeth at the back of the jaw are visible just below the cheekbones and the orbit. These teeth were used to hold prey in place before swallowing (BELOW) An illustrated panel about Tylosaurus made for our Instagram account (BELOW) A Tylosaurus skull on display at the KU Natural History Museum. This particular skull is very large. Its crushed appearance is a result of taphonomic processes which changed the shape of the bones while it was in the ground (BELOW) A skull of a smaller species of Tylosaurus, Tylosaurus kansasensis, on display at the Sternberg Museum (BELOW) A Tylosaurus tooth from the Campanian of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo Dolichosaurs (Dolichosauridae) Snake-like Swimming Lizards Dolichosaurids were a group of small to medium-sized marine lizards that were related to mosasaurs and possibly closely related to the evolutionary lineage leading to modern snakes. They had elongated, snake-like bodies with reduced limbs and flexible necks. Despite their aquatic adaptations, they retained many features of their terrestrial lizard ancestors such as feet with small toes. Fossils of dolichosaurids such as Coniasaurus have been recovered from marine sediments in Kansas, confirming their presence within the seaway's diverse faunal communities. They preyed primarily on fish, crustaceans and other small aquatic organisms. Coniasaurus Temporal Range: 100 - 84 million years ago Geographic Range: Kansas, Germany, parts of the UK Species: C. crassidens, C. gracilodens Diet: Carnivorous, small fish, squid Coniasaurus was a small marine lizard that lived during the Late Cretaceous. These animals measured only about 3 feet in length, far smaller than later marine reptiles, yet they occupied a key evolutionary position near the base of the mosasaur lineage. Fossils of Coniasaurus have been found in Europe and North America, including deposits from the Western Interior Seaway of Kansas. One of the most distinctive features of Coniasaurus was its elongated body plan. It had a long neck, an extended trunk, and a slender tail which would have given it a serpentine appearance. Its skull was relatively long and narrow, and its dentition was highly specialized. The species Coniasaurus crassidens would have had enlarged hind teeth that differed markedly from those of many living lizards. 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University of Alberta. Polcyn, M. J., and Everhart, M. J., (2008), “Description and phylogenetic analysis of a new species of Selmasaurus (Mosasauridae: Plioplatecarpinae) from the Niobrara Chalk of western Kansas: In: Proceedings of the Second Mosasaur Meeting”.Fort Hays Studies, Special Issue number 3, p. 13-28. Russell, D. A. (1988). “A check list of North American marine Cretaceous vertebrates including fresh water fishes.” Royal Tyrrell Museum Zietlow, Amelia R.; Boyd, Clint A.; Van Vranken, Nathan E. (2023). "Jormungandr walhallaensis: A New Mosasaurine (Squamata: Mosasauroidea) from the Pierre Shale Formation (Pembina Member: Middle Campanian) of North Dakota". Bulletin of the American Museum of Natural History. 464 (1).

An online illustrated guidebook about ancient life in the Midwest with extinct animal field guides about life in the Western Interior Sea, the Pleistocene epoch and more. Extinct Animal Field Guides Explore the ancient past of the Midwest! These illustrated online guides are filled with information about the types of extinct animals who were once found in the Midwest and the Great Plains. New guides coming soon! Pleistocene Megafauna Pleistocene epoch 2.5 million years ago - 11,700 years ago Large land mammals and reptiles of the Ice Ages found in the Great Plains, Great Lakes region and the Ohio River valley Explore Western Interior Sea Cretaceous Period 100 - 66 million years ago Marine reptiles, birds and invertebrates found in the seas which covered the Great Plains during the Cretaceous period Explore Recent Extinctions 250 years ago- Present Over a dozen species of Midwestern animals have become extinct over the last 250 years, including birds, fishes, insects and snails Explore Timeline Of Prehistoric Life In The Midwest The landscape of the Midwest has taken on many forms over the course of Earth's history. At various times in the distant past parts of the Midwest have been covered in rainforests, in shallow seas and in mile-thick sheets of solid ice. Explore the Timeline Welcome to Prehistoric Midwest: an illustrated guide to the ancient past This website is intended to be a catalogue of information about ancient life in the Midwest, as well as a collection of paleoart featuring our many types of extinct organisms! About our project Anatomical Coloring Pages Color in these anatomical and skeletal diagrams to learn more about the finer details of ancient fossils! See our coloring pages 3D Scans and Models Browse 3-dimensional scans of fossil specimens produced by the Virtual Curation Lab Learn More Further Reading Find more sources of information about prehistoric life in the Midwest Learn More The Drawing Process Learn about how all of the illustrations for our website are made! Learn More Updates

Explore the different types of ray-finned fishes who once lived in the Western Interior Sea during the Cretaceous period, including animals like Xiphactinus, Pachyrhizodus and Cimolichthys Actinopterygii Ray-Finned Fishes Ray-finned fishes, or actinopterygians, are a type of fish who have fins supported by thin bony rays known as lepidotrichia rather than the fleshy lobes found in other “lobe-finned” groups of bony fishes. Most have lightweight skeletons, scales, gills protected by a bony operculum, as well as a swim bladder that helps keep them buoyant in the water. The Late Cretaceous Western Interior Sea was home to many types of ray-finned fishes, including giant predatory animals like Xiphactinus and Protosphyraena , filter feeders like Bonnerichthys and smaller fish like Omosoma and Kansius who would have made their homes along the bottom of the ancient seafloor. Anatomy (BELOW) Features of the skeletal anatomies of ray-finned fishes, in this case a pycnodont fish. Skeletal Anatomy Ray-finned fishes are named for the skeletal structures of their fins which are supported by thin, flexible bony rays called lepidotrichia. Unlike the fleshy muscular fins of lobe-finned fishes, these rays are only connected to supporting skeletal elements known as radials. Ray-finned fishes also have a skull composed of many articulating small bony plates, a column of vertebral centra, ribs, and a series of bones and radial cartilages that support their fins. In many species, the bones of the jaws are highly mobile and specialized for the preferred feeding strategy of each fish, such as suction feeding. They are often able to hinge forward and extend the reach of the mouth as part of the biting motion of the fish. The gills of ray-finned fishes are protected by a bony covering called the operculum, which also helps move water across the gill surfaces as they breathe. Many ray-finned fishes also have a gas-filled swim bladder that helps control buoyancy and allows them to remain suspended in the water with minimal effort. External Anatomy Ray-finned fishes have bodies which can usually be divided into three main regions: the head, trunk, and tail. The outer surface of the body of the fish is usually covered with scales that provide protection while also helping to reduce resistance as the fish moves through the water. These scales are coated with a thin mucus layer that reduces friction while swimming and that can also help protect the fish from parasites and/or infections. Some groups of ray-finned fishes, such as catfish, do not have these scales but they are common in other groups. The head carries the mouth, eyes, and nostrils of the fish, which are used for feeding, vision and detecting chemical cues in the surrounding water. The trunk of the fish is used mostly for locomotion and carries the animal’s fins. The paired pectoral and pelvic fins are primarily used for steering, maneuvering, and maintaining balance. Along the midline of the body the dorsal fin and anal fin help stabilize the fish during swimming and prevent uncontrolled rolling. The caudal fin is found at the base of the tail of the fish and is the animal’s main source of propulsion. Another important external feature is the lateral line which is a sensory system that runs along the sides of the body and detects vibrations, water currents, and nearby movement. Ray-Finned Fishes Order Ichthyodectiformes Gillicus Ichthyodectes Prosaurodon Saurocephalus Saurodon Xiphactinus Order Pycnodontiformes Anomoeodus Coelodus Gyrodus Micropycnodon Palaeobalistum Order Semionotiformes Hadrodus Order Crossognathiformes Apsopelix Elopopsis Pachyrhizodus Order Elopiformes Laminospondylus Family Pachycormidae Bonnerichthys Protosphyraena Order Aulopiformes Apateodus Cimolichthys Enchodus Leptecodon Stratodus Order Beryciformes Caproberyx Kansius Order Tselfatiiformes Bananogmius Enischnorhynchus Ferrifrons Luxilites Martinichthys Niobrara Pentanogmius Pseudanogmius Syntegmodus Thryptodus Zanclites Order Polymixiiformes Omosoma Order Albuliformes Paralbula Pollerspoeckia Family Apogonidae Apogonidarum Order Amiiformes Cyclurus Paraliodesmus? Order Anguiliformes Anguilavus Urenchelys Family Ariidae Vorhisia Order Aspidorhynchiformes Belonostomus Family Lepisosteidae Atractosteus Lepisosteus Next Page Ichthyodectiformes Giant Carnivores and Filter Feeders Ichthyodectiform fishes were a group which contained some of the best known fishes of the Western Interior Sea. This included animals like Xiphactinus, Ichthyodectes and Gillicus who were characterized by sleek, streamlined bodies, powerful tails and large mouths designed for capturing prey or filter feeding. Many species were fast-moving hunters capable of pursuing a wide variety of marine prey. Cretaceous ichthyodectiform fishes are generally split into two groups; the saurodontids, who had longer and narrower skulls, often with a large bar or spike on their lower jaw, and the ichthyodectids, who had shorter faces and wider mouths. Gillicus Temporal Range: 100 million years ago - 72 million years ago Geographic Range: Central North America, the UK, Japan Species: G. arcuatus, G. serridens Diet: Filter feeder or suction feeder, small fish/invertebrates Gillicus was a possibly filter feeding member of the ichthyodectiform order. Although it was outwardly pretty similar in appearance to animals like Ichthyodectes, it is currently considered to be part of the same group as animals like Saurodon . Gillicus had many very small teeth lining its jaws. These teeth were so tiny that the jaws could appear almost toothless at first glance. Paleontologists infer that Gillicus captured food by rapidly opening its mouth and sucking in smaller fish and some researchers have even suggested that it may have filtered small organisms out of the water. Gillicus was frequently preyed upon by larger marine animals, most famously its distant relative Xiphactinus. Several famous fossils preserve entire Gillicus individuals inside the stomachs of Xiphactinus . (BELOW) A famous skeleton housed at the Sternberg Museum of Natural History in Hays, Kansas, preserves the remains of a Xiphactinus with a complete skeleton of a smaller Gillicus in its stomach cavity. One reason for this type of preservation is that the large size of the Gillicus caused the death of the Xiphactinus after swallowing its prey. Photo taken by Corbin Rainbolt (BELOW) A skeleton of Gillicus on display at the KU Natural History Museum. In addition to the animal's bones, this particular specimen also preserves fossils of the animal's skin and scales. Photo by Corbin Rainbolt. (BELOW) Closeup view of the skull of a Gillicus . Note the presence of only very small teeth along the animal's jaws. These small teeth are often suggested to be a sign that these animals were filter feeders. Photo by Corbin Rainbolt. Ichthyodectes Temporal Range: 85 million years ago Geographic Range: Kansas, Alabama, South Dakota, Alberta, Manitoba, the UK Species: I. ctenodon, I. tenuidens Diet: Carnivorous, fish, marine reptiles Ichthyodectes was a large ichthyodectiform fish known for its unique dentition. Unlike Xiphactinus , which had large fang-like teeth for capturing large prey, Ichthyodectes had numerous small, uniformly sized teeth, giving rise to its species name “ctenodon ,” which translates roughly to “comb tooth.” These teeth suggest a feeding strategy focused on grasping or sucking in smaller fish rather than overpowering large prey. Fossil evidence in the form of a specimen preserved with the remains of another fish in its stomach, confirms that Ichthyodectes was piscivorous and actively hunted other fishes. (BELOW) A skeleton of Ichthyodectes on display at the KU Natural History Museum. Note the position of the dorsal fin. As we see in Xiphactinus , many ichthyodectiform fishes had dorsal fins which were positioned further along their backs than in other fishes. The tails of these animals were broad and heavily forked. Photo by Corbin Rainbolt (BELOW) An Ichthyodectes skull on display at the Sternberg Museum of Natural History. The teeth of Ichthyodectes are much smaller relative to the size of the rest of the skull than the teeth of Xiphactinus . Note the enlarged orbit, these animals probably had large eyes which could have allowed them to see in low light conditions. Photo by Corbin Rainbolt (BELOW) An Ichthyodectes vertebra from the Niobrara Chalk Formation in front view and side view. Fossil collected by Kris Super, photo by Corbin Rainbolt Prosaurodon Temporal Range: 83 million years ago Geographic Range: Kansas Species: P. pygmaeus Diet: Carnivorous, small prey, fish Prosaurodon was a relatively uncommon member of the ichthyodectiform fishes. It was closely related to Saurodon and Saurocephalus and would have had a slight extension to the length of its lower law’s predentary bone. Like other saurodontids, Prosaurodon was most likely an active marine predator with a streamlined body suited for pursuing prey in the open waters of the Late Cretaceous seas. It was smaller than most species of Saurodon or Saurocephalus and it seems to be one of the most basal members of this group, without many of the specialized jaw features seen in many of its relatives. It was originally considered a species of Saurodon , classified as “Saurodon pygmaeus ”. Saurocephalus Temporal Range: ~120 million years ago - 64 million years ago Geographic Range: Kansas, New Jersey, Jordan Species: S. lanciformis, S. ferox, S. albensis, S. dispar, S. dentatus Diet: Carnivorous, fish, squid Saurocephalus was a member of the saurodontid lineage of ichthyodectid fishes. Unlike its relative Saurodon it did not have a large pointed spike on its lower jaw and instead it seems to have depended largely on its specialized tooth arrangement when hunting. Both of its jaws were equipped with densely packed, sharp teeth that interlocked to form an efficient cutting mechanism. Rather than relying on large fangs to seize prey, as seen in Xiphactinus , or the smaller suction-feeding teeth found in Gillicus and Ichthyodectes , Saurocephalus had teeth specialized for slicing and shearing flesh. This arrangement allowed it to tackle relatively large prey by cutting it into manageable pieces before swallowing. Saurocephalus also had a slight extension of a part of the lower jaw known as the predentary bone. Although similar structures occurred in related saurodontid fishes, the blockier squared-off shape and proportions of this element gave Saurocephalus a unique skull profile. Saurocephalus lanciformis may have been among the final surviving ichthyodectiforms, persisting into the earliest Paleocene after most members of the group had become extinct. (BELOW) The dentary and maxilla of a Saurocephalus showing the pointed conical teeth found in these animals. The front of the dentary would have had a reinforced extension in life instead of the spines seen on the jaws of some of its close relatives. This fossil is on display at the McWane Science Center. Photo by Skye McDavid, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Saurodon Temporal Range: 84 million years ago - 72 million years ago Geographic Range: Kansas, Italy, Jordan Species: S. leanus, S. elongatus, S. intermedius Diet: Carnivorous, fish, squid Saurodon was a type of Ichthyodectiform fish well known for the large spike on its lower jaw. A long, pointed predentary bone projected well beyond the extent of the upper jaw which would have given these animals a prominent underbite unlike that seen in most other ichthyodectiform fishes. This unusual structure may have improved its ability to capture fast-moving prey and likely reflects a specialized feeding strategy. It may have used this bony spike to spear or slice through the smaller fish it hunted. Its narrow body and the streamlined shapes of its dorsal fin both suggest that these animals were built for moving very quickly though the water. (BELOW) A Saurodon skeleton on display at the LA County Museum. The skull of the animal is somewhat dissarticulated but it is positioned on the right-hand side of the image. Note the long, narrow shape of the animal's body. Photo by Jonathan Chen, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license (BELOW) A Saurodon skull fossil on display at the Sternberg Museum of Natural History. This skull is particularly well preserved and the teeth and the jaw spike are all present in association with the other elements of the skull. Photo by Neil Pezzoni, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Xiphactinus Temporal Range: 100 million years ago - 66 million years ago Geographic Range: the US, Canada, Venezuela, Argentina Species: X. audax, X. vetus? Diet: Carnivorous, fish, marine reptiles Among the ichthyodectiform fishes of the Cretaceous, Xiphactinus was one of the most distinctive and formidable predators. It could grow to be nearly 20 feet long which would have made it much larger than many of its relatives, including Gillicus and Ichthyodectes . Its elongated, streamlined body and powerful tail made it a fast-swimming hunter capable of pursuing prey in open water. One of the most striking features of Xiphactinus was its enormous jaws, which were lined with large, fang-like teeth designed for gripping slippery prey. Unlike some smaller ichthyodectiforms that had relatively small teeth and may have relied on suction feeding, Xiphactinus was adapted to seize and swallow large fishes whole. Fossil specimens have famously been discovered with entire prey animals preserved inside their stomachs, even including a six-foot long fossil of the related fish Gillicus arcuatus . Another characteristic that set Xiphactinus apart was its ecological dominance. While most ichthyodectiforms were predatory, Xiphactinus occupied some of the higher levels of the marine food web within the Western Interior Sea, the same levels occupied by mid-sized mosasaurs and the largest ancient sharks. (BELOW) A Xiphactinus skull on display the the Sternberg Museum of Natural History. The opercular bones seen on Xiphactinus are particularly large and very sturdy. This particular specimen also preserves a series of vertebrae which can be seen to the right of the orbit in this picture. Their vertebral centra were circular and their sides were covered in a ridged texture. Fossils of ichthyodectiform vertebrae are common finds in some parts of Kansas. Photo - Corbin Rainbolt (BELOW) A Xiphactinus tooth. The largest teeth in the mouths of these fish were found at the front of their mouths. Fossil collected by Kris Super, photo by Corbin Rainbolt (BELOW) A Xiphactinus skeleton. This fossil preserves the remains of a smaller fish, whose skeleton can be seen underneath the spine of the Xiphactinus . Several Xiphactinus skeletons have been found preserved with smaller fish in their stomachs. Photo by Chip Clark, Smithsonian, National Museum of Natural History, accessed via Wikimedia Commons, Public Domain image. Pycnodontiformes Cretaceous Shell Crushing Fish Cretaceous pycnodont fishes were a group of Mesozoic ray-finned fishes who mostly had specialized durophagous diets, meaning that they used their flattened teeth to crush hard-shelled prey. Some types of pycnodonts may have also evolved more unusual tooth structures capable of capturing soft-bodied prey and may have preferred to hunt fish and squid. These fishes were especially common in shallow coastal and reef ecosystems, though certain species also inhabited brackish and possibly even freshwater settings. Many species had nearly circular body profiles which is an adaptation that we see in many living reef fish that likely allowed them to move efficiently through fields of giant clams, rudist shell beds and other structurally complex environments. Their most recognizable feature was a battery of rounded, pavement-like teeth set within strong jaws, which were built for crushing hard-shelled prey such as clams, oysters, crabs and shrimp. They are often compared to living triggerfishes, a separate group whose members also have deep bodies and teeth used to crush hard shells. Anomoeodus Temporal Range: 100 million years ago - 66 million years ago Geographic Range: North America, Europe, northern Africa, central Asia Species: A. latidens, A. phaseolus, A. pauciseriale, A. wolfi, A. mississippiensis, A. angustus, A. willetti, A. subclavatus Diet: Durophagous, bivalves, crabs, shrimp Anomoeodus was a pycnodont fish that lived in the Cretaceous seas and was especially well adapted for feeding on hard-shelled prey. Like other pycnodonts, it had a deep and laterally compressed body that allowed it to swim easily through shallow marine habitats. Its most characteristic features were its strong jaws and broad crushing teeth which ranged from rounded to kidney-shaped in form and were arranged in multiple rows. These specialized dental batteries helped these animals crack and process tough-shelled organisms. It was primarily a durophagous predator, feeding on different types of marine mollusks and crustaceans. Its fossils are found in many deposits from shallow seas found across the world, mostly representing individual teeth or sets of tooth plates. (BELOW) A tooth of Anomoeodus . The teeth of these fish were arranged in complex tooth plates with rounded or oval-shaped teeth. Image A shows the chewing surface, Image B shows the lower surface of the tooth. Original photo taken by Dylan Falkner/Prehistoric Alabama (BELOW) A partial pycnodont tooth plate from the Early Cenomanian of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo Coelodus Temporal Range: ~90 million years ago Geographic Range: Worldwide distribution Species: C. saturnus, C. stantoni Diet: Durophagous, bivalves, crabs, shrimp Coelodus was a pycnodont fish that looked broadly similar to Amomoedus , although these two types of fish had fairly distinct tooth arrangements. Coelodus had tooth sets consisting of rows of smooth, rounded crushing teeth adapted for processing hard food items. In some species, such as Coelodus stantoni , the teeth were more elongated and kidney-shaped. These animals would have made their homes at the bottoms of shallow Cretaceous seas and would have used their broad teeth to crush up the shells of many types of benthic animals. Given the relative scarcity of arthropod remains in the places where its fossils are found in the Western Interior Sea, it is likely that its diet would mostly have been primarily composed of small bivalves, relatives of today’s clams and oysters. (BELOW) A Coelodus costai skeleton on display at the Natural History Museum of Milan, Italy. This skeleton shows the distinctive crosshatch-like pattern of bones along the neural spines behind the head of the fish, a feature seen in many pycnodonts. Photo by Giovanni Dall'Orto, accessed via Wikimedia Commons, distributed under a CC BY-SA 2.5 IT license (BELOW) A Coelodus saturnus tooth plate. Photo accessed via Wikimedia Commons, uploaded by user Ghedo and distributed under a CC BY-NC-SA 4.0 license Gyrodus Temporal Range: 169 million years ago - 120 million years ago Geographic Range: Possible North American record, Germany, Argentina, Italy, Spain Species: G. circularis, G. hexagonus, G. huiliches Diet: Durophagous, bivalves, crabs, shrimp Gyrodus has a set of teeth which differs somewhat from the arrangement seen in many other pycnodont genera. Its teeth had a distinctive depressed pit on their chewing surfaces and the lower jaw held