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.
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
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
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.
