Misc. Invertebrates
Corals, Bryozoans, Brachiopods, Scaphopods and Sponges
In addition to the more common remains of bivalves, cephalopods, gastropods and sea urchins, rare fossils of sponges, stony corals, bryozoans and brachiopods are sometimes recovered from deposits formed by the Western Interior Sea. These animals seem to have fared poorly in the lime-rich mud which coated the floor of the seaway. Notably this ancient body of water preserves no remains of large stony coral-based reef systems like the ones we are used to seeing in today’s tropical seas. Most of the reefs built in this ecosystem were formed by bivalves like rudists or oysters and the corals that we do find in these areas are usually considered solitary corals. Nevertheless many of these types of invertebrates found as fossils formed by the Western Interior Sea are part of genera which still contain extant species. Many of them seem to have changed very little in the intervening years, which indicates that by the Late Cretaceous many of these animals had already moved into niches similar to those occupied by their modern relatives.
Brachiopods
Temporal Range: ~530 million years ago - Present
Diet: Filter feeders: plankton, organic waste
Genera in the Western Interior Sea: Terebratulida, Lingula
Brachiopods are hard-shelled animals who appear outwardly similar to bivalves, despite being very distantly related to one another. These animals have a soft inner body sealed within two hard valves which are hinged at their base and often supported by a type of fleshy tube called a pedicle. Brachiopods are generally split into two groups based on the form of their hinges into articulate brachiopods and inarticulate brachiopods. Articulate brachiopods like Terebratulida have small tooth-like structures near their hinges and usually have a small opening in their lower valve from which the pedicle can extend. Inarticulate brachiopods like Lingula on the other hand have no tooth structures on their hinges. Their pedicles usually come out from the apex point of their shells.
(ABOVE) Brachiopod genera from the Western Interior Sea: (A) Lingula, (B) Terebratulida
Bryozoans
Temporal Range: ~518 million years ago - Present
Diet: Filter feeders: plankton, organic waste
Genera in the Western Interior Sea: Membranipora, Pyripora
Bryozoans are a group of typically colonial animals whose forms are composed of many small individual organisms called zooids. These zooids are specialized to perform specific functions used to keep the rest of the colony alive. This can include helping to expel waste or catching food. Fossil bryozoans found in rocks from the Western Interior Sea come in a few different shapes, with some resembling fans or pieces of netting while others look like small chains. Many of these Cretaceous bryozoa would attach themselves to rocks or to the soft substrate of the seafloor.
(ABOVE) Genera of bryozoans known from the Western Interior Sea, (A) Membranipora, (B) Pyripora
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Temporal Range: ~340 million years ago - Present
Tusk Shells (Scaphopoda)
Diet: Detritivore: organic waste, benthic microinvertebrates
Genera in the Western Interior Sea: Cadulus, Dentalium
Scaphopods are a class of mollusks whose fossil record dates back to the early Carboniferous period. They are an exclusively marine group and their fossils do not appear in depositional environments formed by areas covered in freshwater. These animals spend their lives searching for small pieces of food by sifting through the sediment on the seafloor with their thin hairlike tentacles. Their closest relatives on the mollusk family tree seem to be the cephalopods.
Sponges (Porifera)
Temporal Range: 544 million years ago - Present
Diet: Filter feeders: plankton, organic waste, some genera eat larger prey
Genera in the Western Interior Sea: Actinostromaria, Cliona
Sponges are some of the oldest groups of animals with their origins possibly going back to even before the start of the Cambrian period. They use a special set of choanocyte cells to circulate water through their bodies and use this process in order to strain most of their food out of the water. Some sponges who live in nutrient poor environments can become carnivorous but most are filter feeders. They tend to be mostly stationary animals although some types of mature sponges are able to move over the seafloor at very slow speeds. Their fossils are fairly rare in deposits from the Western Interior Sea. Most of the sponges found in these Cretaceous rocks are the fossils of burrowing or boring sponges like those in the genus Cliona.
(ABOVE) Genera of sponges known from the Western Interior Sea, (A) Actinostromaria, (B) Cliona
Stony Corals (Scleractinia)
Temporal Range: 240 million years ago - Present
Diet: Filter feeders: plankton, organic waste
Genera in the Western Interior Sea: Thamnasteria, Astrangia
Stony scleractinian corals are some of the primary reef building organisms found in today’s oceans. They are usually colonial animals who form from an original polyp which begins to form the rest of the mass of a larger piece of coral by replicating itself asexually. This process is called budding. As the coral grows the polyps begin to secrete compounds which form into a hard skeletal structure composed of the mineral calcium carbonate. These mineralized skeletons tend to fossilize easily and so the fossil record for stony corals, which began to appear in the middle Triassic, is fairly complete. However, during the Cretaceous period stony corals were replaced as reef builders in some parts of the world by rudist bivalves. Fossils of these stony corals are fairly rare in the Western Interior Sea and are mostly known from deposits left behind by nearshore environments like those of the Woodbine formation in Texas.
(ABOVE) Genera of stony corals known from the Western Interior Sea, (A) Thamnasteria (B) Astrangia
Suggested References: Misc. Invertebrates
Bayer, F. M., et al. (1956). “Systematic Descriptions, in Moore, R. C., eds., Treatise on Invertebrate Paleontology, Part F, Coelenterata, Volume 1.” The University of Kansas and Geological Society of America. F16-F20 pp.
Emig, Christian C. (May 2003). "Proof that Lingula (Brachiopoda) is not a living-fossil, and emended diagnoses of the Family Lingulidae". Carnets de Géologie / Notebooks on Geology. CG2003 (L01): 1–8.
Hooper, J. (2018). "Structure of Sponges". Queensland Museum.
"WoRMS - World Register of Marine Species - Cliona Grant, 1826"
Moore, R.C. (1965). Brachiopoda. Treatise on Invertebrate Paleontology. Vol. Part H., Volume 1. Boulder, Colorado/Lawrence, Kansas: Geological Society of America/University of Kansas Press. pp. H263.
WoRMS - World Register of Marine Species - Scleractinia".