within it a set of four highly specialized dentary teeth. While most pycnodonts are thought to have been closely associated with shallow coastal or reef environments, evidence suggests that Gyrodus may have spent considerable time in more open-water, pelagic settings. Its body form and fin structure appear to support this interpretation. Although fossils of Gyrodus found in North America are usually fairly small, some fossils from Europe seem to have belonged to animals that were well over 3 feet long, making them rather large by the standards of the pycnodont group. (BELOW) A fossil skeleton of a Gyrodus circularis. This particular fossil is from the Jurassic of Germany. On display at the Natural History Museum of Vienna. Photo by Thaler Tamas, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license (BELOW) A Gyrodus cuvieri tooth plate fossil. This particular fossil is from the Jurassic of France. Scale bar is measured in centimeters. Part of a collection produced by the Muséum d'histoire naturelle de Neuchâtel, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Micropycnodon Temporal Range: 85 million years ago Geographic Range: North America, Europe, western Asia Species: M. kansasensis Diet: Durophagous, bivalves, crabs, shrimp Micropycnodon was a small type of pycnodont known primarily from fossils found in parts of central and western Kansas. It had a set of specialized incisor-like teeth at the front of its jaws and crushing tooth plates farther back. Compared with many other pycnodonts, Micropycnodon is known from relatively complete fossil material that reveals details of its body armor and skeletal structure. It had a roughly diamond-shaped body and was covered in thick scales. It also would have had a few rows of larger raised scale-like features on some parts of its body, similar to the hardened denticles found in many other types of fish. (BELOW) Micropycnodon 's range wasn't limited to North America! This tooth fossil from a Micropycnodon kansasensis was found in the Menuha Formation in the Negev desert. Photo by Andrew Retzler, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Palaeobalistum Temporal Range: 112 million years ago -48 million years ago Geographic Range: central US, Brazil, Lebanon Species: P. goedeli, P. flavellatum, P. geiseri, P. orbiculatum, P. rectidens, P. dossantosi Diet: Durophagous, bivalves, crabs, shrimp Palaeobalistum is a pycnodont fish known from many sites around the world, including parts of the central US, Brazil and Lebanon. Its fossil record even extends past the end of the Cretaceous period and into the Cenozoic era with possible fossils belonging to this genus dating to as recently as the Eocene epoch around 48 million years ago. Like many other pycnodont fishes it had column-shaped teeth adapted for a durophagous diet of hard-shelled animals. (BELOW) A photo of a skeleton of Palaeobalistum goedeli from the Late Cretaceous of Lebanon, on display at the Museo Civico di Storia Naturale di Milano. Photo accessed via Wikimedia Commons, uploaded by user Hectonichus distributed under a CC BY-NC-SA 4.0 license Semionotiformes Semionotiform fishes were members of an extinct order of fishes who were distant relatives of modern gars. They came in many forms, including some animals who had long, narrow bodies while others were more heavily built. In the Western Interior Sea this order is possibly represented by a type of fish called Hadrodus , which is sometimes also classified as a member of the pycnodont fish group. It seems to have been a mid-sized durophagous fish adapted for crushing hard shells and exoskeletons. Hadrodus Temporal Range: 83 million years ago - 73 million years ago Geographic Range: Central and eastern North America Species: H. priscus Diet: Durophagous, bivalves, crabs, shrimp Hadrodus was an enigmatic fish from the Cretaceous period which is sometimes considered to be part of the larger pycnodont group, or alternatively considered a member of its own separate family, Hadrodontidae, within the order Semionotiformes. These animals had individual separate teeth in their mouths instead of the fused tooth plates seen in pycnodonts. They also had highly specialized blunt branchial teeth along the insides of their throats. These teeth probably helped break down food after it had been crushed by the jaws as a way to further break up its food while swallowing. (BELOW) A Hadrodus tooth and jaw fragment from the Campanian of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo Crossognathiformes Crossognathiform Fishes of the Midwest Crossognathiform fishes were a diverse group of early teleost fishes that lived in marine environments throughout much of the Cretaceous Period. They were typically streamlined, fast-swimming predators with elongated bodies adapted for life in open water. During the Late Cretaceous, the group diversified and moved into many new ecological niches occupied by animals like tuna today. Their fossils are found in Cretaceous marine deposits around the world. They typically had elongated opercular bones covering their gills and specialized sensory canals along the upper parts of their skulls. Some genera such as Apsopelix and the members of the family pachyrhizodontidae, developed sleek body shapes that were well suited for active predation, although they varied widely in size. Some grew to be as large as a bluefin tuna, while others were only a few feet long. Apsopelix Temporal Range: 95 million years ago - 80 million years ago Geographic Range: Kansas, Nebraska, Texas, western Europe, Japan Species: A. anglicus, A. sauriformis, A. miyazakii Diet: Carnivorous, small fish, squid Apsopelix was a small crossognathiform fish that lived in the Late Cretaceous seas, including the Western Interior Sea. In contrast to many of its relatives, which were adapted for hunting other fishes, Apsopelix appears to have specialized in feeding on very small organisms suspended in the water. Fossils show that it had long, densely packed gill rakers that would have been useful for filtering plankton and other microscopic food particles. Its remains are sometimes found in the stomachs of other larger animals from the Western Interior Sea. (BELOW) A photo of an Apsopelix skeleton, on display at the Sternberg Museum of Natural History. Photo by James St. John. accessed via Wikimedia Commons and distributed under a CC BY-NC-SA 4.0 license Elopopsis Temporal Range: 100 million years ago - 90 million years ago Geographic Range: Kansas, Nebraska, Colorado, Saskatchewan, parts of Europe Species: E. crassus, E. fenzli, E. microdon, E. smithwoodwardi, E. ziegleri Diet: Carnivorous, small fish, squid Elopopsis is a crossignathiform fish classified as part of the family Pachyrhizodontidae. It was a streamlined predatory fish that lived across a broad geographic range extending throughout Europe, North Africa and North America, including the Western Interior Sea. Its name, meaning “Elops-like face,” reflects its superficial resemblance to modern ladyfish, Elop s, and related fishes. Fossil evidence suggests that Elopopsis preyed on smaller fishes and occupied an important role as a mid-level marine predator despite its fairly small body size. Pachyrhizodus Temporal Range: 94 million years ago - 61 million years ago Geographic Range: North and South America, Australia, Europe Species: P. caninus, P. minimus, P. leptognathus, P. curvatus, P. kingi Diet: Carnivorous, fish, marine reptiles Pachyrhizodu s was a genus of predatory crossognathiform fish that lived in ocean environments from the Late Cretaceous into the Paleocene. It had a streamlined body, heavily built jaws and large conical teeth that were well suited for capturing and holding prey. Some species grew to be very large and were fearsome predators within ancient marine ecosystems. The best known species of Pachyrhizodus from the Western Interior Sea is Pachyrhizodus caninus . Among its most distinctive characteristics were its heavy lower jaws and strongly developed premaxillary bones, adaptations that gave it a powerful bite. A much smaller species, Pachyrhizodus minimus , is known from very complete body fossils which preserve their scaly skin as well as their skeletal remains. (BELOW) A small skeleton of Pachyrhizodus on display at the Sternberg Museum of Natural History in Hays, Kansas. This particular specimen is notable since it not only preserves the animal's bones but also a section of its digestive tract. Photo by Corbin Rainbolt (BELOW) A Pachyrhizodus caninus vertebra from the Middle Turonian of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo (BELOW) A photo of a Pachyrhizodus jaw . The shape of the teeth and strong jaws of these fish led them to occasionally have their remains mistaken for those of mosasaurs. Photo by user "the_paleobear". accessed via Wikimedia Commons and distributed under a CC BY 2.0 license Elopiformes The elopiform fishes are an order of animals which includes today’s tarpons and ladyfishes. These animals typically have long, streamlined bodies, smooth scales, forked tails, and large mouths. One of their main identifying features is an extra set of bones in the throat, which helps distinguish them from other fish groups. They also have a unique larval stage called a leptocephalus. In the Western Interior Seaway during the Late Cretaceous, elopiforms included extinct animals like Laminospondylus. Laminospondylus Temporal Range: 87 million years ago Geographic Range: Texas Species: L. transversus Diet: Carnivorous, fish, squid Laminospondylus was an uncommon Cretaceous fish represented by the species Laminospondylus transversu s, which was first described from Late Cretaceous deposits in Texas. The genus is classified as either a crossognathiform fish or early elopiform fishes, part of a lineage that includes the ancestors of modern ladyfish and tarpon-like fishes. Its most distinctive characteristic was its unusual layered and thickened vertebral anatomy, which inspired the genus name and served as one of the primary features used to distinguish it from related fishes. Pachycormidae Ancient Swordfish Mimics Pachycormid fishes were a group of very bizarre Mesozoic fishes which had elongated pectoral fins, reduced pelvic fins and a reduced bony skeleton with many elements replaced with cartilage. Some species evolved streamlined, powerful bodies resembling modern tunas or swordfish and became active predators, while others developed into enormous filter-feeding fishes that consumed plankton and other tiny organisms suspended in the water. Within the Western Interior Sea we find fossils of both filter feeding and macropredatory pachycormid fishes. Bonnerichthys Temporal Range: 89 million years ago - 66 million years ago Geographic Range: Kansas, Alabama, South Dakota Species: B. gladia Diet: Filter feeder, plankton Bonnerichthys was a giant filter-feeding pachycormid which could grow to be much larger than other Cretaceous pachycormids like Protosphyraena . It was a giant suspension feeder that got its food by filtering plankton from seawater rather than actively pursuing prey. Unlike the sharp-toothed, fish-eating pachycormids, Bonnerichthys did not have large grasping teeth and instead had a specialized filtering system made up of gill arches and a series of closely spaced gill rakers. These structures formed a sieve used for capturing tiny organisms from the water column. Bonnerichthys occupied an ecological niche comparable to that of modern whale sharks, basking sharks and baleen whales. Its specialized feeding strategy shows us that giant plankton-feeding fishes had already evolved millions of years before the appearance of today's largest marine filter feeders. Remains from this animal were described during the nineteenth century but were mistakenly assigned to several different genera, including Portheus (Xiphactinus) and Protosphyraena . As additional and more complete material became available, researchers recognized that these fossils represented a distinct type of pachycormid. In 2010, the genus Bonnerichthys was formally established and named in honor of the Bonner family and their work collecting Kansas fossils. Protosphyraena Temporal Range: 100 million years ago - 66 million years ago Geographic Range: Kansas, parts of the UK, Australia Species: P. ferox, P. perniciosa, P. nitida Diet: Carnivorous, fish, squid Protosphyraena' s anatomy has often been compared to that of a modern swordfish, Despite this it was actually a Pachycormid and not closely related to either swordfish or the barracudas for which it was named (Name roughly translates to "early barracuda). In contrast to many pachycormids, which were adapted for filter feeding or general predation, Protosphyraena evolved into a highly efficient open-water hunter. It had a long, pointed snout, large forward-facing teeth, a torpedo-shaped body and elongated pectoral fins. In certain species, these fins were strengthened and lined with serrations along their leading edges. They would have had very wide crescent-shaped tails. Their fossils have been found in parts of Kansas since as far back as the 1870s and became famous thanks to the impressive fossils of their large teeth and sword-like fins. (BELOW) A tooth of a Protosphyraena from the front of the animal's mouth viewed from the front and back (A, B) and from the side (C). Original photo taken by Dylan Falkner/Prehistoric Alabama (BELOW) A photo of an Protosphyraena skull, on display at the University of Michigan Museum of Natural History. Photo by Jonathan Chen. accessed via Wikimedia Commons and distributed under a CC BY-NC-SA 4.0 license (BELOW) A photo of a Protosphyraena ferox fin showing the serrations on the front edges of the front-most fin ray. Photo by user Ghedoghedo. accessed via Wikimedia Commons and distributed under a CC BY-SA 3.0 license Aulopiformes Prehistoric Relatives of Barracudas Aulopiformes is an order of fish which includes today’s barracudas and lizardfishes. Although this group has many living members still found today, several extinct aulopiform lineages are known from fossils dated to the Cretaceous period. The most successful aulopiforms found in the Western Interior Sea were the Enchodontoidei, including the well-known saber-toothed fish Enchodu s, the long-bodied predator Cimolichthys and the sleek, pelagic fish Apateodus . Apateodus Temporal Range: 87 million years ago - 72 million years ago Geographic Range: central and southern North America, the UK, India, Russia Species: A. crenellatus, A. busseni Diet: Carnivorous, fish, squid Apateodus was a medium-sized fish with a slender, streamlined body adapted for active swimming in open marine environments. The genus is primarily recognized from cranial remains. Its jaws carried sharp teeth adapted for capturing prey, but compared with the saber-toothed Enchodus, Apateodus lacked the enormous fang-like palatine teeth and instead had a more generalized dentition. Its skull was narrower than the crania seen in most Enchodus species. Cimolichthys Temporal Range: 95 million years ago - 66 million years ago Geographic Range: Parts of the US and Canada, western and central Europe Species: C. nepaholica, C. levesiensis Diet: Carnivorous, fish, squid Cimolichthys was an aulopiform with a long, streamlined body similar to that of a barracuda. This lance-shaped body probably made it a fast swimmer as it moved through open water. The skull of Cimolichthys was relatively lightly constructed but supported multiple rows of sharp, forward-leaning teeth that were just the right shape for capturing and holding active prey such as fish and cephalopods. This genus is also known for the sets of robust bony scutes along its dorsal and ventral sides. These armored structures provided protection while still allowing the fish to remain an effective swimmer. It seems to have had two rows of scutes along the top and bottom of its body. (BELOW) A fossilized skeleton of Cimolichthys on display at the KU Natural History Museum in Lawrence, Kansas. Note the rows of bony scales running along the dorsal and ventral sides of the animal and the deeply forked tail. Photo was taken by Corbin Rainbolt (BELOW) A fossilized set of Cimolichthys hypurals found in the Niobrara Chalk Formation. These bony rays support the tail fin lobes of most types of ray-finned fishes, fossil collected by Kris Super, photo by Corbin Rainbolt (BELOW) A fossilized Cimolichthys vertebra found in the Niobrara Chalk Formation. Side facing upwards (A) and lower portion showing leftover chalk matrix (B). Hourglass-shaped vertebrae are often seen in aulopiform fishes. Fossil collected by Kris Super, photo by Corbin Rainbolt Enchodus Temporal Range: 105 million years ago - 66 million years ago Geographic Range: Worldwide distribution Species: E. petrosus, E. dirus, E. ferox, E. gladiolus, E. machairus, E. shumardi Diet: Carnivorous, fish, squid Enchodus is one of the most recognizable predatory fishes of the Late Cretaceous and is best known for the enormous fang-like teeth that earned it the nickname “saber-toothed herring.” This genus had a pair of large fangs positioned near the front of the mouth, often accompanied by additional enlarged teeth in the upper and lower jaws. These spear-like teeth were used for seizing and holding slippery prey and are among the most commonly preserved Enchodus fossils. The genus name itself translates to “spear tooth,” which reflects this particular adaptation. In addition to its enlarged fangs, Enchodus had large eyes and a lightly built skull, which hints at an active predatory lifestyle. Most species grew to be only a few feet long. Some species also had particularly large fan-shaped pectoral fins. (BELOW) An Enchodus fang viewed from the front (A) and the side (B). Original photo taken by Dylan Falkner Leptecodon Temporal Range: 85 million years ago Geographic Range: Kansas Species: L. rectus Diet: Carnivorous, fish, small invertebrates Leptecodon is a genus of small aulopiform fishes which were related to animals like Enchodus. They had mouths with small pointed teeth and seem to have preyed upon smaller fishes and invertebrates living near the seafloor. Their fossils are often found in association with large Inoceramid clams and they are thought to have used the insides of these mollusks as a place to shelter from possible predators. Its fossils are sometimes found in the same places as those of the small Cretaceous beardfish Omosoma . Stratodus Temporal Range: 66 million years ago - 50 million years ago Geographic Range: Kansas, Alabama, South Dakota, western Africa, western Asia Species: S. apicalis, S. indamanensis, Diet: Carnivorous, fish, squid Stratodus was a giant aulopiform fish with an eel-like body, an elongated dorsal fin and a strange set of tiny teeth unlike those seen in most other predatory fishes. One of its most unique features was the presence of several rows of small teeth lining the upper jaw, along with more rows of larger inward-curving conical teeth. This dental arrangement was probably used for seizing and retaining prey and inspired the genus name, which means “layered tooth.” Stratodus also stands out among aulopiform fishes because of its large size. Fossil evidence indicates that some individuals grew to be well over 14 feet long, which would make it among the largest known members of the order Aulopiformes. Its body was slender and elongated, similar to an enlarged and stretched out lizardfish and was protected by thick, spiny scutes along the flanks. See the rest of our entries about Cretaceous ray-finned fishes by continuing to the next page Next Page Media Gallery Suggested References: Ray-Finned Fishes Alvarado-Ortega, Jesús; Porras- Múzquiz, Héctor ( 2009). "On the occurrence of Gillicus arcuatus (Cope, 1875) (Pisces, Ichthyodectiformes) in Mexico". Boletín de la Sociedad Geológica Mexicana. 61 (2): 215–224. Bardack, D. (1965). "Anatomy and evolution of chirocentrid fishes". The University of Kansas Paleontological Contributions. 10: 1–88. Bell, G. L., Jr. (1986). “A pycnodont fish from the upper Cretaceous of Alabama”. Journal of Paleontology 60(5):1120-1126, 2 figs. Bice, K. N.; Shimada, K.; Kirkland, J. I. (2013). "Late Cretaceous Marine Fishes from the Upper Greenhorn Limestone in Southeastern Nebraska". Transactions of the Kansas Academy of Science. 116 (1–2): 22–26. Cope, E. D, (1872), “On the families of fishes of the Cretaceous Formation of Kansas”: Proceedings of the American Philosophical Society, v. 12, p. 327-357. Everhart, M. J. (2007). “Remains of a pycnodont fish (Actinopterygii: Pycnodontiformes) in a coprolite; An upper record of Micropycnodon kansasensis in the Smoky Hill Chalk, western Kansas”. Kansas Academy of Science, Transactions 110(1/2): 35-43. Everhart, M. J. (2010). “Bonnerichthys gladius – The largest bony fish and first known planktivore from the Late Cretaceous”. Kansas Academy of Science, Transactions 113(1-2):123-124. Fielitz, C.; Stewart, J. D.; & Wiffen, J. (1999). “Aethocephalichthys hyainarhinos gen. et sp. nov., a new and enigmatic Late Cretaceous actinopterygian from North America and New Zealand”. Mesozoic Fishes 2 – Systematics and Fossil Record, G. Arratia & H.-P. Schultze (eds.): pp. 95-106, 7 figs. Fielitz, Christopher; González-Rodríguez, Katia A. (2010). "A New Species of Enchodus (aulopiformes: Enchodontidae) from the Cretaceous (albian to Cenomanian) of Zimapán, Hidalgo, México". Journal of Vertebrate Paleontology. 30 (5): 1343–1351. Bibcode:2010JVPal..30.1343F. Fielitz C, Shimada K. (1999).”A new species of Bananogmius (Teleostei; Tselfatiformes) from the Upper Cretaceous Carlile Shale of western Kansas”. Journal of Paleontology 73(3): 504-511. Friedman, Matt; Beckett, Hermione T.; Close, Roger A.; Johanson, Zerina (2016). "The English Chalk and London Clay: two remarkable British bony fish Lagerstätten". Geological Society, London, Special Publications. 430 (1): 165–200. Grande, Lance; Bemis, William E. (1998). "A Comprehensive Phylogenetic Study of Amiid Fishes (Amiidae) Based on Comparative Skeletal Anatomy. an Empirical Search for Interconnected Patterns of Natural History". Journal of Vertebrate Paleontology. 18 (sup1): 1–696. Kanarkina, A.; Zverkov, N.; Polyakova, I. (2024). "New evidence of the global distribution of the swordfish-like pachycormid Protosphyraena in the late Early Cretaceous and a review of global records of the genus". Cretaceous Research. 166. Lucas, Spencer G.; Sullivan, Robert M. (2006). “Late Cretaceous Vertebrates from the Western Interior: Bulletin 35”. New Mexico Museum of Natural History and Science. Maisey, J. G., (1996). “Discovering Fossil Fishes”. Henry Holt and Company, New York. McIntosh, A. P.; Shimada, K.; Everhart, M. J. (2016). "Late Cretaceous marine vertebrate fauna from the Fairport Chalk Member of the Carlile Shale in southern Ellis County, Kansas, U.S.A." Transactions of the Kansas Academy of Science. Near, Thomas J.; Thacker, Christine E. (2024). "Phylogenetic Classification of Living and Fossil Ray-Finned Fishes (Actinopterygii)". Bulletin of the Peabody Museum of Natural History. 65 (1): 3–302. Nelson, Luke E.; Murray, Alison M.; Sims, Megan E. (2026). "A new species of Enchodus from the Western Interior Seaway of North America and a taxonomic revision of Enchodus dirus, Leidy, 1857". Journal of Vertebrate Paleontology. Russell, D. A. (1988). “A check list of North American marine Cretaceous vertebrates including freshwater fishes”. Royal Tyrrell Museum Shimada, Kenshu (2016). "A new species of the Late Cretaceous 'sail-finned' bony fish, Pentanogmius (Actinopterygii: Tselfatiiformes), from Texas, USA". Cretaceous Research. 61: 188–198. Shimada, K. and M. J. Everhart. (2004). “Shark-bitten Xiphactinus audax (Teleostei: Ichthyodectiformes) from the Niobrara Chalk (Upper Cretaceous) of Kansas”. The Mosasaur 7, p. 35-39. Shimada, K. (2015). “Body form and paleoecology of the large Late Cretaceous bony fish, Pachyrhizodus caninus”. Cretaceous Research, 52, 286-291. Stewart, A. (1899). “Pachyrhizodus minimus, a new species of fish from the Cretaceous of Kansas”. Kansas University Quarterly, 8(1), 37-38. Taverne, Louis (2001). "Révision du genre Bananogmius (Teleostei, Tselfatiiformes), poisson marin du Crétacé supérieur d'Amérique du Nord et d'Europe". Geodiversitas. 23 (1): 17–40. Vavrek, Matthew J.; Murray, Alison M.; Bell, Phil R. (2016). "Xiphactinus audax Leidy 1870 from the Puskwaskau Formation (Santonian to Campanian) of northwestern Alberta, Canada and the distribution of Xiphactinus in North America". Vertebrate Anatomy Morphology Palaeontology. 1: 89.

Explore the different kinds of marine arthropods who lived in the Western Interior Sea during the Cretaceous period, including lobsters, barnacles, clam shrimp and crabs Arthropods Crustaceans and Horseshoe Crabs The fossils of aquatic arthropods, including crabs, lobsters, barnacles and horseshoe crabs, sometimes turn up in deposits left behind by the Western Interior Sea. These animals shared a few common arthropod characteristics, including segmented bodies, jointed limbs, and outer exoskeletons composed of a material called chitin. Decapods, the group containing crabs, lobsters, and similar ten-legged animals went through a diversification event during the Cretaceous period and many more familiar forms of these animals appear in the fossil record around this time. Some remains of horseshoe crabs which appear very similar to their living relatives are also found in layers of sedimentary stone formed in nearshore environments. Arthropod remains are sometimes incomplete because their exoskeletons could separate after death or be discarded during the molting process. Most arthropod groups are fully mobile animals but in the case of some crustaceans such as gooseneck barnacles, the adult forms of these organisms attach themselves to a hard surface such as a rock or a piece of driftwood. Arthropods (Browse by class, order) Horseshoe crabs (Order Xiphosura) Limulus Clam shrimp (Superorder Diplostraca) Cyzicus Seed shrimp (Class Ostracoda) Cytherella Barnacles (Subclass Ciripedia) Stramentum Decapods (Order Decapoda) Ghost shrimp (Familly Callianassidae) Protocallianassa Prawns (Suborder Dendrobranchiata) Penaeus Spiny lobsters (Infraorder Achelata) Linuparis Panulirus Rugafarius Lobsters (Infraorder Astacidae) Hoploparia True crabs (Infraorder Brachyura) Horseshoe Crabs (Xiphosura) Temporal Range: 480 million years ago - Present Diet: Benthic carnivore/scavenger, fish, invertebrates Genera in the Western Interior Sea: Limulus Horseshoe crabs are marine arthropods that are more closely related to other chelicerates like spiders and scorpions than they are to true crabs. A tough, horseshoe-like or crescent-shaped exoskeleton protects their bodies, which consist of three sections: the prosoma, opisthosoma, and telson. The prosoma holds the eyes, mouth, and legs, while the opisthosoma carries movable spines and five pairs of book gills used to breathe underwater. The long telson is a sharp spine on the rear part of the animal. This structure is not venomous or used for stinging. Instead it helps with steering and allows the animal to flip itself over when it ends up lying upside down. They spend most of their lives at sea but will come ashore in order to lay their eggs. (ABOVE) The genus Liumulus, the same genus as the extant horseshoe crab which is also found in Cretaceous deposits Crustaceans Crustaceans are a very diverse order of arthropods which includes animals like isopods, ostracods, lobsters and crabs. Their clearest diagnostic feature is the presence of two pairs of antennae, including one shorter set called the antenulles. A crustacean’s segmented body is generally divided into the head, thorax, and abdomen, similar to what is seen in insects. In some species the head and thorax unite to form a cephalothorax which is often protected by a shell or carapace. Their bodies are supported by a hard, chitinous exoskeleton that must be molted and shed as the animal grows. Aquatic crustaceans usually take in oxygen through their gills, but many forms of crustaceans have independently adapted to spend extended periods of time on land, and some groups have even become fully terrestrial. Clam Shrimp (Diplostraca) Temporal Range: ~410 million years ago - Present Diet: Microscopic filter feeder/carnivore, tiny plants and animals Genera in the Western Interior Sea: Cyzicus Clam shrimps are branchiopod crustaceans whose bodies are protected by a hinged shell made of two valves, giving them a clam-like appearance. Despite this feature and their resemblance to their namesake clams, clam shrimps are still crustaceans rather than bivalve mollusks. They have a series of feathery legs which stick out slightly from the edges of their shells. They also use a specialized set of antennae to push themselves forward when swimming. They are able to seal the valves of their shells closed when they feel threatened. Today these animals usually live in shallow freshwater ponds and temporary pools, although in the distant past these animals seem to have been able to live outside of ephemeral environments and may have survived as fully marine organisms. Many living species endure dry conditions by laying eggs which can stay dormant in the sediment until water returns. (ABOVE) The clam shrimp genus Cyzicus is a genus whose shells are found in Cretaceous deposits but which is also found living in ephemeral pool environments today Seed Shrimp (Ostracoda ) Temporal Range: ~460 million years ago - Present Diet: Detritivore, bacteria, algae, mold, other organic waste Genera in the Western Interior Sea: Cytherella Ostracods, often known as seed shrimp, are microscopic crustaceans whose bodies are surrounded by a hinged shell consisting of two valves. This carapace is mostly made of chitin reinforced with calcium carbonate. The presence of this layer of calcium carbonate allows these small animals to fossilize more easily than some other types of microinvertebrates. Although their shells resemble those of tiny clams, ostracods have jointed limbs like most other arthropods, as well as antennae which they use when they are swimming, crawling, and/or detecting their surroundings. (ABOVE) The ostracod genus Cytherella . These animals are generally quite small, less than 1/2 a millimeter long. Barnacles (Cirripedia ) Temporal Range: ~320 million years ago - Present Diet: FIlter feeder, Plankton Genera in the Western Interior Sea: Stramentum Cirripedia the subclass of crustaceans that contains barnacles. Most adult barnacles remain firmly attached to rocks, shells, or driftwood once they have fully matured. Their bodies are usually surrounded by a hard calcified mantle. Their feather-like limbs called cirri reach through the shell opening to collect plankton and suspended organic material as part of their suspension-feeding process. Although adults are stationary, their larvae swim freely before selecting a surface and becoming attached. Acorn barnacles fasten directly to surfaces, while goose barnacles attach themselves to surfaces along flexible stalks. Gooseneck barnacles, like the genus Stramentum, are the best represented group of barnacles known from deposits formed by the Western Interior Sea. (ABOVE) The genus Stramentum is the most common barnacle found fossilized in the Western Interior Sea. As a gooseneck barnacle it would have been connected to a hard surface by a tube of softer tissue which is rarely preserved. Decapods (Decapoda) Temporal Range: ~376 million years ago - Present Diet: Includes active predators, scavengers, detritivores Genera in the Western Interior Sea: Protocallianassa, Penaeus, Panulirus, Rugafarius, Linuparis, Hoploparia, Cenomanocarcinus, Dakoticancer, Necrocarcinus, Hoplitocarcinus, Zygastrocarcinus, Heus, Raninella, Dromiopsis, Dioratiopus, Xanthosia, Plagiophthalmus Decapods are a group of crustaceans who are named for their tendency to have ten legs. Many members of this group, including crabs, lobsters and ghost shrimp, have limbs which have transformed over time into chelae, or claws, of various sizes. In some other lineages, like the spiny lobsters, the five pairs of legs remain roughly the same size while their antennae grow to be quite large. The segmented bodies of decapods consist of a cephalothorax, formed by the fused head and thorax, and a rear, often segmented, section called an abdomen. A rigid carapace covers the cephalothorax and protects the organs and gills beneath. A series of smaller appendages called pleopods which can be found on the lower surfaces of the tails of some decapods are used to help these animals swim. In some forms of shrimp, spiny lobsters and true lobsters, the muscular sections of their abdomens end in tail fans formed by the telson and uropods. These structures are used to help these animals steer themselves through the water. In crabs, the abdomen is generally reduced and folded beneath the cephalothorax. This abdominal reduction is a feature found in true brachyuran crabs as well as several groups of anomuran “false crabs”. (ABOVE) The genus Protocallianassa from the Late Cretaceous. Note the asymmetrical claws (ABOVE) The genus Penaeus, an extant genus also known from the Late Cretaceous of Kansas. Like many types of prawns these animals have prominent pleopods on their ventral surfaces (ABOVE) Spiny lobsters of the Western Interior Sea: (A) Panulirus, (B) Linuparis, (C) Rugafarius (ABOVE) The genus Hoploparia , a Late Cretaceous lobster. These animals had a fairly wide geographic distribution and could grow to be as large as some extant lobsters. (ABOVE) Types of crabs found in the Western Interior Sea. Remains from these animals are fairly rare in deposits from central North America Crab Genera Family Dakoticancridae: Dakoticancer Family Cenomanocarcinidae : Cenomanocarcinus Family Homolidae: Hoplitocarcinus, Zygastrocarcinus, Plagiophthalmus? Family Raninidae: Heus, Raninella Family Longodromitidae: Dioratiopus Family Necrocarcinidae: Necrocarcinus Family Dromiidae: Dromiopsis Family Xanthidae: Xanthosia Suggested References: Arthropods Digital Atlas of Ancient Life. (n.d.). Maxillopoda. Cretaceous Atlas of Ancient Life | Maxillopoda. https://www.cretaceousatlas.org/classes/maxillopoda/ Digital Atlas of Ancient Life. (n.d.). Branchiopoda. Cretaceous Atlas of Ancient Life | Branchiopoda. https://www.cretaceousatlas.org/classes/branchiopoda/ Digital Atlas of Ancient Life. (n.d.). Malacostraca. Cretaceous Atlas of Ancient Life | Malacostraca. https://www.cretaceousatlas.org/classes/malacostraca/ Dunlop, J. A., Penney, D. & Jekel, D. (2013). “A summary list of fossil spiders and their relatives”. In Platnick, N. I. (ed.) The world spider catalog, version 14.0 Everhart, M.J. (2017). “Oceans of Kansas - A Natural History of the Western Interior Sea,” Second Edition. Indiana University Press, 460 pp. Newman, W. A. et al., 1969. Cirripedia in Moore, R. C., ed., “Treatise on Invertebrate Paleontology, Part R, Arthropoda 4(1), Volume 1”. The University of Kansas and Geological Society of America. 398 pp. Schweitzer, C.E.; Feldmann, R.M. (2001). "New Cretaceous and Tertiary decapod crustaceans from western North America". Bulletin of the Mizunami Fossil Museum. 28: 173–210. Sammy De Grave; N. Dean Pentcheff; Shane T. Ahyong; et al. (2009). "A classification of living and fossil genera of decapod crustaceans" (PDF). Raffles Bulletin of Zoology. Suppl. 21: 1–109.

The Western Interior Sea once spanned from the Gulf of Mexico to the Arctic Ocean and split North America in two during the Late Cretaceous period The Cenozoic Era 66 million years ago - Present The Cenozoic era is the span of time lasting from the end of the Cretaceous period 66 million years ago all the way up to the present day. Often this time period is referred to as the “Age of Mammals” since many of the new ecological niches which opened up after the extinction of the non-avian dinosaurs were taken over by mammals. Despite this, mammals were not the only animals to radiate into new roles over the course of the Cenozoic. Many types of crocodylomorphs, birds, snakes and turtles grew to gigantic sizes at various points during this era. Much like the Mesozoic era, the Cenozoic is divided into three periods; the Paleogene period, The Neogene period, and the Quaternary period. The Paleogene was the first period of the Cenozoic and this span of time saw the first appearances of many of the large mammal groups we are used to seeing today; including the first hoofed mammals and whales, the first primates, and the first carnivorans. The Paleogene period began as a very warm moment in geologic history but as this period gave way to the Neogene, the Earth had started to grow colder. The Neogene saw the expansion of grassland habitats. Much of the Midwest would have been covered in wide savannahs during the Neogene and rhinos, tapirs, camels and early horses would have been abundant in these new open environments. The Quaternary period is the last period in the Cenozoic and is the geologic period which includes the present day. This time period was characterized by the advance and retreat of huge continental ice sheets which at various times in the last 2.5 million years had managed to cover nearly the entire Midwest. Animals like mammoths, mastodons, giant sloths and bison would have been common in North America during this period. Humans arrived in the Midwest towards the end of the Quaternary and their arrival is thought to have been one of the reasons for the extinction of the large land mammals which used to be found in this region. Previous Section The Mesozoic Era Return to Home Home Page The Paleogene Period: 66 - 23 million years ago In the immediate aftermath of the end-Cretaceous mass extinction most of the animals who had managed to survive the effects of the asteroid impact were small generalists or seed eaters. With most of the large animals gone, mammals, crocodylomorphs, squamates and birds were able to move into new niches and grew rapidly to much larger sizes. Rocks which date back to the Paleogene period are found in parts of the upper Great Plains, particularly in North Dakota. Some of the earliest large Cenozoic animals to appear in the fossil record are found in these deposits, such as the strange pantodont mammal Titanoides . The Paleogene is divided into three epochs; the Paleocene, the Eocene and the Oligocene. For most of the Paleocene and Eocene epoch the climate of North America was generally warm and humid. Huge rainforests spanned across the entire continent and large crocodilians and other cold-sensitive reptiles were found at very high latitudes in the northern hemisphere. Over the course of the Eocene and Oligocene epochs the Earth’s climate began to cool and become more dry. By the end of the Paleogene period the large rainforests had fragmented and were increasingly divided by larger expanses of more open environments. Many different groups of animals crossed over repeatedly between Eurasia and North America during the Paleogene, including early horse ancestors and several groups of carnivorous hoofed mammals. Camels first appeared in North America during the Paleogene and they would become one of the most abundant and diverse mammal groups endemic to the continent during the next period in geologic time; the Neogene period. (ABOVE) A scene set in a dense rainforest during the Eocene epoch of the Paleogene period. A pair of large rhino-like uintatheres stand beside a small stream. These forest are home to many types of birds and hoofed carnivorous mammals. (ABOVE) A giant Megacerops titanothere with a forked nasal horn moves in to protect its young calf from a pair of cat-like sabertoothed nimravids. Fossils of Megacerops are often found in parts of the White River badlands in South Dakota Paleogene Animals Basilosaurus an ancient whale Archaeotherium an entelodont Dissacus a mesonychid Agriochoerus a hoofed mammal The Paleogene Period 23 million years ago The Neogene Period The Neogene Period: 23 - 2.58 million years ago The Neogene period is divided into two epochs; the Miocene epoch (23-5.3 million years ago) and the Pliocene epoch (5.3-2.58 million years ago). In many parts of North America, grasslands took over the landscape during the Neogene period. Although the first grasses found in the fossil record date all the way back to the Cretaceous period, these plants became much more abundant in the Neogene as forests began to retreat as a result of a cooling climate. In broad terms the Midwest was still warmer during the Neogene period than it is today. Many parts of the Great Plains were covered in savannah-like habitats which were home to relatives of elephants, ancient rhinos, horses, tapirs, dogs, camels and early cats. Some of the best preserved fossils from this time period are found within deposits linked to the Ogallala formation/group which today extends from Texas to South Dakota. The fossils found in these deposits show us that Neogene life in North America would have been just as diverse as the biotas of today’s African savannahs. A particularly notable series of sites in Nebraska, the Ashfall Fossil Beds, preserve the remains of an entire herd of Teleoceras rhinos, as well as the fossils of dozens of other types of animals, preserved in fine ash along the edge of what was once a watering hole. These animals were victims of the ash generated by an ancient eruption of the Yellowstone supervolcano which formed an ash plume extending across most of North America. The western parts of North America seem to have been very volcanically active over the course of much of the Cenozoic and many Neogene fossil localities in the western US are linked to the deposition of ash as a result of these catastrophic eruptions. Despite this, none of these volcanic events seem to have resulted in large-scale mass extinction events. Towards the end of the Neogene period, North and South America became connected by the isthmus of Panama. This allowed many types of animals to move between the continents. North America got its first armadillos and sloths, while South America would receive its first carnivorans, horses, proboscideans and deer. (ABOVE) A representation of the landscape of central North America during the Miocene epoch. The warmer climate which characterized this moment in the Neogene period, along with the spread of prairie grasses, led to the formation of savannah-like grassland environments in parts of Kansas, Nebraska, Oklahoma, South Dakota and Texas. Neogene Animals Andrias a giant salamander Alligator mefferdi an alligatorid Balearica a crowned crane Harrisonsaurus an iguana Ceratogaulus a mylagaulid rodent Teleoceras a rhinoceros Synthetoceras a protoceratid Ischyrocyon a bear-dog Borophagus a borophagine dog Gomphotherium a proboscidean Aepycamelus a camel Daeodon an entelodont The Neogene Period 2.58 million years ago The Quaternary Period The Quaternary Period: 2.58 million years ago - Present The Quaternary period is the final and shortest period of the Cenozoic era. It is also the period of geologic time which includes the present day. The span of time within the Quaternary period between 2.58 million years ago and 11,700 is known as the Pleistocene epoch, while the length of time from 11,700 years ago to the present is known as the Holocene epoch. The Pleistocene saw large ice sheets advance and retreat across the Midwest over the course of many successive warm and cold cycles lasting tens of thousands of years. At their largest extent, these ice sheets extended as far south as Kansas City. The presence of such large volumes of ice had huge impacts on the world’s geography. Sea levels were very low during much of the Pleistocene and many parts of the world which are currently covered by shallow seas would have been coastal lowlands at various times throughout this epoch. The movement of ice sheets also reshaped the landscape of the Midwest, scouring the surface of the land, changing the drainage patterns of rivers, and even creating the conditions necessary to form the Great Lakes. North America was also home to many unique forms of large animals during the Pleistocene. These extinct Pleistocene animals, often collectively known as “ice age megafauna”, included mammoths, mastodons, deer, bison, ground sloths, camels and tortoises. Many of these large land animals became extinct towards the end of the Pleistocene, possibly as a result of increased hunting from humans who arrived in North America towards the end of the Pleistocene epoch. Our Pleistocene Field Guide (ABOVE) Forest and wetland animals found in the Midwest during the Pleistocene epoch; (1) Cervalce s, (2) Castoroides , (3) a Mastodon We have an online field guide about all the different kinds of animals who lived in the Midwest during the Pleistocene epoch! Click the link to learn more! Learn More Extinct Pleistocene Animals American Mastodon Mammut americanum Giant Bison Bison latifrons Western Tortoise Hesperotestudo Giant Beaver Castoroides ohioensis Scott's Horse Equus scotti Stag Moose Cervalces latifrons Giant camel Titanotylopus nebrascensis Saber-toothed Cat Smilodon fatalis (BELOW) The skyline of Kansas City as it appears today at the meeting point of the Kansas and Missouri rivers. The path followed by the Missouri River today was shaped by the presence of large ice sheets in this region around 700,000 years ago which pushed the river further south. The advance of these ice sheets also caused many rivers in the area to flow from east to west instead of north to south. Previous Section The Mesozoic Era Return to Home Home Page

The Western Interior Sea once spanned from the Gulf of Mexico to the Arctic Ocean and split North America in two during the Late Cretaceous period The Mesozoic Era 252 million years ago - 66 million years ago The Mesozoic era is the span of time lasting from the start of the Triassic period 252 million years ago to the end of the Cretaceous period 66 million years ago. It is composed of three time periods; the Triassic, the Jurassic and the Cretaceous. This era in Earth’s history saw the appearance of many of the groups of animals and plants we see around us today with the first birds, flowering plants, wasps, bees, true sharks, lizards, turtles and mammals appearing in the Mesozoic. Fossils from the Triassic and Jurassic periods are very rare in the Midwest but deposits from the Cretaceous period are found all across the Great Plains. These Cretaceous rocks preserve the remains of ancient ocean animals who lived in an expansive body of water called the Western Interior Sea which split North America in two between about 110 and 70 million years ago. Fossils of mosasaurs, plesiosaurs, sharks, massive clams and giant carnivorous fishes are found in deposits linked to this ancient seaway. Other Cretaceous deposits were formed on land, including the Dakota sandstone which is famous for its well-preserved leaf fossils. Previous Section The Paleozoic Era Next Section The Cenozoic Era The Triassic Period: 252 - 201 million years ago The Triassic period is not well represented in the fossil record of the Midwest. This time period began in the aftermath of the most devastating mass extinction in Earth’s history; the end-Permian extinction, also known as the Great Dying. Only a small fraction of the species who lived in the late Permian managed to survive into the early Triassic and as a result the survivors of the extinction were able to radiate into newly opened niches. The Triassic period was when we started to see the first marine reptiles, distant crocodile relatives and dinosaurs appear in the fossil record. Some of the largest reptiles of all time lived during this period, with massive ichthyosaurs reaching sizes comparable to those of living sperm whales. On land large dicynodont synapsids still occupied the roles of giant herbivores. Pseudosuchians filled both predatory and herbivorous niches, with a particularly strange plant-eating group called the aetosaurs developing long shoulder spines and thick dorsal armor composed of large bony plates. The first dinosaurs appeared around 233 million years ago as small-to-medium-sized carnivores and by the end of the Triassic some of the firstlong-necked herbivorous sauropodomorphs, like Plateosaurus, had begun to grow to be around the size of a rhinoceros. The Triassic period ended with another volcanism-driven mass extinction event. The survivors of this extinction event, including dinosaurs and mammaliaforms, would play a major role in the next period, the Jurassic. (ABOVE) A scene set at the end of the Triassic period. A group of large ichthyosaurs has washed up on a beach and are being scavenged by small theropod dinosaurs. Most of the larger types of ichthyosaurs did not make it through the end of the Triassic period and their relatives who made it into the Jurassic and Cretaceous periods would never again reach the same massive sizes the group had once reached during the Triassic. The Triassic Period 201 million years ago The Jurassic Period The Jurassic Period: 201 - 143 million years ago Rocks from the Jurassic period are not very common in the Midwest. Some parts of the Great Plains which extend into parts of Oklahoma and eastern Colorado do have outcrops of rocks from the Morrison formation. The fossils in these rocks date back to the very end of the Jurassic period and include the remains of some famous types of dinosaurs such as Apatosaurus, Allosaurus, Diplodocus and Stegosaurus. The climate in the eastern part of the Morrison formation would have been seasonally arid, with patches of forests and greenery found near rivers and floodplains while drier open areas were found in areas with less water. In the world’s oceans, marine reptiles began to diversify as well, with many types of long necked plesiosaurs, short-necked macropredatory pliosaurs, and fish-like ichthyosaurs found in marine deposits from this time. Alongside these ocean reptiles, some of the first true sharks also appear during the Jurassic period, as do the first members of the stingray group. Fossils of birds also show up in rocks from the Jurassic period as these flying dinosaurs seem to have already developed the advanced flight feathers seen in their living relatives. (ABOVE) A scene set in the Late Jurassic of western North America. several species of long-necked sauropods such as Diplodocus (left) and Apatosaurus (right) are found in deposits dated back to this moment in time. A pair of large allosauroid theropod dinosaurs stalks along the edges of the floodplain in the background. (ABOVE) A giant Torvosauru s fends off a group of juvenile Allosauru s in a Jurassic forest. Many types of large meat-eating dinosaurs lived in the same areas during the Jurassic period. Prey animals were abundant and there were many niches available for large, medium, and small theropods. Jurassic Animals Ceratosaurus a theropod Diplodocus a sauropod Allosaurus a theropod Camarasaurus a sauropod The Jurassic Period 143 million years ago The Cretaceous Period The Cretaceous Period: 143 - 66 million years ago The Cretaceous is the final period of the Mesozoic era. For most of the Cretaceous the Earth was much warmer than it is today. Ice caps were virtually nonexistent at the poles and as a result, sea levels were very high. As the continents began to move into the arrangement we know today, Europe and America became further separated and the Atlantic ocean grew increasingly wide over the course of the Cretaceous. Across the area we now call the Great Plains, a combination of elevated sea levels and a steady local downwarping of the Earth’s crust resulted in the formation of the Western Interior Sea, a body of water which once spanned from the Arctic Ocean to the Gulf of Mexico. Deposits linked to this ancient seaway are found from central Mexico to northern Canada. They preserve the fossils of mosasaurs, giant sharks, plesiosaurs and huge cephalopods. Along the coastlines of this seaway dinosaurs continued to flourish. On the western side of the Interior Sea the ancient continent of Laramidia was home to horned dinosaurs, azhdarchid pterosaurs and giant tyrannosaurs. Eastern North America formed the ancient continent of Appalachia where fossils of terrestrial animals are fairly rare. Deposits from the Cretaceous found in eastern North America often preserve the remains of hadrosaurs, crocodilians and early flowering plants. (ABOVE) A baby horned dinosaur comes across the decaying skeleton of a large Albertosaurus in the middle of a dense conifer forest. Many types of horned dinosaurs were very common across the ancient landmass of Laramidia which makes up most of what is now western North America. Albertosaurus and other tyrannosauroids were some of the region's largest terrestrial carnivores. (ABOVE) A giant long-necked Styxosaurus plesiosaur and a group of diving Pteranodon chase down a school of small Apsopelix fish. Pteranodon had very fragile skeletons but may have been able to swim underwater for brief periods of time to catch fast-moving aquatic prey. (ABOVE) A pair of Nyctosaurus pterosaurs flying over the Western Interior Sea in an area which will eventually become central Kansas. These bizarre flying reptiles had very long forked head crests. Our Western Interior Sea Field Guide (ABOVE) Animals found in the Western Interior Sea: (1) Belonostomus , (2) Mosasaurus , (3) Plesioelasmosaurus , (4) Dolichorhynchops, (5) Squalicorax , (6) Xiphactinus We have an online field guide about all the different kinds of animals who lived in the waters covering the Great Plains during the Late Cretaceous! Click the link to learn more! Learn More Cretaceous Animals Mosasaurus a mosasaur Cretoxyrhina a shark Tyrannosaurus a theropod Triceratops a ceratopsian Parapuzosia an ammonoid Onchopristis a sawskate Pteranodon a pterosaur Plesiopleurodon a polycotylid plesiosaur Previous Section The Paleozoic Era Next Section The Cenozoic Era

The Western Interior Sea once spanned from the Gulf of Mexico to the Arctic Ocean and split North America in two during the Late Cretaceous period The Western Interior Sea The seas which once covered the Great Plains were made possible by the elevated sea levels of the Late Cretaceous as well as a downward shift in the elevation of central North America. The waters of this seaway were generally warm and fairly shallow. Traces of a world covered in water The Late Cretaceous was a very warm time in the history of the Earth. There were virtually no ice sheets at the poles and accordingly the world's sea levels were much higher than they are today. We are used to living on a planet where roughly 75 percent of the surface is covered in water but during the Campanian and Cenomanian stages of the Cretaceous period roughly 85 percent of the Earth's surface was covered in water. Huge swaths of low-lying land in the southern, eastern and central parts of North America were covered in warm shallow seas at this time. One of the best studied bodies of water from the Late Cretaceous is the Western Interior Sea, which once spanned from the Arctic Ocean to the gulf coast. Fossils of ancient marine life recovered from deposits of shale and chalk left behind by this body of water were first studied in the mid-late 19th century as teams of paleontologists and collectors from scientific institutions in the eastern US became increasingly interested in the fossils of giant marine reptiles, pterosaurs and carnivorous fishes. Since then the fossils of many types of world famous air and sea animals have been found in these marine deposits, such as those of Archelon, Tylosaurus, Pteranodon and Xiphactinus. Geography The early development of the Western Interior Sea is strongly linked to a phase of mountain building known as the Sevier orogeny which took place in the western part of North America during the Jurassic and Cretaceous periods. The shallow subduction of a plate of rock called the Farallon plate caused the middle part of the North American continent to warp downwards. At the same time, sea levels remained very high during the Late Cretaceous. This combination of factors led to the formation of large bodies of water in the middle of the continent, beginning with extensions of the Arctic Ocean moving south into a body of water called the Mowry sea which covered parts of central Canada during the middle Cretaceous. This body of water would eventually link up with some of the waters of the Gulf of Mexico in order to form a fully connected seaway at various times during the Cretaceous. The Western Interior Sea is best known as a body of water which split North America in two, with the continent of Laramidia in the west and Appalachia in the east. However, it should be noted that this contiguous seaway stretching across the entire continent was only present at the points when this body of water was at its maximum size and there were several phases of expansion and regression which saw the waters of the Interior Sea rise and fall over the course of the Late Cretaceous. The evidence for these changes in the extent of the seaway can be seen in the cyclothems found in the layers of rock below the Great Plains, with bands of marine sediment being separated by bands of rock which formed in terrestrial environments (ABOVE) Pleistocene woodlands were often found near rivers and wetland habitats and were home to many types of browsing and semi-aquatic mammals. Cervalces (1), Castoroides (2), American mastodon (3) (ABOVE) A map of North America as it appeared during the Campanian stage of the Late Cretaceous, around 75 million years ago. Image adapted from a figure from Sampson et. al. (2010) The Western Interior Sea became fully connected for the first time during the late Albian stage of the Early Cretaceous, around 105 million years ago. The seaway was narrower at this point than it would be later in the Cretaceous and by the start of the Cenomanian (~100 million years ago) it had retreated somewhat. Areas in Kansas, Texas and Colorado which had previously been submerged were now above the water once again. This regression phase was followed by yet another expansion of the Interior Sea from around 95-85 million years ago during which the seaway became fully connected once again and most of Kansas, Nebraska and the Dakotas were submerged. The last 10 million years of the Cretaceous would see the Western Interior Sea slowly recede as the mountain building process further west continued to push the middle of North America upwards. By the very end of the Cretaceous very little was left of the seaway, aside from a line of wetlands, lakes and deltas running from the Dakotas all the way up to the Arctic Ocean (ABOVE) A map of North America during the Campanian stage of the Cretaceous with an overlay of modern borders The seaway was a very large body of water in terms of surface area but it wasn't particularly deep. The entire sea was positioned over the continental shelf and would most likely have been no more than 2,500-3000 feet deep. The waters of this body of water were very warm with estimated surface temperatures ranging from between 90-100 degrees Fahrenheit during the earlier phases of the seaway and around 65-70 degrees towards the very end of the Cretaceous period. Deeper waters near the seafloor were most likely considerably colder. Stratigraphy The history of the Western Interior Sea extends over the course of around 40 million years. Consequently, no single layer of rock can give us the whole picture of life in these Cretaceous waters. Scientists studying marine fossils from this area reconstruct the story of the seaway by looking at many different overlapping geological formations which were deposited by the Western Interior Sea, each of which have outcrops in different areas. By looking for changes in the types of animals found in a given area, as well as changes in the types of rocks surrounding them, paleontologists can see how this ancient environment shifted over time and the ways that prehistoric animals adapted to be able to survive in a constantly changing landscape. Broadly speaking, the Western Interior Sea can be said to have existed from around 110 million years ago during the middle Albian stage of the Early Cretaceous, all the way up until the later part of the Maastrichtian stage, around 68-67 million years ago. Geological formations linked to the presence of the seaway are known from outcrops spanning from northern Canada to central Mexico. (ABOVE) The duration of the Western Interior Seaway as compared with the entire span of the Cretaceous period. The oldest deposits linked to the Western Interior Sea are from formations like the Kiowa Shale formation in Kansas and the Clearwater Formation in Alberta. The Kiowa Shale preserves the remains of some types of plesiosaur fossils as well as fossils of oysters and ammonoids, while the Clearwater Formation contains the fossils of some of the last known Ichthyosaurs, like Athabascasarus. Younger layers of rock like the Niobrara Chalk and Greenhorn Limestone preserve many of the best known fossils from the Western Interior Sea, including animals like Xiphactinus, Pteranodon and Tylosaurus. The youngest layers of rock from the Western Interior Sea are found in formations like the Pierre Shale and Bearpaw Shale formations. These layers were formed as the seaway was beginning its retreat further north towards the Arctic Ocean. In the case of the Bearpaw Shale, the younger layers of this formation seem to mostly preserve the remains of animals living in a estuarine or river delta environment instead of an expanse of open water. (ABOVE) Formations deposited by the Western Interior Sea are found across central North America and date from the late Albian stage all the way up to the early Maastrichtian, spanning around 40 million years of time. The types of rocks which make up the layers found in these formations vary depending on the depositional environment which existed at the time/place when they were formed. Some layers are formed from mudstones made of compressed seafloor substrate while others are made of limestone. Limestone/chalk is a rock type that is fairly unique to marine depositional environments and is formed from the calcium carbonate accumulated from the remains of trillions of marine microorganisms, in particular a group of algae called the coccolithophores. The Niobrara Chalk Formation Dated to around 87 - 82 million years ago this formation has outcrops in parts of Kansas, Nebraska and North Dakota. This formation sits above the Carlile Shale Formation and below the Pierre Shale Formation. Its main members are the Smoky Hill Chalk and the Fort Hays Limestone. The Niobrara Chalk preserves the remains of animals who lived in the Western Interior Sea at the height of its size, including the fossils of sharks, elasmosaurs, pachycormid fishes, pterosaurs and sea turtles. Some dinosaur bones are also known from the Niobrara Chalk, namely Niobrarasaurus, which is one of only a few dinosaurs known from fossils found in the state of Kansas. It seems as though this animal would have died closer to shore before being washed out into the Interior Sea and deposited on the seafloor. (BELOW) Selected taxa known from fossils found within the rock layers of the Niobrara Chalk Formation Niobrarasaurus Protosphyraena Ptychodus Ichthyodectes Cimolichthys Nyctosaurus Toxochelys Cretoxyrhina Xiphactinus Stratodus The Pierre Shale Formation Some of the largest animals known from the Western Interior Sea are found in deposits which formed late in the sea's history. In the Pierre Shale formation, which extends from Manitoba to New Mexico, we find fossils of some animals like Mosasaurus missouriensis and the strange long-necked polycotylid plesiosaur Serpentisuchops. The Pierre Shale is also the formation which contains the remains of the largest known turtle, a giant protostegid sea turtle called Archelon . The Pierre Shale Formation was deposited over a very long time, starting in the Campanian stage at the height of the extent of the seaway, and lasting all the way up until the Interior Sea had nearly fully receded at the beginning of the Maastrichtian stage. Some of the best known fossils recovered from the Pierre Shale are those of ammonoids like the large horn-shaped heteromorph genus Baculites. (BELOW) Selected taxa known from fossils found within the rock layers of the Pierre Shale Formation Mosasaurus Archelon Globidens Martinectes Serpentisuchops The Carlile Shale Formation One expansive formation found across much of the central and western US is the Carlile shale which dates back to the early days of the Western Interior Sea during the Turonian stage of the Cretaceous. One of the most famous fossils recovered from these rocks is the skeleton of a pliosauroid plesiosaur called Megacephalosaurus . This huge marine reptile was one of the last members of the pliosauroid group which had previously been the top predators of the worlds oceans during the Jurassic and Early Cretaceous. Shark tooth fossils are also commonly found in the Carlile Shale. Squalicorax is well represented in these rocks, as is a unique type of sand tiger shark named Johnlongia, which seems to have been a close relative of the filter feeding odontaspid Pseudomegachasma . Fossils of marine crocodylomorphs like Terminonaris are also found in this formation. (BELOW) Selected taxa known from fossils found within the rock layers of the Carlile Shale Formation Squalicorax Johnlongia Terminonaris Megacephalosaurus Cretodus The Bearpaw Shale Formation The Bearpaw Shale is a formation which contains fossils of animals from the last few million years of the Western Interior Sea. This formation extends throughout parts of Montana and the southern parts of Alberta and Saskatchewan and is a particularly productive source of the mineral ammolite which is formed from the shells of ancient ammonoids. Vertebrate fossils in the lowest layers of the Bearpaw Shale represent the remains of mosasaurs and plesiosaurs while the younger rock layers above preserve the fossils of terrestrial animals such as theropod dinosaurs and hadrosaurs. The Bearpaw formation gives us a good idea of what kinds of environmental transitions took place as the Interior Sea began to retreat northwards (BELOW) Selected taxa known from fossils found within the rock layers of the Carlile Shale Formation Plioplatecarpus Meristodonoides Prognathodon Albertonectes Nakonanectes Marine and Coastal Environments The Western Interior Sea supported a wide range of habitats, both within its waters and along its coastlines. The seaway itself was quite large, around the same size as the Mediterranean sea today, and at any given time during the Late Cretaceous the climate would likely have been quite different in the far northern parts of the seaway as compared to the warmer southern waters where the Interior Sea entered the Gulf of Mexico. Estimates for the maximum depth of the seaway vary somewhat but they range between estimates of around 1,000 feet of maximum depth to around 3,000 feet of depth. In either case these depth estimates indicate that the deepest parts of the Interior Sea, despite being fairly shallow when compared with the deepest oceans, would still have been deep enough that they would have experienced very little if any sunlight. Large colonies of bivalves seem to have survived very well at these depths where they sustained populations of smaller vertebrates and invertebrates. (ABOVE) A chart showing the zones found in the water column of the ocean. Since most of the Western Interior Sea sat atop the continental shelf, even the deepest parts of the sea would not have gone any deeper than the mesopelagic zone. Along the shorelines of the Western Interior Sea there would have been extensive systems of river deltas, lagoons and estuaries. Some formations in eastern Texas which seem to have been formed in these types of estuarine systems preserve the remains of many types of gastropods and shallow water bivalves, including the remains of large oyster beds. In the shallow waters of the seaway stony coral reefs were notably absent. Instead the main reef building organisms in these environments seem to have been the rudists; a unique group of bivalves who grew in large colonial clusters in both shallow and deeper waters. These rudists were a group which only lived during the Mesozoic era and became extinct at the end of the Cretaceous but owing to their dominance at this time, scleractinian coral fossils are very rare in deposits from the Western Interior Sea. Fossils of the remains of pieces of driftwood are also found in deposits from the Interior Sea. These pieces of wood are often found full of the traces of burrows left behind by shipworm bivalves. Above the seas, pterosaurs and seabirds would have been a common sight as they hunted for fish or combed the beaches in search of carrion. (ABOVE) Coastal and near-shore environments found alongside the Western Interior Sea included deltas, estuaries, lagoons, shallow bays and sandbars. Some animals were also able to fly above the seas for great distances The open waters of the Western Interior Sea would have been one of the most dangerous marine environments ever to exist in the entire history of the Earth. Mosasaurs, giant sharks and plesiosaurs would have roamed these waters in search of food. Schools of fish, both large and small would also have been found in the sunlit surface waters, or they may have possibly migrated up from deeper waters under the cover of night for protection from predators. These sunlit waters would also have supported a large volume of plankton, which in turn was a food source for some of the large Cretaceous filter-feeding fishes whose fossils are known from these areas. The giant pachycormid fish Bonnerichthys and the filter-feeding sand tiger shark Pseudomegachasma might have occupied the same ecological niches as today's whale sharks, basking sharks and baleen whales. (ABOVE) Environments found further out in the open waters of the Western Interior Sea and in the deepest areas of the seafloor The very deepest parts of the Western Interior Sea may have experienced near or total darkness even during the brightest parts of the day. At these depths the waters of the seaway would have been much colder than they were at the surface. The lower layers of substrate found along much of the seafloor in the seaway would not have been composed of sand, but rather made up of a lime-rich ooze. Very little was able to survive while anchored to this ooze, outside of some very tenacious (and in some cases very large) bivalves like the giant Inoceramid clams which are hallmarks of the fossil beds found in parts of western Kansas. Fauna Hundreds of different genera of ancient animals are known from deposits linked to the Western Interior Sea, ranging from tiny zooplankton all the way up to the mightiest mosasaurs. However, these animals were often separated from one another by millions of years of time and even some contemporaneous taxa may not have come across one another simply because they lived in two very different areas. Yet thanks to the stunning preservation of the remains of the animals found in these rock formations, we can piece together a lot of information about what kinds of animals would have lived alongside one another, competing for resources or becoming the unfortunate prey of a larger carnivore. Estuaries (ABOVE) Estuarine and terrestrial animals whose fossils are found in deposits linked to the Western Interior Sea The estuaries and deltas found where rivers met the Interior Sea would have been full of all sorts of life. Gastropod remains from ancient snails (1) are often found in places like the Woodbine formation in Texas, where they lived in coastal habitats along side bryozoans (2), solitary corals (3) , sponges (4) and brachiopods (5). Giant crocodylomorphs like Deinosuchus (6) roamed the swamps along the edge of the seaway. These crocs grew so large that they could very easily have preyed upon unlucky dinosaurs who wandered too far into the water. Much like their modern relatives, ancient sea turtles (7) most likely came ashore in order to lay their eggs. Scavengers also picked their way along the edges of the sea and animals like Pteranodon (8) and Ichthyornis (9) would have scrounged for any sort of detritus or carrion which might have washed up on the beach. Dinosaurs were of course very common in these coastal areas and animals like Niobrarasarus (10) were occasionally washed out to sea where they became preserved and fossilized on the seafloor, possibly hundreds of miles away from their point of origin by powerful currents. Above the seaway (ABOVE) Flying animals whose fossils are found in deposits linked to the Western Interior Sea Fossils of pterosaurs like Nyctosaurus (1) and Pteranodon ( 2) have been found in places which would once have been very far out at sea, meaning that these animals were no doubt capable of flying for great distances over open water in search of food. They may have even been able to sit atop the surface of the water and plunge their heads beneath the surface to grab ahold of fish. Seabirds were also able to take advantage of the abundance of food in the Interior Sea. Some animals like Iaceornis (3) were able to fly just like living seagulls while others like Baptornis (4) were completely flightless and had very short wings, heavy bodies and long webbed hind feet used to swim while underwater. Shallow water (ABOVE) Shallow water animals whose fossils are found in deposits linked to the Western Interior Sea The sunlit shallow waters near the shoreline of the Western Interior Sea would have been very productive marine environments. These areas were home to a variety of pycnodont fishes (1) which used their crushing teeth to break open the shells of bivalves and small ammonoids (2). Small plesiosaurs (3) would have lived in these areas as well, possibly using these sheltered shallow waters as nurseries to care for their young before moving out to more open waters later in life. Other types of durophagous fishes like Hadrodus (4) are also known from areas which would have been covered in shallow waters. Basal mosasaurs (5) likely got their start living in these types of nearshore environments and some animals like Dallasaurus and Russellosaurus continued to occupy this sort of semi-aquatic transitional niche for millions of years. Open water (ABOVE) Pelagic animals whose fossils are found in deposits linked to the Western Interior Sea The open waters of the Interior Sea were home to some small animals, such as the aspidorhynchiform false needlefishes (1) as well as some of the largest vertebrates of the entire Cretaceous period, including the massive Mosasaurus missouriensis (2). Long-necked elasmosaurid plesiosaurs (3) were also commonly found in these open areas, using their long necks to obfuscate the shape and size of their bodies as they picked through shoals of fish. Smaller polycotylid plesiosaurs (3) would have been more adapted for pursuit and used their long snouts to grab ahold of soft bodied prey such as fish, belemnites or squid. Some animals in these waters were scavengers instead of hunters. Squalicorax (5) teeth have been found embedded in the bones of many types of large marine reptiles, seemingly picking over these giant carcasses as they floated at the ocean's surface. Ravenous carnivorous fishes such as Xiphactinus (6) were also commonly found in these pelagic environments, where they would have hunted everything from sharks to marine reptiles to seabirds. The seafloor (ABOVE) Benthic and deep-water animals whose fossils are found in deposits linked to the Western Interior Sea The depths of the Western Interior Sea were mostly devoid of sunlight but were nevertheless home to many different types of vertebrates and invertebrates. Just as in today's oceans giant sixgill sharks (1) were found living in the depths of the seaway, preying upon cephalopods or searching for sunken carrion in the deep twilight. Rudists (2) and Inoceramid clams (3) were a common sight on the seafloor. Other types of mollusks found in these areas included large muensterellid octopods (4) and car-sized ammonoids like Parapuzosia (5), possibly one of the largest mollusks of all time. Smaller animals like beardfishes (6) would have taken shelter in the shells of large clams and rudists, benefiting from the nearly impenetrable walls of their shell valves as a form of protection from benthic predators like the angelshark Squatina . (7) Find out more about Cretaceous ocean life by exploring our Western Interior Sea field guide Learn More

The Western Interior Sea once spanned from the Gulf of Mexico to the Arctic Ocean and split North America in two during the Late Cretaceous period The Paleozoic Era 538.8 million years ago - 252 million years ago The Paleozoic era is the span of time lasting from the start of the Cambrian period 538.8 million years ago and lasting up until the end of the Permian period around 252 million years ago. This era lasted longer than the entire lengths of the Mesozoic and Cenozoic eras combined and saw life on Earth move into many of the familiar niches we associate with living things found all around us today. Fossils of Paleozoic organisms preserve the remains of the first carnivorous animals, the first arthropods, the first land plants, the first trees, the first vertebrates and the first animals to move out of the water and onto the land. Many deposits dating back to periods of time during the Paleozoic are found in the Midwest. Some of the best known of these fossil bearing rock exposures are the late Devonian Cleveland Shale from Ohio which contains the fossils of giant arthrodires like Dunkleosteus, and the exceptionally preserved Carboniferous-aged Mazon Creek Formation in Illinois, where fossils of hundreds of types of small aquatic soft-bodied organisms are preserved pressed within small rocky nodules. In deposits from the Great Plains, such as those at the Hamilton quarry and Elmo quarry sites in Kansas, we find fossils of a huge variety of insects, as well as the skeletons of some of the oldest members of the reptile family tree. Previous Section The Early Earth Next Section The Mesozoic Era The Cambrian Period: 538.8 - 486 million years ago The Cambrian is a time period lasting from 538 to 486 million years ago and is the first time period of the Paleozoic era. Most Cambrian fossils come from marine deposits, although some rarer localities occasionally preserve the remains of animals which were deposited along the edges of beaches. At the Blackberry Hill site in Wisconsin, for example, traces of ancient jellyfishes who happened to wash to shore during the middle-late Cambrian period are preserved as circular ridge-like marks formed on top of the wavy patterns left behind by the movement of tidal waters. Small sand stromatolites are also found in these areas, as are traces of what seem to be trackways left behind by a scuttling arthropod, most likely an aquatic myriapod, and the slip marks deposited by the movement of a snail-like mollusk. These trackways seem to have been laid down outside of the water and are some of the first known traces in the fossil record of complex animals leaving the ocean to crawl out onto land. Other types of Cambrian fossils are found in other parts of the upper Midwest, with the remains of corals, trilobites, sponges, brachiopods, jellyfishes, graptolites and mollusks turning up in parts of Michigan, Wisconsin, Illinois and Iowa. Much of the Midwest would have been outside of the water on top of the ancient paleocontinent of Laurentia during the early part of the Cambrian period. These terrestrial areas would have been nearly devoid of all complex life, covered in vast featureless deserts with the only organisms found in these areas being hardy bacteria and archaea. By the later part of the Cambrian however, the seas to the south of Laurentia expanded and covered many of the low-lying parts of the ancient landmass, bringing shallow water reef ecosystems into a vast embayment stretching across the area between what is now Ohio and central Minnesota. (ABOVE) Opabinia was one of the strangest animals known from fossils dated to the Cambrian period. These animals had five eyes, a long trunk-like proboscis, and a series of paddle-like appendages running along the sides of their bodies. Their fossils are found in the Burgess Shale in British Columbia. (ABOVE) An extant brachiopod in the genus Lingula . The first brachiopods appeared during the Cambrian period. Although they seem superficially similar to bivalves like clams and oysters they are actually part of their own separate phylum. The Cambrian Period 486 million years ago The Ordovician Period The Ordovician Period: 486 - 443.1 million years ago Fossils of marine life dating back to the Ordovician period are commonly found across much of the northern and eastern Midwest. Fossils of bryozoans, rugose corals and brachiopods are among the most common remains dated to this time period but some deposits also preserve the remains of new types of trilobites which were going through a diversification event during the Ordovician as they moved into new niches and developed new external features such as elaborate tubercles and spines. One of the best known Ordovician trilobites is the genus Flexicalimene, which is notable for having been able to fully roll itself up into a ball, similarly to a modern woodlouse. Its remains are particularly common in parts of western Ohio and eastern Indiana. These animals lived in some of the largest reef ecosystems ever to exist in the history of the Earth, although most of these reefs were composed of colonies of rugose corals. Modern stony corals would not appear until much later during the Triassic period. The first simple land plants also appeared during the Ordovician. These were small low-lying organisms who were well-adapted for spending their lives in the rocky, nutrient-poor sun-baked terrestrial landscape. The world’s climate at the start of the Ordovician was fairly warm and much of the Midwest was initially covered in ocean. Over the course of the Ordovician, a series of ice ages would lock up large amounts of water at the poles. These ice sheets caused sea levels to fluctuate wildly over the course of the Ordovician. Around 470 million years ago most of the Midwest was above water once again but over the course of the rest of the Ordovician the oceans would slowly encroach over the Laurentian landmass until by around 450 million years ago the entire region was once again covered in a shallow sea. Glaciation events at the end of the Ordovician would, however, see the water retreat from areas around Wisconsin and Minnesota. (ABOVE) Conodont animals were some of the most common chordates found in the Paleozoic seas, becoming very abundant during the Ordovician period. They are mostly known from fossils of their jaw elements which look like small serrated blades. Most of these animals were only a few inches long. (ABOVE) Giant cephalopods were some of the largest carnivores found in the Ordovician seas. Animals like Cameroceras and Endoceras had long cone-shaped shells which could grow to be over 12 feet long in some cases! The Ordovician Period 443.1 million years ago The Silurian Period The Silurian Period: 443.1 - 419.3 million years ago The Silurian period is known from layers of sedimentary rocks found all across the Midwest, including in parts of Indiana, Illinois, Wisconsin and Iowa. These deposits were typically formed in marine or coastal environments, with layers linked to deep ocean waters, shallow reefs and beach environments. The Silurian period saw many major milestones in the history of life. On land, some of the first vascular plants appeared during the middle Silurian. Fungi also began to diversify around this time and some of these ancient organisms grew to be the size of small trees. Several groups of arthropods managed to fully transition over to terrestrial life, and some of the first land-dwelling members of the millipede group appeared in the Silurian. Vertebrates were very common at this time as well, although they were still confined to the water. The first jawed vertebrates appeared during the Silurian, including early members of the cartilaginous fish group, distant ancestors of sharks and stingrays. Eurypterid sea scorpions were at the peak of their diversity during the Silurian. These animals were distantly related to horseshoe crabs, although some of these chelicerates were powerful carnivores who could grow to be well over 8 feet long. Sea levels were highly variable during the Silurian and cycles of rising and falling water levels took place over millions of years as global temperatures rose and fell. Shallow ocean waters would have covered many of the low-lying areas across the Midwest, leading to the development of some of the largest reef systems ever to form in this region. Further to the east the Caledonian Orogeny, a collision between the European continental plates and the North American landmass, resulted in the continuation of a period of mountain building in parts of eastern North America, the Scottish highlands, Greenland and Norway. (ABOVE) Some sea scorpions like Carcinosoma had long grabbing appendages lined with sharp spines instead of the crab-like claws seen in some of their relatives. These ancient arthropods could grow to be very large and some genera would have been over 8 feet long. (ABOVE) Trilobites continued to be very successful during the Silurian period. Some of these animals even developed large spines, tubercles and tall eye stalks. The Silurian Period 419.3 million years ago The Devonian Period The Devonian Period: 419.3 - 358.8 million years ago The Devonian period is sometimes called the “Age of Fishes” and for good reason. Fossils of ancient armored placoderms and sharks are some of the most distinctive fossils found in Devonian rock layers. The Cleveland Shale in Ohio, for instance, preserves the remains of several dozen ancient fish species, including the famous arthrodire Dunkleosteus terrelli. These armored fishes did not have actual teeth in their mouths. They would have instead used extensions of their bony cranial armor to shear through prey or to crush hard-shelled animals. Ammonoids also appeared for the first time during the Devonian. These early ammonoids had much more simple suture patterns than their later relatives from the Mesozoic era. These early ammonoids had spiral-shaped shells and soft bodies with long, complex arms similar to those of octopi or cuttlefish. One of the major steps taken by vertebrate life during the Devonian was the start of the tetrapod transition onto land. These tetrapods were lobe-finned fishes who developed over time into animals who were able to spend more and more time outside of the water. Many of the earliest tetrapodomorphs still had fins but later members of this group would develop fully functional limbs and feet able to support the weight of their bodies outside of the water for extended periods of time. Land plants also went through a major diversification event during the Devonian. By the middle of this period large forests of tree-like Archaeopteris and Wattezia plants were found all around the world. These tall vascular plants were some of the first plants to develop a structure resembling a tree trunk. Devonian Animals Ctenacanthus a ctenacanthiform cartilaginous fish Stethacanthus a symmoriiform cartilaginous fish Bungartius an arthrodire Cladoselache a cladoselachiform chimaera relative The Devonian Period 358.8 million years ago The Carboniferous Period The Carboniferous Period: 358.8 - 298.9 million years ago The Carboniferous period lasted from 358.8 million years ago to 298.9 million years ago. In North America the period is often split into an earlier Mississippian subperiod (358.8 - 323 million years ago) and the later Pennsylvanian subperiod (323 - 298.9 million years ago). Rocks from the Carboniferous period are particularly common in the midcontinental region of the Midwest, especially rocks deposited in marine deposits from the Pennsylvanian which outcrop in eastern Kansas, northern Missouri and parts of Iowa, Illinois and Indiana. Older rocks from the Mississippian are also present in parts of southeastern Kansas and southern Missouri. Within these rock layers the most common fossils are those of brachiopods, bryozoans and crinoid columnals. Rocks from coastal wetland environments are also present in the Midwest. Sites like the Mazon Creek fossil beds in Illinois and Hamilton Quarry in Kansas preserve the remains of a wide variety of small freshwater and brackish invertebrates, lungfishes, cartilaginous fishes and insects. Tetrapod fossils are also known from this period. Temnospondyl amphibians were some of the largest animals found on land, usually occupying the niche of aquatic ambush hunters although some of these animals seem to have been almost completely terrestrial. Amniotes also show up during the Carboniferous and early on in their evolution they split into two lines; the sauropsids and the synapsids. The sauropsid group contains all true reptiles and their closest relatives while synapsids include all of today’s mammals as well as a wide variety of superficially reptile-like stem mammals. Early members of these groups were fairly similar to one another and would have had rather lizard-like body plans. Despite this even the fossils of these most basal forms can be distinguished from one another by the shapes of the openings at the back of their skulls. During the Carboniferous period many of the Earth’s continents had begun the process of moving together to form the supercontinent Pangaea. Parts of western North America were covered in water at this time and many parts of the Midwest would have been covered in shallow water reef ecosystems or tropical coastal forests. Sea levels rose and fell many times, particularly during the later part of the Carboniferous as the climate cooled and large ice sheets around the South Pole advanced and retreated over the course of tens of thousands of years. In many areas with Carboniferous rocks in the Midcontinent region the effects of these changes in sea level can be seen in the rock layers themselves, with bands of marine limestone separated by layers of shale and other mudstones which formed on land or in freshwater environments. Fossils of plants are quite common in terrestrial Carboniferous deposits, including the remains of scale trees, giant horsetails, clubmosses, early conifers and seed ferns. Although scale tree forests were very common during the early and middle Carboniferous, by the end of the period these rainforests became increasingly fragmented and were replaced in many areas with more open environments. Carboniferous Life Calamites a giant horsetail Cordaites an early conifer relative Actiobates a trematopid amphibian Arthropleura a giant millipede Iniopteryx a flying shark Tullimonstrum a mysterious animal from Mazon Creek Whatcheeria a tetrapodomorph Listracanthus a cartilaginous fish The Carboniferous Period 298.9 million years ago The Permian Period The Permian Period: 298.9 - 252 million years ago The Carboniferous period did not end in a major extinction event and the lifeforms found at the start of the Permian period were fairly similar to the kinds of organisms whose fossils are found in rocks from the late Carboniferous. Over the course of the Permian the Earth’s climate generally became cooler and drier, with large deserts expanding across the interior regions of the Pangean supercontinent. Volcanic activity seems to have intensified towards the end of the Permian with a particularly intense series of eruptions taking place in Siberia at the very end of the period. The volcanic activity around the Siberian traps is usually seen as the main driver of the end-Permian mass extinction event which wiped out over 90% of marine species and 70% of terrestrial species. Most Permian deposits in the Midwest date back to the first half of the Permian period. At this time much of what is now central North America was still close to the equator and was very warm and humid all year round despite the cooler global temperatures. The lycopsid rainforests which had dominated much of the Carboniferous period still existed in small pockets during the early Permian but they faced increased competition from conifers which were better adapted to cooler, drier climates.The giant lycopsids had completely vanished by the middle of the Permian period. Amniotes continued to radiate into new forms with the largest animals on land being members of the synapsid branch. Pelycosaurs were particularly successful during the early part of the Permian, with many animals like Dimetrodon and Edaphosaurus developing large sails along their backs. These pelycosaurs were replaced by the more derived therapsids later on in the Permian which occupied both carnivorous and herbivorous ecological niches. Temnospondyls also continued to be very successful, with some animals, like the genus Prionosuchus from Brazil, growing to be as large as saltwater crocodiles. This would likely make them the largest amphibians of all time. Flying insects also diversified quite a lot during the Permian, and the first members of the beetle order Coleoptera show up in the fossil record around 299 million years ago. Permian Life Adelophthalmus a eurypterid Diplocaulus a nectridean amphibian Labidosaurikos a captorhinid Fadenia a eugeneodontid Meganeuropsis a griffinfly Dimetrodon a pelycosaur Psaronius a tree fern Noeggerathia an extinct spore-bearing plant Previous Section The Early Earth Next Section The Mesozoic Era

An online guide to the animals found in the Western Interior Sea, including mosasaurs, plesiosaurs, pterosaurs and giant Cretaceous sharks Guide To The Western Interior Sea An ancient seaway once covered the Great Plains Between 100 and 70 million years ago during the Cretaceous period, the part of North America we now call the Great Plains was in the process of sinking while sea levels rose to some of the highest levels ever seen in Earth’s history. The result of these two processes working in tandem was the slow ingression of the waters of the Arctic Ocean and the Gulf of Mexico into the American midcontinental region, with inland seas forming over the continental shelf in the North and South. Eventually these northern and southern seas joined together, forming a long narrow body of water we now call the Western Interior Sea. At its height, this sea spanned from southern Texas to the Canadian Arctic, splitting the continent in two. The waters of this seaway were warm and shallow, estimated to have been no more than 3,000 feet deep. Today we find the fossils of the ancient denizens of these prehistoric waters in the layers of limestone and shale located beneath much of the Great Plains, including the remains of mosasaurs, plesiosaurs, giant fish, sharks and even some rare fossils of dinosaurs whose bodies were washed out to sea. Browse animals by taxonomic group Or scroll to the bottom of the page to browse by genus Vertebrate fauna Mosasaurs and kin (Squamata) Mosasaurs, Dolichosaurids Explore Plesiosaurs (Plesiosauria) Elasmosaurids, Polycotylids, Pliosaurs Explore Ichthyosaurs (Ichthyosauria) Fish-Shaped Marine Reptiles Explore Archosaurs (Archosauria) Dinosaurs, Pterosaurs, Crocs, Seabirds Explore Turtles (Testudines) Protostegids And Other Sea Turtles Explore Shallow Water Coelacanths Explore Latimiriid Coelacanths (Latimiriidae) Explore Lamniform Sharks, Carpet Sharks, etc. Sharks (Selachii) Other Cartilaginous Fishes Rays, Sawskates, Guitarfish Explore Explore Gars and Teleost Fishes Ray-Finned Fishes (Actinopterygii) Invertebrate fauna Ammonoids (Ammonoidea) Monomorph and Heteromorph Ammonites Explore Coleoids and Nautiloids Octopods, Belemnites, Nautilids Explore Snails (Gastropoda) Marine And Estuarine Aquatic Snails Explore Bivalves (Bivalvia) Rudists, Clams, Mussels, Oysters Explore Arthropods (Arthropoda) Crabs, Horseshoe Crabs, Barnacles, Shrimp Explore Sea Lilies, Starfishes, Brittle Stars, Urchins Explore Crinoids, Sea Urchins (Echinodermata) Explore Misc. Invertebrates: Bryozoa, Sponges, Brachiopods, Corals The Geography of the Western Interior Sea Although the Western Interior Sea spanned all the way from the Arctic circle to the Caribbean, the waters of the ocean were very shallow. The ocean floor wasn't covered in corals in the same way as today’s tropical coastal waters. It was instead composed of a thick layer of ooze and slime, home to many different species of large bivalves. Explore the Interior Sea Browse vertebrates by genus Squamates Clade Mosasauria Clidastes Dallasaurus Ectenosaurus Eonatator Globidens Gnathomortis Jormungandr Latoplatecarpus Mosasaurus Clade Mosasauria (continued) Platecarpus Plesioplatecarpus Plioplatecarpus Prognathodon Russellosaurus Selmasaurus Tylosaurus Family Dolichosauridae Coniasaurus Plesiosaurs Pliosaurs (Suborder Pliosauroidea) Brachauchenius Megacephalosaurus Elasmosaurs (Family Elasmosauridae) Albertonectes Elasmosaurus Libonectes Nakonanectes Plesioelasmosaurus Styxosaurus Terminonatator Thalassomedon Wapuskanectes Leptocleidids (Family Leptocleididae) Nichollssaura Polycotylids (Family Polycotylidae) Dolichorhynchops Edgarosaurus Martinectes Pahasapasaurus Plesiopleurodon Polycotylus Serpentisuchops Trinacromerum Unktaheela Ichthyosaurs Athabascasaurus Maiaspondylus Archosaurs Non-avian dinosaurs Claosaurus Niobrarasaurus Birds (Class Aves) Apatornis Baptornis Fumicollis Guildavis Hesperornis Iaceornis Ichthyornis Parahesperornis Pasquiaornis Pterosaurs (Order Pterosauria) Aetodactylus Alamodactylus Cimoliopterus Nyctosaurus Pteranodon Clade Crocodylomorpha Deinosuchus Terminonaris Sea Turtles Archelon Chelosphargis Ctenochelys Desmatochelys Kansastega Lophochelys Porthochelys Prionochelys Protostega Toxochelys Coelacanths Megalocoelacanthus Sharks Order Orectolobiformes Chiloscyllium Cretorectolobus Orectoloboides Plicatoscyllium Order Lamniformes Archaeolamna “Carcharias” Cardabiodon Cretolamna/Cretalamna Cretodus Cretoxyrhina Dallasiella Johnlongia Leptostyrax Odontaspis Paranomotodon Protolamna Pseudocorax Pseudomegachasma Ptychodus Scapanorhynchus Squalicorax Order Carcharininiformes Archaeotriakis Galeorhinus Palaeogaleus Squatigaleus Order Synechodontiformes Paraorthacodus Synechodus Order Squatiniformes Squatina Order Heterodontiformes Heterodontus Order Squaliformes Squalus Order Hexanchiformes Hexanchus Unclear Affinities Aquilolamna Rays, Hybodonts and Batomorphs Stingrays (Order Myliobatiformes) Dasyatis Myliobatis Rhombodus Skates (Order Rajiformes) Ischyrhiza Onchopristis Ptychotrygon Texatrygon Walteraja Chimaeras (Order Chimaeriformes) Edaphodon Ischyodus Hybodonts (Order Hybodontiformes) Lonchidion Meristodonoides Electric rays (Order Torpediniformes) Proplatyrhina Guitarfishes (Rhinopristiformes) Pseudohypolophus Rhinobatos Ray-Finned Fishes Order Ichthyodectiformes Gillicus Ichthyodectes Prosaurodon Saurocephalus Saurodon Xiphactinus Order Pycnodontiformes Anomoeodus Coelodus Gyrodus Micropycnodon Palaeobalistum Order Semionotiformes Hadrodus Order Crossognathiformes Apsopelix Elopopsis Pachyrhizodus Order Elopiformes Laminospondylus Family Pachycormidae Bonnerichthys Protosphyraena Order Aulopiformes Apateodus Cimolichthys Enchodus Leptecodon Stratodus Order Beryciformes Caproberyx Kansius Order Tselfatiiformes Bananogmius Enischnorhynchus Ferrifrons Luxilites Martinichthys Niobrara Pentanogmius Pseudanogmius Syntegmodus Thryptodus Zanclites Order Polymixiiformes Omosoma Order Albuliformes Paralbula Pollerspoeckia Family Apogonidae Apogonidarum Order Amiiformes Cyclurus Paraliodesmus? Order Anguiliformes Anguilavus Urenchelys Family Ariidae Vorhisia Order Aspidorhynchiformes Belonostomus Family Lepisosteidae Atractosteus Lepisosteus Browse invertebrates by family, order or class Ammonoids (Browse by family) Family Acanthoceratidae Acanthoceras Burroceras Calycoceras Conlinoceras Cunningtoniceras Dunveganoceras Eucalycoceras Mammites Morrowites Neocardioceras Paraconlinoceras Plesiacanthoceras Plesiacanthaceratoides Pseudaspidoceras Spathites Tarrantoceras Watinoceras Family Anisoceratidae Allocrioceras Family Baculitidae Baculites Sciponoceras Family Binneyitidae Borissiakoceras Family Brancoceratidae Mortoniceras Oxytropidoceras Family Coilopoceratidae Coilopoceras Family Collignoniceratidae Collignoniceras Menabites Prionocyclus Texanites Family Desmoceratidae Moremanoceras Parapuzosia Family Dipoloceratidae Hystoceras Family Engonoceratidae Engonoceras Metengonoceras Family Forbesiceratidae Forbesiceras Family Gaudryceratidae Anagaudryceras Family Hamitidae Stomohamites Hamites Family Muniericeratidae Tragodesmoceras Family Nostoceratidae Cirroceras Didymoceras Exiteloceras Eubostrychoceras Family Pachydiscidae Eopachydiscus Pachydiscus Family Placenticeratidae Placenticeras Proplacenticeras Family Scaphitidae Clioscaphites Desmoscaphites Discoscaphites Haresiceras Hoploscaphites Jeletzkytes Scaphites Trachyscaphites Yezoites Family Schloenbachiidae Schloenbachia Family Sphenodiscidae Sphenodiscus Family Turrilitidae Mariella Ostlingoceras Turrilites Family Vascoceratidae Vascoceras Coleoids and Nautiloids (Browse by subclass) Subclass Coleoidea Belemnites Enchoteuthis Muensterella Niobrarateuthis Subclass Nautiloidea Stenzeloceras Eutrephoceras Cymatoceras Gastropods (Browse by subclass) Subclass Caenogastropoda Family Aporrhaidae Anchura Drepanochilus Lispodesthes Family Buccinidae Aliofusus Family Cancellariidae Caveola Family Capulidae Trichotropis Family Cassiopidae Gymnentome Family Cerithiidae Cerithium Hemicerithium Levicerithium Macrocerithium Voysa Vascellum Family Cerithopsidae Monroea Family Epitoniidae Epitonium Family Fasciolariidae Bellifusus Fasciolaria Fusinus Graphidula Paleosephaea Piestochilus Family Gyrotropidae Lirpsa Family Naticidae Euspira Gyrodes Lunatia Natica Family Pseudomelaniidae Psedomelania Family Sarganidae Hillites Family Thiaridae Pyrgulifera Family Turritellidae Cragina Mesalia Turritella Caenogastropoda (continued) Family Tylostomatidae Tylostoma Family Vanikoridae Vanikoropsis Family Viviparidae Campeloma Family Volutidae Parvivoluta Tovula Subclass Heterobranchia Family Acteonidae Fictoacteon Pirsila Family Pyramidellidae Turbonilla Family Ringiculidae Cinulia Ringicula Family Siphonariidae Anisomyon Subclass Neritimorpha Family Neritidae Nerita Subclass Patellogastropoda Family Acmaeidae Acmaea Subclass Vetigastropoda Family Eucyclidae Amberleya Family Fissurelidae Diadora Family Pleurotomariidae Pleurotomaria Family Turbinidae Turbo Bivalves (Browse by order) Order Adapedonta Leptosolen Panope Panopea Siliqua Superorder Anomalodesmata Anatyma Cuspidaria Geniomya Laternula Pholadomya Psilomya Order Arcida Breviarca Cuculea Idonearca Limopsis Parallelodon Protarca Order Cardiida Callistina Cardium Cyclorisma Nelltia Protocardia Protodonax Solyma Tancredia Tellina Tellinimera Trachycardium Order Carditida Crassatella Opis Venericardia Order Hippuritida Durania Eoradiolites Hippurites Proradiolites Sauvagesia Order Lucinida Clissocolus Ctena Lucina Nympholucina Sexta Order Myalinida Cremnoceramus Inoceramus Myalinida Mylitoides Volviceramus Order Myida Caryocorbula Corbula Opertochasma Parmicorbula Pholas Ursirivus Order Mylitida Brachidontes Botula Crenella Lithophaga Modiolus Volsella Order Nuculanida Malletia Nuculana Yoldia Order Nuculida Acila Nucula Order Ostreida Crassostrea Exogyra Gervilliopsis Gryphaea Lopha Nanostrea Pseudoperna Pycnodonte Tenuipteria Texigryphaea Order Pteriida Phelopteria Pteria Order Pectinida Anomia Lima Pecten Plicautula Neithea Syncyclonema Order Solemyida Solemya Order Trigoniida Trigonia Order Venerida Arctica Callistina Corbicula Cyclorisma Cyprimeria Dentonia Flaventia Legumen Arthropods (Browse by class, order) Horseshoe crabs (Order Xiphosura) Limulus Clam shrimp (Superorder Diplostraca) Cyzicus Seed shrimp (Class Ostracoda) Cytherella Barnacles (Subclass Ciripedia) Stramentum Decapods (Order Decapoda) Ghost shrimp (Familly Callianassidae) Protocallianassa Prawns (Suborder Dendrobranchiata) Penaeus Spiny lobsters (Infraorder Achelata) Linuparis Panulirus Rugafarius Lobsters (Infraorder Astacidae) Hoploparia True crabs (Infraorder Brachyura) Echinoderms (Browse by class) Sea Urchins (Class Echinoidea) Codiopsis Cottaldia Goniopygus Cassidulus Echinobrissus Nucleolites Globator Tetragramma Crassiholaster Holaster Anorthopygus Holectypus Gauthiera Phymosoma Goniophorus Salenia Hemiaster Sea Urchins (continued) Heteraster Holanthus Mecaster Micraster Sea Lilies (Class Crinoidea) Dunnicrinus Lakotacrinus 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Explore the different types of Cretaceous sea turtles who lived in the Western Interior Sea, including Archelon, the largest turtle of all time Sea Turtles Sea Turtles of the Great Plains Testudines, the order that includes all turtles, tortoises, and terrapins, have one of the most unusual body plans among vertebrates. Members of this order have a protective shell formed from modified ribs and vertebrae that is permanently fused to their skeletons. Their shell consists of an upper carapace and a lower plastron, while their keratin-covered beaks fill a similar function as teeth. Many terrestrial and semi-aquatic species can retract their heads into their shells, although this ability is not present in sea turtles. The layers of rock deposited by the Western Interior Sea preserve fossils of several types of prehistoric marine turtles, including car-sized giants like Protostega a nd Archelon. Although many of these animals appear superficially similar to today’s sea turtles, some of these animals do not seem to be part of groups of turtles which were the direct ancestors of living sea turtles. Others, like Prionochelys and Ctenochelys , seem to have been close relatives of the superfamily which would eventually lead to all seven of today’s living sea turtle species. Anatomy Skeletal Anatomy Sea turtles descend from animals who once lived on land and as a result, most features of their skeletons are derived from structures which originally helped their ancestors move around in terrestrial habitats. The bones found in their flippers are the same bones found in the legs and feet of other turtles. In sea turtles the phalanges supporting the outer parts of each flipper are longer than those of their terrestrial relatives, while their other leg bones are proportionally shorter. The skulls of some types of Cretaceous sea turtles, namely the protostegids, grew to be very large and were equipped with long hooked beaks. The shell structures of the carapaces of these ancient turtles were linked to their ribs. The lower section of the shell, known as the plastron, was formed from wide, somewhat flattened bony plates. In protostegid sea turtles the bony shell structure of the carapace was reduced and the only remaining sets of scute bones were those found along the margins of the shell. The shells of protostegid sea turtles were covered in a thick layer of skin instead of the scutes seen in most turtles. (BELOW) Features of the skeletal anatomy of the Cretaceous sea turtle Protosteg a. As a member of the protostegid lineage, the bones of the carapace are reduced in this genus. The edges of the bones of the plastron have a distinctive saw-toothed shape in protostegids and the central part of the plastron is unfused beneath the bones of the pectoral girdle. Shell Plates Turtle shells are composed of a bony inner layer covered by a series of plates called scutes. The bony core of the shell is connected to the turtle's ribs. Although the shapes of the outer layers of scutes can vary wildly between different types of turtles, they all tend to be organized in a similar way. There is a series of five central scutes along the turtle's spine. In front of the first central scute at the base of the neck there is an additional plate called the precentral scute, while a pair of postcentral scutes sits at the rear of the shell just above the position of the tail. The central scutes are flanked on each side by five pairs of lateral scutes. The edges of a turtle's shell are marked by a line of smaller marginal scutes. (BELOW) The arrangement of scutes found on a turtle's carapace. Some sea turtles do not have central or lateral scutes along the upper parts of their carapaces, although the ribs of these animals are still somewhat wider and more heavily built than those of most types of tetrapods in order to support an outer covering of soft tissue. External Anatomy The shell of a sea turtle is composed of two halves. The dorsal half of the shell is called the carapace while the ventral side is known as the plastron. Sea turtles have four flippers. Along the front edge of the fore flippers there is often a spur of tougher material called a claw. Some sea turtle genera have more than one of these claws on each of their fore flippers. In some genera, such as Ctenochelys , the hind flippers can be rather large, which is thought to be a feature which would let these animals push themselves along the seafloor or the bottoms of rivers. Sea turtles tend to have very robust beaks, although the exact shape of these structures can vary markedly between families of sea turtles. Protostegids, for instance, have very long hook-shaped beaks while Toxochelys and its relatives have shorter triangular beaks. (ABOVE) External antomical features of the Cretaceous sea turtle Ctenochelys, a distant relative of the sea turtle groups found in today's oceans. Some Cretaceous sea turtles had ridges running along the central scutes of their carapaces. Sea Turtles Archelon Chelosphargis Ctenochelys Desmatochelys Kansastega Lophochelys Porthochelys Prionochelys Protostega Toxochelys Archelon Temporal Range: 80 million years ago - 74 million years ago Geographic Range: South Dakota Species: A. ischyros Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Archelon was the largest known turtle to have ever lived and could grow to be the size of a small truck. In contrast to many other Cretaceous sea turtles it did not have a fully solid, heavily ossified shell. Instead, its body was supported by a lighter skeletal framework covered by leathery tissue. It would have had very large front flippers used to push itself through the water, as well as a long skull ending in a powerful, hooked beak suited for handling a variety of prey. These animals probably preferred to feed on soft-shelled prey in open waters, much in the same way that living sea turtles feed on jellyfish today. With a body length exceeding 12 feet, Archelon dwarfed most contemporary sea turtles. Fossils of Archelon first turned up in 1895 when a rather complete skeleton was found in the Pierre Shale formation of South Dakota. The following year the species Archelon ischyros would be officially described based on those remains. Owing to their large size, Archelon has remained one of the most famous prehistoric turtles throughout the ensuing years all the way up until the present. (BELOW) An Archelon skeleton on display at the Natural History Museum of Vienna. The skull of the animal is in the center of the photo. The saw-toothed edges of the plates of the plastron are visible through the ribs and bones of the carapace. Photo by Paul Hermans, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license (BELOW) A photo of an Archelon skull which appeared in the American Journal of Science, volume 9, in 1900. Note the very long hook-shaped beak which is characteristic of protostegid sea turtles. accessed via Wikimedia Commons, public domain image Chelosphargis Temporal Range: 94.3 million years ago - 89.3 million years ago Geographic Range: Kansas, Alabama Species: C. advena Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Chelosphargis was a sea turtle which was unique amongst its relatives because it had a well-developed keel extending along the center of its carapace. This feature has been used by paleontologists to differentiate Chelosphargis from closely related genera, particularly Calcarichelys , which exhibits a similar overall body plan but differs in the details of its shell morphology. These turtles also seem to have had a very deep shell, which would have made the animals appear much more rounded in profile than many other types of Cretaceous sea turtles. Their shells are usually found disarticulated and somewhat crushed so exact measurements of the range of shell shapes found in this genus are not entirely known. It is currently considered to be a member of the protostegid sea turtle family, along with animals like Archelon and Desmatochelys . (BELOW) A sketch of the holotype fossil of Chelosphargis advena from Oliver Hay's "Fossil turtles of North America" (1908), which refers to the animal as "Protostega advena ". The holotype is composed of a partial plastron, a series of marginal scutes and some other shell and skull fragments. Public domain image Ctenochelys Temporal Range: 89 million years ago - 70 million years ago Geographic Range: Central North America, Alabama Species: C. stenoporus, C. acris Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Ctenochelys was a fairly small turtle whose name means "comb turtle" after the series of spine-like projections on the scutes running along the back of its shell. Unlike the enormous protostegid turtles which would have been some of its contemporaries, Ctenochelys belonged to a lineage more closely aligned with the ancestors of modern sea turtles. It is currently recovered as one of the earliest close relatives of the Chelonioids, part of the lineage that leads to all seven of today’s sea turtle species. When fossils of this animal were first found in 1904, the species now known as Ctenochelys stenoporus was initially placed within Toxochelys before being given its own classification years later. (BELOW) A photograph of a Ctenochelys shell and a diagram of its components. Photo is from "The life of a fossil hunter", by Charles H. Sternberg and refers to the animal as "Toxochelys bauri ". The diagram appeared in the American Journal of Science, volume 20 in 1905. Note the raised area along the midline of the shell. Public domain images Desmatochelys Temporal Range: 120 million years ago - 83.5 million years ago Geographic Range: North America, Japan, Columbia Species: D. lowi, D. padillai Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Desmatochelys was among the earliest known sea turtles and part of the protostegid turtle family. In contrast to later giant sea turtles such as Protostega and Archelon , Desmatochelys retained a number of more basal anatomical characteristics seen in the shells of many types of semi-aquatic and terrestrial turtles. Despite these more basal features, Desmatochelys also had modified paddle-like limb bones adapted for life in marine environments. The genus includes two recognized species, Desmatochelys lowii and Desmatochelys padillai. The latter is particularly important because it is considered to be the oldest known sea turtle, with fossils dating to more than 120 million years ago. Fossil discoveries of these animals found all around the world show us that Desmatochelys occupied a broad geographic range and inhabited coastal and marine ecosystems associated with the Western Interior Sea and other Cretaceous oceans. (BELOW) A skeleton of Desmatochelys padillai , a species from the Early Cretaceous of Columbia. These animals would have been some of the earliest animals we would recognize as sea turtles. Photo accessed via Wikimedia Commons and uploaded by user Sernapi, distributed under a CC BY-SA 3.0 license Kansastega Temporal Range: ~84 million years ago Geographic Range: Kansas Species: K. copei Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Kansastega was a protostegid sea turtle which was at various times classified within the genera Protostega and Archelo n. Later research showed that it had distinctive shell features that warranted recognition as a separate genus. One of the most important differences is that Kansastega did not have the highly reduced costal plates along the sides of the shell that are found in both Protostega and Archelon. Fossils belonging to Kansastega were first described in 1909 under the name Protostega cope i. Later studies reassigned it to Archelon, and in 1998 it was given its own genus, Microstega , after it was found that these animals showed significant anatomical differences from other protostegids. However, because the name Microstega had already been assigned to a genus of moths it could not be retained. To correct this problem the replacement name Kansastega was given to it in 2023. Lophochelys Temporal Range: ~75 million years ago Geographic Range: Central North America Species: L. natatrix Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Lophochelys was a marine turtle from the Late Cretaceous that belonged to the family Toxochelyidae, a group which also includes animals like Toxochelys . Within this family, Lophochelys is considered to be a part of its own subfamily, Lophochelyinae. While fossil material has suggested that Lophochelys was adapted to life in marine environments, many aspects of its anatomy remain incompletely understood because the available fossil material for this animal is relatively limited. Paleontological research and taxonomic reviews have generally continued to recognize Lophochelys as a member of the Toxochelyidae since its discovery and have retained several species within the genus. It seems to have been a rather small animal, comparable to other smaller toxochelyids rather than the huge sizes reached by some contemporary protostegid turtles. These animals may have been hunted by durophagous sharks such as Ptychodus, as well as shell-crushing mosasaurs. Porthochelys Temporal Range: Santonian stage Geographic Range: Kansas Species: P. laticeps Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Porthochelys was among the earlier representatives of the sea turtle lineage and was well adapted to life in the shallow marine environments of the Western Interior Sea. Its shell was broad and roughly circular, a little over three feet across. This shell was sturdy and composed of highly fused plates. This makes its shell much more similarly built to living sea turtles than to those of other Cretaceous marine turtles who tended to have much more lightly-built shell bones. Its fossil remains were found in Kansas and to date it is only known from a very limited number of specimens, although we are lucky enough to have a shell for this animal which is more than half complete and gives us a sense of the general form of these animals. (BELOW) The remains of the shell, skull and a limb bone of Porthochelys. This genus is known from limited material and this particular fossil at the KU Natural History Museum gives us our best picture of the shape of the shell and skull of this animal. Prionochelys Temporal Range: 84 million years ago - 73 million years ago Geographic Range: Kansas, Alabama Species: P. matutina, P. galeotergum Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Prionochelys is an extinct sea turtle which would have had a series of small bony structures, called epineurals, positioned along the dorsal midline of its carapace, a characteristic rarely observed in other turtle groups. These spiny structures give this genus its name, which translates to “saw turtle”. Its fossils are best known from the Mooreville Chalk of Alabama but its remains also turn up in deposits from the Western Interior Sea. Its fossils were first found in 1953 and for many years it was considered to be a close relative of animals like Toxochelys and placed within the family Toxochelyidae. More recent work conducted in the last decade has shown that Prionochelys was actually more closely related to Ctenochelys and Peritresius, leading to its placement within the family Ctenochelyidae. This would make Prionochelys a closer relative of today’s sea turtles than we had previously thought. Protostega Temporal Range: 89 million years ago - 80 million years ago Geographic Range: Kansas, Alabama, central Canada, Russia Species: P. gigas Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Protostega was among the largest marine turtles of all time, exceeded only by Archelon. The remains of these turtles were first found in 1871 in a chalk deposit in western Kansas. The species was formally described and named Protostega gigas in 1872 by Edward Drinker Cope. Because the earliest specimens were incomplete, researchers initially struggled to reconstruct the animal accurately. Over time we learned more about the particular shell structures found in many types of Cretaceous sea turtles as well as the extent of skin coverings on the shells of some of these animals. Protostega gave its name to the family Protostegidae, which also includes animals like Archelon and Desmatochelys. These turtles were initially considered close relatives of today’s leatherback sea turtles but further research suggests that they were part of a separate branch of the turtle family tree. Rather than having a heavily ossified carapace like many turtle species, Protosteg a had a reduced shell structure that likely improved its mobility in the water. The animal also had large forelimbs adapted into powerful flippers, a rather wide shoulder girdle, a very large head and a long hook-shaped beak (BELOW) A fossil Protostega shell on display at the Sternberg Museum of Natural History. The bones of the plastron are visible on the other side of the ribs. The shell bones of protostegids were less heavily built than those of today's sea turtles. (BELOW) The fossils of the hind limb bones of a Protostega . In life the femur would be tucked below the animal's shell. Photo taken at the KU Natural History Museum in Lawrence, Kansas Toxochelys Temporal Range: 89 million years ago - 66 million years ago Geographic Range: Central North America, Alabama Species: T. latiremis, T. moorevillensis Diet: Carnivorous, soft-shelled prey, fish, cephalopods, jellyfish Toxochelys was a fairly small Cretaceous sea turtle and by far the most common prehistoric turtle whose fossils are found in deposits from the Western Interior Sea. It would have measured only a little over 3 feet in length, which made it much smaller than giant sea turtles such as Archelon and Protostega . What makes Toxochelys particularly important is its position in sea turtle evolution. It is considered to be a transitional animal that combined ancestral turtle characteristics with adaptations for life in the ocean. Its limbs had already evolved into short swimming paddles, yet the structures of the toe bones within those paddles closely resemble those of earlier semi-aquatic turtles. Toxochelys is considered to be closely related to the common ancestor of most Cretaceous sea turtle genera. Compared with other sea turtles these animals had rather small heads and broad, flat shells. Its plastron is mostly composed of two hourglass-shaped bones fused along the animal’s midline. (BELOW) A replica skeleton of Toxochelys on display at the Indianapolis Childrens Museum. The digits of the hands and feet of these animals still appear rather similar to those of more basal semi-aquatic turtles despite being encased within flippers Media Gallery Suggested References: Sea Turtles Carrino, M.H. (2007). “Taxonomic comparison and stratigraphic distribution of Toxochelys(Testudines: Cheloniidae) of South Dakota”. pp. 111-132 in Martin, J.E. and Parris D.C. (eds.), The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas. Geological Society of America, Special Paper 427. Derstler, K. (1994). “Evolutionary history of sea turtles”. Journal of Vertebrate Paleontology 15(suppl. to no. 3):23A. Gentry, Andrew D. (2018). "Prionochelys matutina Zangerl, 1953 (Testudines: Pan-Cheloniidae) from the Late Cretaceous of the United States and the evolution of epithecal ossifications in marine turtles". PeerJ. 6 e5876. Hirayama, R. (1997). “Distribution and diversity of Cretaceous cheloniids”. pp. 225-241 in Callaway, J.M. and Nicholls, E.L. (eds.), Ancient Marine Reptiles, Academic Press, San Diego. Hooks, G. E., III. (1998). “Systematic revision of the Protostegidae, with a redescription of Carcarichelys gemma” Zangerl, 1957. Journal of Vertebrate Paleontology, 18(1):85-98. Kear, B.P. (2006). “First gut contents in a Cretaceous sea turtle”. Biological Letters 2:113-115. Lucas, Spencer G.; Sullivan, Robert M. (2006). “Late Cretaceous Vertebrates from the Western Interior: Bulletin 35”. New Mexico Museum of Natural History and Science. Matzke, A.T. (2007). “An almost complete juvenile specimen of the cheloniid turtle Ctenochelys stenoporus (Hay, 1905) from the Upper Cretaceous Niobrara Formation of Kansas, USA”. Palaeontology 50(3):669-691. Matzke, A.T. (2009). “Osteology of the skull of Toxochelys (Testudines, Chelonioidea) (with 25 text-figures). Palaeontographica Abteilung A 288(4):93-150. McDavid, S. N., & Hooks, G. E. (2023). “Kansastega, nom. nov., a replacement name for Microstega Hooks, preoccupied by Meyrick”. Journal of Vertebrate Paleontology, 43(1). McIntosh, A. P.; Shimada, K.; Everhart, M. J. (2016). "Late Cretaceous marine vertebrate fauna from the Fairport Chalk Member of the Carlile Shale in southern Ellis County, Kansas, U.S.A." Transactions of the Kansas Academy of Science.

Explore the different types of plesiosaurs found in the Cretaceous waters of the Western Interior Seaway, including animals like Elasmosaurus, Styxosaurus, Dolichorhynchops and Megacephalosaurus Plesiosauria Plesiosaurs in the Great Plains The plesiosaurs were one of the dominant groups of marine reptiles found in the world’s oceans during the Mesozoic. Their ancestry is somewhat mysterious but they seem to have been related to many other types of marine reptiles who independently became aquatic during the Mesozoic era. It should be noted that plesiosaurs were not closely related to the mosasaurs, another group of marine reptiles who became very successful during the Late Cretaceous and whose fossils are also found in rocks in central North America. The Western Interior Sea supported a diverse range of plesiosaurs during the Late Cretaceous. The best known members of this group of ocean reptiles are the elasmosaurids, known for their extremely long necks, small heads, cone-shaped teeth and powerful flippers. Species such as Elasmosaurus likely used their long necks to approach schools of fish and squid from below with minimal disturbance. Another important group was the family polycotylidae, which had short necks, long crocodile-like heads, and streamlined bodies. Predators like Dolichorhynchops were adapted for speed and active pursuit of prey, occupying a different ecological niche from the elasmosaurs. The Interior Sea was also home to members of older plesiosaur lineages, such as some of the latest surviving examples of the short-necked pliosaurs. These families filled multiple roles within the ecosystem, from giant hunters with bone-crushing bites to fast-swimming predators. Anatomy (BELOW) A labeled diagram of the skeleton of the polycotylid short-necked plesiosaur Dolichorhynchops from Kansas Skeletal Anatomy Plesiosaurs came in many different shapes and sizes and their skeletons varied quite a bit depending on their specific ecological roles. Despite this variation many shared broad characteristic features can be seen in the remains of most types of plesiosaurs. Their bodies tended to be wide, streamlined and supported by a sturdy rib cage and a relatively rigid vertebral column that helped give them extra stability while swimming. Plesiosaurs also had a very distinctive neck structure. Their necks were built from dozens of short interlocking cervical vertebrae and the number of neck vertebrae in an animal's neck could vary wildly between species even within a single taxonomic family. Some plesiosaurs had short, muscular necks, while others evolved long and flexible ones. These animals are often shown in older media with coiled snake-like necks but this degree of flexibility does not actually seem to have been attainable for these animals. Even animals with 50 or more cervical vertebrae would have had fairly rigid necks, The skulls of plesiosaurs were typically equipped with an array of sharp, conical teeth well suited for capturing fish, cephalopods, and other marine prey. Openings at the back of their skulls and along their cheekbones provided attachment points for strong jaw muscles. The pectoral and pelvic girdles around the hip and shoulder regions were heavily built and served as strong anchors for the limbones. All four limbs had developed over time into large, paddle-shaped flippers composed of elongated and flattened bony blocks formed from the bones of the arms and wrists. These limb bones can be generally sorted into two types; propodials (humeri and femurs) and podials (forearm bones, lower leg bones, leg and manus bones). The exact swimming motion that would have been used by these ancient animals is still something of a mystery but they are often shown in media as flapping their foreflippers and hindflippers up and down at opposite intervals. (BELOW) Some plesiosaurs had skeletons which included dozens of cervical vertebrae in their very long necks. (BELOW) The waters of the Western Interior Sea were home to both long-necked and short-necked plesiosaurs. The pliosaurs went extinct not long after the Interior Sea first spanned across North America External Anatomy Late Cretaceous plesiosaurs included both animals who had very long necks and others who had extremely large skulls and short necks. The top predators of the seas of the early Cretaceous period were the Pliosaurs, a group of plesiosaurs whose heads measured up to a quarter of their entire body length. Elasmosaurids on the other hand tended to have longer necks and fairly small heads while Polycotylid plesiosaurs had medium-sized necks and long toothy snouts. Plesiosaurs Pliosaurs (Suborder Pliosauroidea) Brachauchenius Megacephalosaurus Elasmosaurs (Family Elasmosauridae) Albertonectes Elasmosaurus Libonectes Nakonanectes Plesioelasmosaurus Styxosaurus Terminonatator Thalassomedon Wapuskanectes Leptocleidids (Family Leptocleididae) Nichollssaura Polycotylids (Family Polycotylidae) Dolichorhynchops Edgarosaurus Martinectes Pahasapasaurus Plesiopleurodon Polycotylus Serpentisuchops Trinacromerum Unktaheela Pliosaurs (Pliosauroidea) Large Skulls and Crushing Bites Pliosaurs were among the most formidable marine reptiles to inhabit the Western Interior Seaway. Unlike their long-necked plesiosaur relatives, pliosaurs had short, powerful necks, massive skulls, and large conical teeth designed for seizing and crushing prey. Their streamlined bodies and four strong flippers made them fast and agile swimmers capable of pursuing large prey. One notable pliosaur from North America was Brachauchenius , an ancient predator from the middle Cretaceous that represented one of the last surviving members of its group. Its robust jaws and strong bite allowed it to occupy the role of an apex predator within the marine ecosystem. Large eyes and keen senses likely helped pliosaurs locate prey in a variety of underwater conditions. Brachauchenius Temporal Range: 100 million years ago - 90 million years ago Geographic Range: Kansas, Morrocco Species: B. lucasi Diet: Carnivorous, large prey, fish, sharks, marine reptiles Brachauchenius was one of the final representatives of the pliosaur lineage. These animals retained many of the adaptations that had made their ancestors successful ocean predators for millions of years during the Jurassic and early Cretaceous periods. Its most notable feature was its large, heavily built head, armed with powerful jaws and sharp, cone-shaped teeth ideal for restraining its prey. Compared to many plesiosaurs, its head was proportionally larger, emphasizing its macropredatory lifestyle. The history of the discovery of Brachauchenius in the Great Plains began in 1884 when a large marine reptile fossil was uncovered near Delphos, Kansas. The specimen was excavated by fossil collector Charles H. Sternberg with support from paleontologist Othniel Charles Marsh and later preserved in museum collections. In 1903, Samuel W. Williston formally described and named the animal Brachauchenius lucasi , identifying it as a short-necked pliosaur based on its skull, vertebrae, and ribs. This animal seems to have been closely related to another pliosaur, Megacephalosaurus. (BELOW) A Brachauchenius tooth from the upper Cenomanian Britton Formation of Texas. Fossil collected and original photo taken by Jared Cooke/ jcookepaleo (BELOW) A photo of the holotype specimen of Brachauchenius lucasi, preserving a skull, cervical and thoracic vertebrae as well as ribs, accessed via Wikimedia Commons, public domain image by the Smithsonian Institute (BELOW) A view of the holotype specimen of Brachauchenius lucasi in which the lower parts of the skull and the palate are visible. Accessed via Wikimedia Commons, public domain image by the Smithsonian Institute Megacephalosaurus Temporal Range: 93.9 million years ago - 92.9 million years ago Geographic Range: Kansas Species: M. eulerti Diet: Carnivorous, large prey, fish, sharks, marine reptiles Megacephalosaurus was a genus of pliosaurs whose name translates to “large-headed lizard.” The skull of Megacephalosaurus is among the largest known for a North American pliosaur and would have made up around a fifth of the animal’s total body length Fossils of Megacephalosaurus were first discovered in 1950 when Frank and Robert Jennrich found a large fossil skull while collecting shark teeth near Fairport, Kansas. The specimen was excavated with the help of paleontologist George F. Sternberg and later donated to the Sternberg Museum of Natural History. Initially, scientists believed the fossil belonged to the pliosaur Brachauchenius lucasi and it remained classified as that species for many years. Further study revealed important anatomical differences between the skull and known specimens of Brachauchenius . In 2013 paleontologists Bruce Schumacher, Kenneth Carpenter, and Michael Everhart formally described the fossil as a new genus and species, naming it Megacephalosaurus eulert i. (BELOW) The original holotype skull of Megacephalosaurus eulerti on display at the Sternberg Museum of Natural History, FHSM VP-321. This is the largest skull known for a North American plesiosaur. (BELOW) A three-dimensional model of the shape of the skull of Megacephalosaurus accounting for taphonomic compression, on display near the entryway of the Sternberg Museum. Model produced by the Rocky Mountain Dinosaur Resource Center Elasmosaurs (Elasmosauridae) Plesiosaurs with Extremely Long Necks Elasmosaurids were some of the most easily recognizable reptiles that inhabited the Western Interior Sea during the Late Cretaceous. Their best known defining feature was an extremely long neck composed of dozens of vertebrae which often made up more than half of the animal’s total body length. This adaptation allowed elasmosaurs to approach prey with their small heads while keeping their large bodies at a distance and hidden in the darkness of the water below, potentially reducing the chance of alarming schools of fish and squid. Unlike the powerful, short-necked pliosaurs that hunted through brute force, elasmosaurs relied on agility and precision. Their four strong, paddle-like flippers provided efficient propulsion, enabling them to swim gracefully through the seaway’s waters. Species such as Elasmosaurus , Styxosaurus , and Thalassomedon were among the most notable members of this family. Although nearly all members of the group had very long necks, fossils show us that they still had a great deal of variation between genera in size and neck proportions, which may have allowed different animals to exploit their own separate niches. Albertonectes Temporal Range: 73.5 million years ago Geographic Range: Alberta Species: A. vanderveldei Diet: Carnivorous, small fish, squid Albertonectes is known from fossils found in Alberta, Canada. This marine reptile is well known for having one of the longest necks ever documented in a plesiosaur. With roughly seventy-six cervical vertebrae, its neck was even more elongated than those of many other well-known elasmosaurids. The first fossil of Albertonectes was discovered in 2007 during ammolite mining near the town of Lethbridge, Alberta, when workers uncovered the remains of a large marine reptile in layers of rocks falling within the Bearpaw Formation. Although part of the fossil was accidentally damaged during excavation, most of the skeleton was preserved and later transferred to the Royal Tyrrell Museum for study. The specimen consisted of an almost complete skeleton, including 132 vertebrae, portions of the shoulder and pelvic girdles, nearly complete limbs, and numerous gastroliths. In 2012, paleontologists Tai Kubo, Mark Mitchell, and Donald Henderson formally described the fossil as a new species. (BELOW) A photo of a cluster of stomach stones, or gastroliths, found in association with a skeleton of Albertonectes near the inside of the animal's right, dorsal ribs. Photo by Donald M. Henderson, Sue Sabrowski, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Elasmosaurus Temporal Range: 80 million years ago - 77 million years ago Geographic Range: Kansas, possible material known from Germany, Japan Species: E. platyurus Diet: Carnivorous, small prey, fish Elasmosaurus is the namesake of the family elasmosauridae and was one of the first members of this group to be discovered in the 19th century. Although it grew to impressive lengths, Elasmosaurus possessed a relatively small head armed with numerous slender, sharp teeth. These teeth were well suited for gripping fish, squid, and other small marine creatures. Rather than overpowering prey with force it probably relied on swift and accurate strikes to capture its meals. Four large flippers gave these animals the propulsion they needed for efficient swimming and allowed them to maneuver through the waters of the ancient seaway with ease. It may have also had a small diamond-shaped fluke along its tail. Elasmosaurus was first discovered in 1867 near Fort Wallace, Kansas, by Army surgeon Dr. Theophilus H. Turner. The fossil was excavated and transported to Philadelphia, where it was studied by paleontologist Edward Drinker Cope. In 1868, Cope named the animal Elasmosaurus platyurus but mistakenly reconstructed the skeleton with the head attached to the tail. This famous error was later corrected and became a well-known episode in the rivalry between Cope and Othniel Charles Marsh during the Bone Wars. Libonectes Temporal Range: 93 million years ago Geographic Range: Northern Texas, Morocco Species: L. morgani, L. atlasense Diet: Carnivorous, small fish, squid Libonectes was a genus of elasmosaurid plesiosaur whose remains are found in parts of Texas and Morocco. It seems to have been a close relative of Thalassomedon. When compared with related genera such as Thalassomedon , Libonectes had taller neural spines on the back of its vertebrae and more elongated supporting structures. It also had nostrils positioned slightly farther forward on its skull. Another noteworthy feature of Libonectes is the preservation of gastroliths, or stomach stones, within some fossil specimens. These stones are fairly common in well-preserved fossil skeletons of elasmosaurs and they may have been used to regulate the animal’s buoyancy as it swam through the water. (BELOW) A photo of the holotype skull of Libonectes morgani from the Late Cretaceous of Texas. Photo by Kenneth Carpenter, accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license (BELOW) A photo of the skull of the Moroccan species Libonectes atlasense , which has a full set of interlocked teeth. Photo by Benjamin P. Kear, Dennis Larsson, Johan Lindgren, Martin Kundrát, accessed via Wikimedia Commons, distributed under a CC BY 2.5 license Nakonanectes Temporal Range: 73 million years ago Geographic Range: Montana Species: N. bradtii Diet: Carnivorous, small fish, squid Nakonanectes an elasmosaurid that lived towards the end of the Late Cretaceous. Fossils discovered in Montana's Bearpaw Formation indicate that it was among the last known elasmosaurids to live in North America. Nakonanectes is especially distinctive because of its relatively short neck. The animal’s neck contained only about 39 to 42 cervical vertebrae. This is considerably fewer than the number found in many classic long-necked elasmosaurids, some of which had 60 cervical vertebrae. This shortened neck would have given it a noticeably different body profile and probably hints at a different type of ecological niche for this animal. Nakonanectes was also fairly small when compared to some of its relatives. It would have only reached an estimated length of roughly 5 to 6 meters, half as long as other elasmosaurids. The species is also known from an unusually well-preserved skull, allowing scientists to study several unique characteristics of its head and jaws which most famously include its distinctive shortened snout. (BELOW) A photo of the holotype skull of Nakonanectes from the Bearpaw Shale. Photo accessed via Wikimedia Commons, uploaded by user Montanaceratops, distributed under a CC BY-NC-SA 4.0 license Plesioelasmosaurus Temporal Range: 94.3 million years ago - 89.3 million years ago Geographic Range: Kansas Species: P. walkeri Diet: Carnivorous, small prey, fish Plesioelasmosaurus was an elasmosaurid that lived in the Western Interior Seaway during the Late Cretaceous. Fossils discovered in Kansas suggest that these would have been a type of marine reptile with a body plan that blended features seen in both early and more specialized elasmosaurids. Although it had the long neck typical of the group, its neck was a bit shorter in proportion than those of highly specialized forms such as Elasmosaurus . The first fossils belonging to this animal were found in 1931 near Holyrood, Kansas. It was informally known as the “Holyrood elasmosaur” for many years before it was finally assigned to its own new genus and species in 2022. Styxosaurus Temporal Range: 100 million years ago - 72 million years ago Geographic Range: Kansas, South Dakota, Nebraska Species: S. snowii, S. brownii, S. rezaci Diet: Carnivorous, small fish, squid Styxosaurus is a famous member of the elasmosaurid family of plesiosaurs. Despite the fact that elongated necks were a defining feature of many elasmosaurids, Styxosaurus belonged to a lineage that developed some of the longest necks known among vertebrates. In addition to its remarkable neck length, it would have had a unique shape to its skull, including forms of ridges and jaw structures that help separate it from closely related elasmosaurs. In 1890, Samuel W. Williston described the original fossils of Styxosaurus , which consisted of a skull and associated neck vertebrae from Kansas, under the name “Cimoliasaurus snowii ”. The species was later reassigned to Elasmosauru s before Samuel P. Welles erected the genus Styxosaurus in 1943. Additional fossils from across the area once covered by the Western Interior Sea were subsequently attributed to the genus. A lot of what we know about more famous animals like Elasmosaurus itself is based on knowledge gained from the study of far more complete fossils of Styxosaurus. (BELOW) The holotype skull of Styxosaurus snowii, KUVP 1301, on display at the KU Natural History Museum Terminonatator Temporal Range: 72 million years ago Geographic Range: Saskatchewan Species: T. ponteixensis Diet: Carnivorous, small fish, squid Terminonatator was one of the last of the elasmosaurids. Its name translates to “last swimmer,” which reflects its position among the later representatives of the long-necked plesiosaur lineage. Although it shared the characteristic elongated neck of other elasmosaurids, it would have had some anatomical features that made it unique within the group, namely having slightly fewer teeth at the front of its mouth. It also had an unusually long femur which may have extended the length of its flippers. This may have helped it navigate both coastal and open-water environments. Like other elasmosaurids, it had a small skull and slender, sharp teeth well suited for capturing fish and soft-bodied prey. Terminonatator ’s first fossils were discovered in the Bearpaw Formation of Alberta, Canada, a rock unit famous for preserving marine reptiles from the final stages of the Western Interior Sea as it began to retreat again at the very end of the Cretaceous period. Thalassomedon Temporal Range: 99.6 million years ago - 93.5 million years ago Geographic Range: Colorado Species: T. haningtoni Diet: Carnivorous, small fish, squid Thalassomedon was among the largest members of the elasmosaurid family to live in the Western Interior Sea. Its name means “sea ruler,” and adults of this genus could reach lengths of more than 40 feet. One of its most distinctive traits was that it had a long neck even for an elasmosaurid, composed of roughly 60 vertebrae. Thalassomedon was discovered in 1939 when fossil bones were exposed by erosion in a layer of shale in a cliff near Pritchett, Colorado. The remains were first spotted by a man named Fred Roth, who called in local paleontologists and geologists to look at his find. Although some bones had been disturbed before scientists arrived, a large portion of the skeleton was successfully recovered from the surrounding rocks, which fell within the Graneros Shale formation. The fossil was later studied by paleontologist Samuel P. Welles, who described and named the new species Thalassomedon haningtoni in 1943. Wapuskanectes Temporal Range: 112 million years ago Geographic Range: Alberta Species: W. betsynichollsae Diet: Carnivorous, small fish, squid Wapuskanectes was one of the earliest known members of the elasmosaurid group on the North American continent. Fossils of these animals were discovered in the Early Cretaceous rocks of the Clearwater formation in Alberta, Canada where it would have lived long before famous later elasmosaurids such as Elasmosaurus, Albertonectes , and Libonectes . It is known from fossils of parts of its spine, shoulder bones and flippers. Like many other types of elasmosaurids from the Western Interior Sea it was found in association with stomach stones, or gastroliths. (BELOW) A photo of a Wapuskanectes skeleton found near the town of Ft. McMurray, Alberta. Photo credit: Sue Sabrowski, Donald Henderson, Royal Tyrrell Museum of Palaeontology. accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Leptocleidids (Leptocleididae) Early Cretaceous Plesiosaurs The leptocleidids were a family of plesiosaurs that lived in parts of the Western Interior Sea during the early Cretaceous. Unlike the giant long-necked elasmosaurids, leptocleidids were generally smaller and more compact in build. They had relatively short necks and robust muscular bodies. One of the most unusual aspects of leptocleidids was their apparent adaptation to shallow coastal environments. Rather than living exclusively in open oceans, they may have moved through estuaries, bays, and nearshore waters. Their strong flippers and streamlined bodies would have allowed them to maneuver efficiently in these habitats while hunting fish and other smaller aquatic prey. Nichollssaura Temporal Range: 110 million years ago Geographic Range: Alberta Species: N. borealis Diet: Carnivorous, small fish, squid Nichollssaura was a member of the family leptocleididae, a more basal family of plesiosaurs than animals like the elasmosaurids. While some plesiosaur groups evolved into large, long-necked marine reptiles, Nichollssaura remained relatively compact at only about 9.5 feet long. Its body proportions also differed from those of the famous elasmosaurids. Nichollssaur a had a shorter neck and a proportionally larger head, giving it a more heavily-built appearance. Another characteristic that sets Nichollssaura apart is its geological age. These animals lived during the early part of the Cretaceous period, when the Western Interior Sea had yet to span across the entire North American continent as it would a few million years later. Nichollsaurua is also known from a rather complete fossil specimen containing around 160 gastroliths, or stomach stones, a very high number even for a Cretaceous plesiosaur, whose fossils are often found with associated gastroliths. Polycotylids (Polycotylidae) Polycotylids in the Western Interior Sea Polycotylid plesiosaurs were highly specialized marine reptiles that inhabited the Western Interior Sea during the Late Cretaceous. Unlike the classic long-necked plesiosaurs, they typically had short necks, large heads, long snouts and streamlined bodies that made them efficient swimmers and active predators. Their four powerful flippers provided them with a great deal of speed and maneuverability which would have allowed them to pursue fast-moving prey such as fish and cephalopods. Long narrow jaws lined with sharp, conical teeth helped them capture slippery animals in open-water environments. Fossils of important genera, including Dolichorhynchops and Polycotylus , have been found in the chalk deposits of Kansas and nearby regions. One of the most significant discoveries was a Polycotylus specimen containing an embryo, providing evidence that these reptiles gave birth to live young. This rare fossil added a lot to scientists’ understanding of plesiosaur reproduction and suggests that polycotylids were fully adapted to a marine lifestyle. Dolichorhynchops Temporal Range: 87 million years ago - 72 million years ago Geographic Range: Kansas, Saskatchewan, possible material from Russia Species: D. osborni, D. herschelensis Diet: Carnivorous, fish, squid Dolichorhynchops is probably the best known genus of polycotylid. The history of Dolichorhynchops research began around 1900, when George F. Sternberg discovered the holotype specimen in the Smoky Hill Chalk of Kansas. The fossil was collected with the assistance of his father, Charles H. Sternberg, and was later described and named by Samuel W. Williston in 1902. These animals were mid-sized members of the polycotylid family and examples of their fossils can be seen on display at the KU Natural History Museum and the Sternberg Museum of Natural History. (BELOW) A skeleton of Dolichorhynchops on display at the KU Natural History Museum. The skull on the mount is a model but the original skull is displayed next to the skeletal mount. See below for a more detailed image. (BELOW) The skull of Dolichorhynchops which is connected to the skeleton in the picture above, photo taken at the KU Natural History Museum. The skull has been somewhat compressed while it was fossilized within a layer of chalk. Edgarosaurus Temporal Range: 100 million years ago Geographic Range: Montana Species: E. muddi Diet: Carnivorous, fish, squid Edgarosaurus was one of the earliest known members of the polycotylid family. While it shared the characteristic short neck and large skull of polycotylids, its head and neck together accounted for an unusually large percentage of its overall body length. The fused front section of the lower jaw contained only six pairs of teeth, fewer than in many related polycotylids and its teeth were large, sturdy and fang-like. This suggests it would have had a feeding strategy different from that of some of its relatives, although the exact implications of these differences in dentition are not certain. Additional distinguishing features included a small pineal opening near the rear of the skull, a nearly vertical back of the cranium, and unusually short, broad bones in its forelimbs. Our understanding of Edgarosaurus comes from a single well-preserved specimen discovered in 1993 in the Thermopolis Shale of Montana. The fossil included a complete skull, a series of neck vertebrae, parts of the backbone, and most of one front paddle. After careful study, paleontologist Patrick Druckenmiller determined that the specimen represented a previously unknown genus and formally named Edgarosaurus muddi in 2002. The genus was named after the nearby town of Edgar, Montana. Because no additional specimens have been described, the holotype remains the primary source of information about this important early polycotylid. Martinectes Temporal Range: 80.5 million years ago - 78 million years ago Geographic Range: Wyoming Species: M. bonneri Diet: Carnivorous, fish, squid Martinectes was a large and distinctive polycotylid plesiosaur that differed from many of its relatives through its robust build and considerable size. Although it shared the short neck and elongated skull common among advanced polycotylids, it was larger than many related genera and possessed strong, interlocking conical teeth adapted for catching fish and other slippery prey. It was initially thought to have been a species of either Dolichorhynchops or Trinacromerum . The fossils of Martinectes have undergone a complex history of classification. Specimens from the Pierre Shale of Wyoming were first described as “Trinacromerum bonneri ”. Later research reassigned the species to Dolichorhynchops in 2008 after comparisons with other polycotylids. Continued study of the skull and skeleton revealed additional unique features eventually leading to the establishment of the new genus Martinectes in 2023. Pahasapasaurus Temporal Range: 90 million years ago Geographic Range: South Dakota Species: P. haasi, P. gillettei Diet: Carnivorous, fish, squid Pahasapasaurus was a polycotylid whose fossils were discovered in South Dakota, as well as a second species recognized from Utah. The scientific history of Pahasapasaurus traces back to 1934, when Charles Haas and his son Arthur uncovered a plesiosaur skeleton in the Black Hills of South Dakota while searching for shark teeth. The specimen was excavated and eventually donated to the local museum in the town of Deadwood. For many years, researchers identified the fossil as part of a different genus, Trinacromerum . However, renewed study during a restoration project in the 1990s revealed important anatomical differences. Paleontologists subsequently determined that the specimen represented a previously unknown genus and formally described Pahasapasaurus haasi in 2007. The name honors both the Black Hills region and the discoverer of the fossil, Charles Haas. Plesiopleurodon Temporal Range: 98 million years ago Geographic Range: Wyoming Species: P. wellesi Diet: Carnivorous, fish, squid, ocean reptiles Plesiopleurodon was a type of polycotylid which was originally thought to have been a part of the pliosaur group owing to its strange proportions which differed significantly from those of other polycotylids. The skulls of these animals were huge and their necks were very short. It also had fewer teeth in the front portion of the lower jaw than many related plesiosaurs. In addition, the skull preserved several ancestral features, including a robustly attached quadrate bone and a backward-sloping profile. These characteristics initially led paleontologists to note similarities with the Jurassic pliosaur Liopleurodo n although this has since been determined to be a case of convergent evolution. The scientific history of Plesiopleurodon is based on a very limited number of fossil remains. The genus was established by paleontologist Kenneth Carpenter in 1996 from a specimen discovered in Cretaceous-aged rocks of Wyoming. The material known from this animal includes a nearly complete skull and lower jaws, associated vertebrae, and a part of the shoulder elements. Polycotylus Temporal Range: 83.6 million years ago Geographic Range: Kansas, Alabama, Russia Species: P. latipinnis Diet: Carnivorous, fish, squid Polycotylus is among the most thoroughly studied of all polycotylids. While it shared many characteristics with other polycotylids, including a streamlined body, large skull, and strong flippers, Polycotylus had a greater number of neck vertebrae than many of its kin which is a trait associated with earlier long-necked plesiosaurs. It is also distinguished by its elongated hip bones, robust teeth marked by a unique banded texture, a narrow upper palate, and a low sagittal crest along the top of the skull behind the eyes. These characteristics allow paleontologists to differentiate Polycotylus from other members of its family. Scientific investigation of Polycotylus began in 1869 when Edward Drinker Cope described the genus from fragmentary remains discovered in the Niobrara Chalk of Kansas. Additional and more complete specimens found later helped researchers develop a clearer understanding of its anatomy and evolutionary relationships. One key discovery was that of a Polycotylus specimen preserving a fetus within the body cavity of an adult. The developing animal was quite large compared with the size of the parent, which would suggest a long gestation period similar to those of today’s whales. (BELOW) A photo of a Polycotylus skeleton preserved with the remains of a fetal skeleton in its abdominal cavity, marked with a white triangle. Fossil on display at the LA County Museum. Photo credit: Jonathan Chen. accessed via Wikimedia Commons, distributed under a CC BY-NC-SA 4.0 license Serpentisuchops Temporal Range: 70 million years ago Geographic Range: Wyoming Species: C. crassidens, C. gracilodens Diet: Carnivorous, fish, squid, Serpentisuchops was a polycotylid which appeared to combine aspects of long-necked plesiosaurs and other shorter-necked polycotylids. It had a very elongated neck and a long, narrow snout. Most advanced polycotylids evolved a body plan characterized by large heads, streamlined jaws, and shortened necks. In contrast Serpentisuchops retained a neck with roughly 32 cervical vertebrae while also possessing the elongated jaws typical of its relatives. Paleontologists have proposed that Serpentisuchops may have used rapid side-to-side strikes of its head and neck to capture fish and other agile marine animals. The fossil record of Serpentisuchops is centered on a single important specimen discovered in 1995 within the Pierre Shale of Wyoming. The fossil, designated GPM 5001, includes portions of the skull, lower jaws, vertebral column, and pelvic bones. Although the remains were collected decades earlier, they were not formally studied and named until 2022. Trinacromerum Temporal Range: 93 million years ago Geographic Range: Kansas, Manitoba, Alabama Species: T. bentonianum, T. kirki Diet: Carnivorous, small fish, squid Trinacromerum was a smaller member of the polycotylid family. It would have had an elongated, narrow snout equipped with numerous fine, needle-like teeth, which were ideal for seizing small fish. Although it shared the short-necked body plan typical of polycotylids, Trinacromerum appears to have been particularly well adapted for speed, thanks to its long, powerful flippers and streamlined build. These features could have possibly allowed it to pursue agile prey in open water rather than relying on ambush tactics. Its anatomy suggests that it occupied a niche as an active predator within the Western Interior Seaway, where quick movement and maneuverability would have been important advantages. Unktaheela Temporal Range: 80 million years ago Geographic Range: Wyoming, South Dakota Species: U. specta Diet: Carnivorous, fish, squid Unktaheela was a small polycotylid with an estimated length of only 8.5 feet, which means that it is considered the smallest known member of its family. Its skull had rather large eye sockets, prominent ridges surrounding the eyes, and broad bony shelves above the orbits that may have reduced glare from sunlight. These characteristics suggest that Unktaheela relied heavily on vision while hunting and may have specialized in pursuing prey in brightly lit surface waters. The scientific study of Unktaheela is relatively recent. Fossils from Wyoming and South Dakota were initially assigned to the genus Dolichorhynchops , and one specimen was described in 1996 as a juvenile individual of that same taxon. Later research demonstrated that these remains represented a distinct genus. In 2023 the genus Untkaheela was named based on two skulls and some additional associated material from other parts of the skeleton. Media Gallery Suggested References: Plesiosauria Angst, Delphine; Bardet, Nathalie (2016). "A new record of the pliosaur Brachauchenius lucasi Williston, 1903 (Reptilia: Sauropterygia) of Turonian (Late Cretaceous) age, Morocco". Geological Magazine. 153 Carpenter, K. (1996). “A review of the short-necked plesiosaurs from the Cretaceous of the Western Interior, North America”. Nues Jahrbuch für Geologie und Paläontologie, Abhandlungen, (Stuttgart), 201(2):259-287. Carpenter, K. (1999). "Revision of North American elasmosaurs from the Cretaceous of the western interior". Paludicola. 2 (2): 148–173. Everhart, M. J. (2002). “New data on plesiosaur remains found as stomach contents of a Tylosaurus proriger (Squamata; Mosasauridae) from the Niobrara Formation of western Kansas”. (Abstract) Transactions of the Kansas Academy of Science Everhart, M.J. (2004). “New data regarding the skull of Dolichorhynchops osborni (Plesiosauroidea: Polycotylidae) from rediscovered photos of the Harvard Museum of Comparative Zoology specimen”. Paludicola 4(3):74-80. Everhart, M.J. (2000). “Gastroliths associated with plesiosaur remains in the Sharon Springs Member of the Pierre Shale (late Cretaceous), Western Kansas”. Kansas Acad. Sci. Trans. 103(1-2):58-69. Everhart, M. J. (2005). “Bite marks on an elasmosaur (Sauropterygia; Plesiosauria) paddle from the Niobrara Chalk (Upper Cretaceous) as probable evidence of feeding by the lamniform shark, Cretoxyrhina mantelli”. PalArch, Vertebrate paleontology 2(2): 14-24. Fischer, V; Benson, R. B. J.; Druckenmiller, P. S.; Ketchum H. F.; Bardet N.(2018). "The evolutionary history of polycotylid plesiosaurians". Royal Society Open Science. 5 (3) 172177. Ketchum, H. F. & Benson, R. B. J. (2010). "Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylogenetic analyses". Biological Reviews. 85 (2): 361–392 Lucas, Spencer G.; Sullivan, Robert M. (2006). “Late Cretaceous Vertebrates from the Western Interior”: Bulletin 35. New Mexico Museum of Natural History and Science. Madzia, D; Sachs, S; Lindgren, J. (2018). "Morphological and phylogenetic aspects of the dentition of Megacephalosaurus eulerti, a pliosaurid from the Turonian of Kansas, USA, with remarks on the cranial anatomy of the taxon". Geological Magazine. 156 McIntosh, A. P.; Shimada, K.; Everhart, M. J. (2016). "Late Cretaceous marine vertebrate fauna from the Fairport Chalk Member of the Carlile Shale in southern Ellis County, Kansas, U.S.A." Transactions of the Kansas Academy of Science. O'Keefe, F. R.; Sander, P. M.; Wintrich, T.; Werning, S. (2019). "Ontogeny of Polycotylid Long Bone Microanatomy and Histology". Integrative Organismal Biology. 1 O'Keefe, F. R.; Chiappe, L. M. (2011). "Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia)". Science. 333 (6044): 870–873. Russell, D. A. (1988). “A check list of North American marine Cretaceous vertebrates including fresh water fishes”. Royal Tyrrell Museum Sato, Tamaki (2003). "Terminonatator ponteixensis, a new elasmosaur (Reptilia: Sauropterygia) from the Upper Cretaceous of Saskatchewan". Journal of Vertebrate Paleontology. 23 (1): 89–103. Schumacher, B. A., (2007), “A new polycotylid plesiosaur (Reptilia; Sauropterygia) from the Greenhorn Limestone (Upper Cretaceous; lower upper Cenomanian), Black Hills, South Dakota”: In: The Geology and Paleontology of the Late Cretaceous marine deposits of the Dakotas, The Geological Society of America, Special Paper 427, p. 133-146. Schumacher, B.A., Carpenter, K. and Everhart, M.J. (2013). “A new Cretaceous Pliosaurid (Reptilia, Plesiosauria) from the Carlile Shale (middle Turonian) of Russell County, Kansas”, Journal of Vertebrate Paleontology 33(3):613-628 Schumacher, B.A. (2008). “On the skull of a pliosaur (Plesiosauria; Pliosauridae) from the Upper Cretaceous (Early Turonian) of the North American Western Interior” Kansas Academy of Science, Transactions 111(3-4):203-218. Serratos, D.J.; Druckenmiller, P.; Benson, R.B.J. (2017). "A new elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana demonstrates multiple evolutionary reductions of neck length within Elasmosauridae". Smith, E.A.; O'Keefe, F.R. (2023). "Occurrence of Styxosaurus (Sauropterygia: Plesiosauria) in the Cenomanian: Implications for relationships of elasmosaurids of the Western Interior Seaway". Journal of Systematic Palaeontology. 21 Storrs, G.W. (1999). “An examination of Plesiosauria (Diapsida: Sauropterygia) from the Niobrara Chalk (upper Cretaceous) of central North America”. The University of Kansas Paleontological Contributions, (N. S.), No. 11, 15 pp. Taylor, M. A. (1993), “Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods”. Phil. Trans. Royal Society London. B 341:163-175. Zhao, R. J. (2026). "Body reconstruction and size estimation of plesiosaurs". PeerJ. 14

An illustrated guide to the types of ammonites whose fossils are found in the Cretaceous deposits formed by the the Western Interior Seaway Ammonoids Prehistoric Cephalopods with Coiled Shells Ammonoids were a diverse group of extinct marine cephalopods that were found all throughout the world’s oceans for millions of years before their extinction at the end of the Cretaceous Period. They are best known from fossils of their coiled external shells, which were filled with a series of gas-filled chambers separated by internal partitions known as septa. The intricate suture patterns created where the septa intersected the shell wall are one of the most distinctive features of ammonoids and are commonly used to distinguish between species. Although they shared similarities with modern squids and octopods, ammonoids had external shells that aided in flotation and protection. Some ammonoids could even grow to enormous sizes, with animals like Parapuzosia known to grow to be the size of a small car. Anatomy Internal Anatomy Ammonoids have shells whose internal structures are often well preserved in the fossil record. The soft body parts of the ammonoid, including most of the animal’s internal organs, were housed in the outermost portion of the shell known as the living chamber. The remaining chambers of the shell formed the phragmocone which was a structure responsible for buoyancy regulation. These internal shell chambers were separated by curved walls called septa and linked by a narrow tube called a siphuncle that controlled the movement of fluids and gases within the shell and allowed the ammonoid to control its ascent or descent as it moved through the water. Fossils of the soft tissues of ammonoids have only recently been found but based on these limited remains we know that they would have differed quite a bit from the soft tissue anatomy of nautiloids. The animal itself likely would have had a well-developed head with large complex eyes, a beak-like pair of jaws and a specialized feeding organ lined with tiny teeth called a radula, a feature found in most types of mollusks outside of bivalves. Surrounding the mouth was a set of 8-10 arms and tentacles that were probably used for capturing prey and handling food. This differs from the soft tissue anatomy seen in nautiloids, which tend to have simple eyes and dozens of smaller arms. Some fossils preserve the position of a digestive tract inside the body chamber of the ammonoid that would have included an esophagus, stomach, intestine, and anus. Beneath the stomach, esophagus and liver sat the mantle cavity, gills for respiration, muscles for locomotion, and reproductive organs. The mantle enclosed much of the body and was responsible for secreting the shell. Like their modern relatives, ammonoids most likely traveled through the ocean by jet propulsion. They would force water through a funnel-like structure called a hyponome to generate movement. External Anatomy Ammonoid shells could vary a lot between species and life stages. These animals had spirally coiled shells made up of successive whorls that increased in size during growth. The degree of coiling differed considerably between ammonoid families, with some shells being tightly wound so that only the outer whorl was visible, while others displayed a more open coil that revealed much of the inner shell. The shell surface was often decorated with ornamental features such as ribs, ridges, grooves, and growth lines. Many ammonoids also developed nodes, tubercles, or spines along the outsides of their shells which would create distinctive patterns that would have given them extra protection from predation and strengthened the shell structure. The aperture, or opening of the shell, represented another important external feature. Its shape varied among ammonoid groups and reflected differences in growth form and ecology. As the shell expanded throughout the animal’s life, growth lines marked the former positions of the aperture which left behind a record of shell development which we can still see in their fossils to this day. Another notable feature was the central area around which the shell coiled, known as the umbilicus. The size and shape of the umbilicus varied a lot between species and their exact shapes are commonly used in classification. Sutures Suture patterns are the best way to classify different groups of ammonoids at a glance. A suture represents the line where an internal chamber wall, or septum, joined the outer parts of the shell. Over millions of years of evolution, these patterns became progressively more elaborate, leading to the development of three principal suture types: goniatitic, ceratitic, and ammonitic. Because these patterns changed through time, they are widely used by paleontologists to classify ammonoids and correlate the ages of sedimentary rocks. The goniatitic suture is the least complex of the three forms and is typically seen in many early ammonoids. These sutures have smooth, undivided lobes and rounded saddles that create a relatively simple wavy or zigzag pattern along the shell. The ceratitic suture represents a more advanced condition. In this type, the saddles remain smooth, but the lobes are finely serrated, giving them a distinctive saw-toothed appearance. The most complex pattern and the only one found in Cretaceous ammonoids from the Interior Sea is the ammonitic suture, which is characteristic of many later ammonoids. In this form, both the lobes and saddles are deeply folded and intricately subdivided, producing highly ornate, branching patterns. The increasing complexity of these sutures may have given the shell extra strength and resistance to stress. Shell Shapes Ammonoid shells came in a lot of different shapes with involute, evolute, and criocone forms representing some of the most distinctive morphologies. These shell types are differentiated by the way the whorls are arranged and by the extent to which earlier coils remain visible. The degree of coiling is an important feature used by paleontologists to classify ammonoids and study patterns of evolutionary change. Involute shells are tightly coiled such that each successive whorl covers most of the preceding one. This overlap conceals the inner whorls, leaving only the outermost coil visible and producing a small, narrow umbilicus. As a result, involute shells have a compact and streamlined appearance. Evolute shells on the other hand, are more loosely coiled. Because the outer whorls do not completely cover the earlier ones much of the inner shell remains exposed. This produces a broad, open umbilicus and allows multiple whorls to be visible, giving the shell a less compact form. Criocone shells represent a more unusual coiling style. Unlike either involute or evolute forms, the whorls are completely detached and do not make contact with one another. The shell therefore forms an open spiral with spaces between successive coils. Criocone ammonoids are considered heteromorphic forms. (BELOW) In addition to the more standard evolute, involute and criocone shell shapes, some ammonite shells can be further classified based on more specific aspects of their spirals and external accessory features. Scaphicone shells, for instance, are almost exclusively found within the family scaphitidae and are known for having a unique lobed section near their apertures which overlaps the lower portion of the umbilicus. Other shell types include turricone shells, baculicone shells and toxocone shells. Most of these shell types can broadly be classified as heteromorph shells, in contrast to the more typical spiral-shaped monomorph shells seen in animals whose shells form the classic ammonoid spiral. (BELOW) Ammonoid shells don't just vary between species and growth stages. Some ammonoid families contain shell types which can be divided into microconch and macroconch varieties. The microconchs are typically much smaller than macroconchs and sometimes present unique features such as spur-like apophyses situated beneath their apertures. Microconch shells seem to be the shells of males while macroconch shells come from female ammonoids. Not all types of ammonoids have such a strong case of dimorphism and in some families of ammonoids the males and females are very similar in terms of general shape and size. Ammonoids (Browse by family) Family Acanthoceratidae Acanthoceras Burroceras Calycoceras Conlinoceras Cunningtoniceras Dunveganoceras Eucalycoceras Mammites Morrowites Neocardioceras Paraconlinoceras Plesiacanthoceras Plesiacanthaceratoides Pseudaspidoceras Spathites Tarrantoceras Watinoceras Family Anisoceratidae Allocrioceras Family Baculitidae Baculites Sciponoceras Family Binneyitidae Borissiakoceras Family Brancoceratidae Mortoniceras Oxytropidoceras Family Coilopoceratidae Coilopoceras Family Collignoniceratidae Collignoniceras Menabites Prionocyclus Texanites Family Desmoceratidae Moremanoceras Parapuzosia Family Dipoloceratidae Hystoceras Family Engonoceratidae Engonoceras Metengonoceras Family Forbesiceratidae Forbesiceras Family Gaudryceratidae Anagaudryceras Family Hamitidae Stomohamites Hamites Family Muniericeratidae Tragodesmoceras Family Nostoceratidae Cirroceras Didymoceras Exiteloceras Eubostrychoceras Family Pachydiscidae Eopachydiscus Pachydiscus Family Placenticeratidae Placenticeras Proplacenticeras Family Scaphitidae Clioscaphites Desmoscaphites Discoscaphites Haresiceras Hoploscaphites Jeletzkytes Scaphites Trachyscaphites Yezoites Family Schloenbachiidae Schloenbachia Family Sphenodiscidae Sphenodiscus Family Turrilitidae Mariella Ostlingoceras Turrilites Family Vascoceratidae Vascoceras Acanthoceratidae Temporal Range: 100 million years ago - 66 million years ago Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Acanthoceras, Burroceras, Calycoceras, Conlinoceras, Cunningtonoceras, Dunveganoceras, Eucalycoceras, Kanabiceras, Mammites, Metoicoceras, Morrowites, Neocardioceras, Paraconlinoceras, Plesiacanthoceras, Plesiacanthoceratoides, Pseudaspidoceras, Spathites, Tarrantoceras, Watinoceras Acanthoceratidae is a family of ammonites who were mostly restricted to the Late Cretaceous, with some material also possibly dating to the Albian stage. Their shells are often covered in a wide array of tubercles, ribs and/or rows of rounded bumps and knobs. Note that these are not universal features and some acanthoceratids had smoother shells, often with fairly simple sutures. Some acanthoceratid shells are preserved with large protruding spiny projections with rounded tips. Their shells were coiled in an open spiral which allowed the earlier whorls to remain visible as the animals grew. (ABOVE) The genus Acanthoceras from the Late Cretaceous, Cenomanian and Campanian stages, possible Maastrichtian record. Found in Texas, New Mexico, Wyoming, Colorado. Species: A. adkinsi, A. amphibolum, A. barcusi, A. bellense, A. cuspidum (ABOVE) The genus Burroceras from the Late Cretaceous, Cenomanian stage. Found in Colorado, Arizona. Species: B. clydense (ABOVE) The genus Calycoceras from the Late Cretaceous, Cenomanian and Turonian stages. Found in Kansas, Texas, New Mexico, Oregon?, Colorado, Wyoming. Species: C. canitaurinum, C. naviculare (ABOVE) The genus Conlinoceras from the Late Cretaceous, Cenomanian stage. Found in New Mexico, Texas. Species: C. tarrantense (ABOVE) The genus Cunningtoniceras from the Late Cretaceous, Cenomanian stage. Found in Texas. Species: C. inerme, C. johnsonanum, C. lonsdalei (ABOVE) The genus Dunveganoceras from the Late Cretaceous, Cenomanian-Coniacian stages. Found in Wyoming, Montana, Iowa, Kansas. Species: D. pondi (ABOVE) The genus Eucalycoceras from the Late Cretaceous, Cenomanian stage. Found in Texas, New Mexico, Colorado. Species: E. dentonense, E. gothicum, E. pentagonum, E. collignoni, E. templetonense, E. pseudobaylei (ABOVE) The genus Mammites from the Late Cretaceous, Cenomanian to Turonian stage. Found in New Mexico, Wyoming, Colorado, Kansas, Texas. Species: M. bellsanus, M. nodosoides (ABOVE) The genus Metoicoceras from the early and Late Cretaceous, Albian to Turonian stage. Found in New Mexico, Wyoming, Colorado, Kansas, Texas, Arizona, Montana, South Dakota. Species: M. crassiocostae, M. frontierense, M. geslinianum, M. latoventer, M. mosbyense, M. ornatum, M. praecox, M. swallovi, M. whitei (ABOVE) The genus Morrowites from the Late Cretaceous, Turonian stage. Found in New Mexico, Wyoming, Colorado, Kansas, Texas. Species: M. bellsanus, M. nodosoides (ABOVE) The genus Neocardioceras from the Late Cretaceous, Cenomanian stage. Found in Texas, New Mexico, Colorado, Arizona, Utah. Species: N. judii, N. densicostatum, N. uptonense, N. laevigatum, N. minutum, N. woodwardi (ABOVE) The genus Plesiacanthoceras from the Late Cretaceous, Cenomanian to Turonian stage. Found in Kansas, Wyoming, Texas, Montana, New Mexico. Species: P. bellsanum, P. wyomingense (ABOVE) The genus Plesiacanthoceratoides from the Late Cretaceous, Cenomanian stage. Found in Texas, Wyoming. Species: P. vetula (ABOVE) The genus Paraconlinoceras from the Late Cretaceous, Cenomanian stage. Found in Texas, Wyoming, Colorado. Species: P. leonensis, P. barcusi (ABOVE) The genus Pseudaspidoceras from the Late Cretaceous, Cenomanian to Turonian stage. Found in New Mexico, Texas. Species: P. footeanum, P. paganum, P. pseudonodosoides, P. flexuosum, P. reesidei, P. armatum, P. cornucostale (ABOVE) The genus Spathites from the Late Cretaceous, Turonian stage. Found in New Mexico. Species: S. rioensis, S. coahuilaensis, S. puercoensis, S. obliquus, S. combesi (ABOVE) The genus Tarrantoceras from the Late Cretaceous, Cenomanian to Turonian stage. Found in New Mexico, Colorado, Texas, Wyoming. Species: T. cuspidum, T. exile, T. multicostatum, T. sellardsi, T. proteus (ABOVE) The genus Watinoceras from the Late Cretaceous, Cenomanian to Turonian stage. Found in New Mexico, Texas, Colorado, Kansas, Minnesota, South Dakota. Species: W. coloradensis, W. hattini, W. reesidei, W. amudariense Gallery: Acanthoceratidae - click to expand images Anisoceratidae Temporal Range: ~110 million years ago - 90 million years ago Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Allocrioceras A family of ammonites whose shells do not form a full spiral. Bumps and tubercles are commonly found on the sides of their shells, some genera also have these tubercles lower down towards the ventral portions of their shells. Sutures are usually simple. Most genera also have shells with tight or loose ribs. Usually thin in cross-section. (ABOVE) The genus Allocrioceras from the Late Cretaceous, Turonian - Coniacian stages. Found in Wyoming, Texas, Utah, Colorado. Species: A. angustum, A. hazzardi, A. larvatum, A. pariense, A. cuvieri Gallery: Anisoceratidae - click to expand images Baculitidae Temporal Range: ~110 million years ago - 66 million years ago Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Baculites, Sciponoceras A family of heteromorph ammonites, usually with mostly straight shells. Most genera have a small spiraling section composed for the first 1-2 whorls at the tips of their shells. Immature individuals can have fully spiral-shaped shells. Shells are usually smooth but some genera have repeating ridges, wave-like patterns or tubercles on the lateral surfaces of their shells. They are usually oval-shaped in cross-section. (ABOVE) The genus Baculites from the Late Cretaceous, Cenomanian - Maastrichtian stages. Found in Wyoming, Texas, Utah, Colorado, Montana, Kansas, Mississippi, New Jersey, Tennessee, Arkansas. Species: B. anceps, B. vertebralis, B. compressus, B. bailyi, B. asper, B. asperformis, B. baculus, B. calamus, B. claviformis, B. clinolobatus, B. codyensis, B. cuneatus, B. grandis, B. meeki, B. jenseni, B. haresi, B. ovatus (ABOVE) The genus Sciponoceras from the Late Cretaceous, Cenomanian - Turonian stages. Found in Wyoming, Texas, New Mexico, Kansas, Colorado. Species: S. gracile, S. baculoides, S. cucullatum Gallery: Baculitidae - click to expand images Binneyitidae Temporal Range: Albian stage - Santonian stage Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Borissiakoceras A family of ammonites with somewhat involute spiral shells. Their shells do not have any ridges or tubercles. Very thin shells in cross-section. (ABOVE) The genus Borissiakoceras from the Late Cretaceous, Cenomanian - Turonian stages. Found in Wyoming, Texas, Kansas, Colorado. Species: B. reesidi, B. auriculatum, B. desmoceratoides, B. inconstans Brancoceratidae Temporal Range: Albian stage - Cenomanian stage Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Mortoniceras, Oxytropidoceras Family of ammonites with evolute spiral shells. Shells usually have ribs, some have tubercles along the inner or outer edges of their spirals. Their cross sections vary from rectangular to oval-shaped depending on the genus but their shells are usually fairly wide. (ABOVE) The genus Mortoniceras from the Early Cretaceous, Albian stage. Images are of a microconch. Found in Texas. Species: M. albense, M. australis, M. barbouri, M. bondanti. M. texanum, M. wintoni (ABOVE) The genus Oxytropidoceras from the Early Cretaceous, Albian stage. Found in Texas, Oklahoma. Species: O. howei, O. carbonarium, O. hubbardi, O. acutocarinatum Gallery: Brancoceratidae - click to expand images Coilopoceratidae Temporal Range: Albian stage - Turonian stage Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Coilopoceras Family of ammonites with involute shells, usually fairly tall. The shells of some genera change shape as they age, immature individuals have flatter sides than adults. Unique sutures with a narrow first saddle and wide first lobe. Sutures are usually fairly narrow. (ABOVE) The genus Coilopoceras from the early and Late Cretaceous, Albian - Turonian stages. Found in Texas, New Mexico, Colorado. Species: C. colleti, C. springeri, C. inflatum Collignoniceratidae Temporal Range: Turonian stage - Campanian stage Diet: Active predator, fish, invertebrates Genera in the Western Interior Sea: Collignoniceras, Menabites, Prionocyclus, Texanites Family of ammonites with involute spiral shells with often very pronounced tubercles on their sides. Their cross sections are rectangular or oval-shaped. Shells are taller than they are wide. (ABOVE) The genus Collignoniceras from the Late Cretaceous, Turonian stage. Found in Texas, Kansas, South Dakota, Nebraska, Wyoming, New Mexico, Colorado. Species: C. casperi, C. hiltensis, C. woolgari, C. praecox, C. hyatti, C. caurinus, (ABOVE) The genus Menabites from the Late Cretaceous. Found in Texas. Species: M. belli, M. campaniense, M. walnutensis (ABOVE) The genus Prionocyclus from the Late Cretaceous, Turonian stage - Santonian. Found in Texas New Mexico, Kansas, Wyoming, South Dakota. Species: P. hyatti, P. macombi, P. wyomingensis (ABOVE) The genus Oxytropidoceras from the Late Cretaceous, Campanian stage. Found in Texas, Mississippi. Species: T. americanus, T. soutoni, T. lonsdalei, T. texanus, T. shiloensis Gallery: Collignoniceratidae - click to expand images See the rest of our entries about Cretaceous ammonoids by continuing to the next page Next Page Suggested References: Ammonoids Brinster, K. F. (1970). “Molluscan Paleontology of the Pierre Shale (Upper Cretaceous), Bowman County, North Dakota (MS).” University of North Dakota. Chirat, R., Goriely, A. & Moulton, D., E. (2021) “The physical basis of mollusk shell chiral coiling” in Proceedings of the National Academy of Sciences of the United States of America - Vol. 118, No 48 - National Academy of Sciences Clark, D., L. (1965) “Heteromorph Ammonoids from the Albian and the Cenomanian of Texas and Adjacent Areas” in Memoirs of Geological Society of America: 95 - The Geological Society of America, Inc. Digital Atlas of Ancient Life. “Ammonoidea”. Cretaceous Atlas of Ancient Life | Ammonoidea. https://www.cretaceousatlas.org/orders/ammonoidea/ Everhart, M.J. (2017). “Oceans of Kansas - A Natural History of the Western Interior Sea”, Second Edition. Indiana University Press, 460 pp. Furnish, W. M., Zuren, Z., & Glenister, B. F. (2009). Part L, Mollusca 4 (Revised), vol. 4, Complete Volume. Treatise on Invertebrate Paleontology. Hoffmann, René; Slattery, Joshua S.; Kruta, Isabelle; Linzmeier, Benjamin J.; Lemanis, Robert E.; Mironenko, Aleksandr; Goolaerts, Stijn; De Baets, Kenneth; Peterman, David J.; Klug, Christian (2021). "Recent advances in heteromorph ammonoid palaeobiology". Biological Reviews. 96 (2): 576–610. Ifrim, C. & Stinnesbeck, W. (2010) “Migration pathways of the late Campanian and Maastrichtian shallow facies ammonite Sphenodiscus in North America” Palaeo (Palaeogeography, Palaeoclimatology, Palaeoecology): 292 Inoue, S., Kondo, S. (2016). “Suture pattern formation in ammonites and the unknown rear mantle structure”. Sci Rep 6, 33689. Kauffman, E.G. (2004). “Mosasaur predation on Upper Cretaceous nautiloids and ammonites from the United States Pacific Coast.” Palaios 19(1):96-100. Kennedy, W.J., Landman, N.H., Cobban, W.A. and Scott, G.R. (2000). “Late Campanian (Cretaceous) heteromorph ammonites from the Western Interior of the United States”. Bulletin of the American Museum of Natural History 251, 88 pp Klug, C., Schweigert, G., Lauer, R. et al. (2025) “Reproductive biology and anatomy of ammonites”. Sci Rep 15, 39621. Kruta, I.; Landman, N.; Rouget, I.; Cecca, F.; Tafforeau, P. (2011). "The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation". Science. 331 (6013): 70–72. Kulicki, C., Tanabe, K. and Landman, N.H. (2007). Primary structure of the connecting ring of ammonoids and its preservation. Acta Palaeontologica Polonica 52(4):823-827. Musick, Glenn (2008). "Utah Ammolite". Rock & Gem. Vol. 38, no. 8. pp. 34–38. Neal L. Larson; Steven D. Jorgensen; Robert A. Farrar & Peter L. Larson (1997).” Ammonites and the Other Cephalopods of the Pierre Seaway.” Geoscience Press, Inc. p. 44 Stephenson, L.W., & Stenzel, H.B. (1952). Larger invertebrate fossils of the Woodbine formation (Cenomanian) of Texas, with Decapod Crustaceans from the Woodbine formation of Texas. Ward, P., D., Haggart, J., W., Mitchell, R., Kirschvink, J., L. & Tobin, T. (2012) “Integration of macrofossil biostratigraphy and magnetostratigraphy for the Pacific Coast Upper Cretaceous (Campanian-Maastrichtian) of North America and implications for correlation with the Western Interior and Tethys.” Geological Society of America Bulletin - v. 124, no 5/6 - The Geological Society of America Westermann, G. E. G. (1996). “Ammonoid life and habitat. In N. H. Landman, K. Tanabe, and R. A. Davis (editors), Ammonoid Paleobiology, pp. 607–707. New York: Plenum Press. Westermann, Gerd E. G. (1996), "Ammonoid Life and Habitat", in Landman, Neil H.; Tanabe, Kazushige; Davis, Richard Arnold (eds.), Ammonoid Paleobiology, Topics in Geobiology, vol. 13, Boston, MA: Springer US, pp. 607–707 Wright, C. W., (1996). Ammonoidea, in Kaesler, R. L., ed., Treatise on Invertebrate Paleontology, Part L, Mollusca 4, Volume 4. The University of Kansas and Geological Society of America. 362 pp.