Fossils of the Great Plains

Mesozoic fossils

The Great Plains of Texas, Oklahoma, and Kansas are dominated by mid to late Mesozoic- and Cenozoic-aged rocks. Little of the bedrock in the region is older than 145 million years.

Triassic and Jurassic

The Triassic rocks of Texas crop out along the border between the Great Plains and Central Lowland regions. A variety of early dinosaurs are found in here, along with phytosaurs (extinct animals that look like crocodiles, although the two groups are not related) and synapsids (mammal-like reptiles). Triassic fossils from this region are covered on the Fossils of the Central Lowland and Interior Highlands page.

The Jurassic is not well represented in this region. However, the western panhandle of Oklahoma includes a small but important exposure of terrestrial shales and sandstones, which contain turtles, crocodiles, freshwater fish, and Oklahoma’s state fossil, the large allosaur Saurophaganax.

Cretaceous

Cretaceous fossils of Kansas

The Cretaceous rocks of Kansas contain abundant remains of late Mesozoic marine life, including some of the best fossils in the world from this time period. Fossils of marine invertebrates indicate that a warm, shallow sea covered the western part of the state, while terrestrial plant fossils and coal seams in central Kansas represent near-shore wetlands. These wetlands mark the shore of the great Western Interior Seaway, which divided North America into two landmasses as it extended from the Gulf of Mexico to the Arctic Ocean. The flat floor of this inland sea provided the basis for the modern topography of the Interior Plains.

Some spectacular fossils, notably ammonites, occur in the Late Cretaceous Carlile Shale that occurs in a northeast-to-southwest oriented exposure in central Kansas. The most well-known Cretaceous fossils, however, are found in western Kansas chalk, a carbonate rock made up primarily of the fossils of microscopic marine algae called coccolithophores. Today, chalk accumulates mainly in the deep sea. During the Cretaceous, when sea levels were much higher than today, chalk accumulated in shallow inland seas in both North America and Europe; these shallow seas were only about 100–300 meters (328–984 feet) deep. The Cretaceous period is named for the abundance of chalk that accumulated during this time (the Latin word for chalk is creta).

In the Western Interior Seaway, chalk formed in marine environments with relatively little wave or current energy, and on seafloors where dissolved oxygen concentrations were low. This led to conditions that were not particularly favorable for bottom-living organisms, but that were exceptionally good for preserving whatever died there.

The Niobrara Chalk Formation of Kansas—especially its Smoky Hill Chalk Member (a member is a subdivision of a formation)—is famous for its spectacularly preserved marine vertebrates, including mosasaurs, plesiosaurs, giant turtles, sharks, and other fishes (such as the enormous Xiphactinus). It also preserves pterosaurs (an extinct group of flying reptiles), and diving birds. Interestingly, coprolites (fossilized feces) are also fairly common.

The few benthic (seafloor-dwelling) organisms that were able to tolerate the low oxygen levels include the stalkless crinoid Uintacrinus as well as rudist and inoceramid bivalves. Other invertebrates include ammonoids and gastropods. 

Cretaceous fossils of Oklahoma and Texas

In Texas, extensive Cretaceous outcrops are preserved in the Edwards Plateau. There, limestone units produce a host of marine invertebrate fossils. Ammonoids, gastropods, bivalves (including reef-forming rudists), echinoderms, corals, and petrified wood (in layers formed on or near land) may also be found. Rare fossil fish, dinosaur, pterosaur, and various marine reptile bones have been found along the Great Plains side of the Balcones Escarpment.

The most famous fossil remains in this area, however, are dinosaur trackways found in more than a dozen locations on and north of the Edwards Plateau. The variety of footprints is attributed to theropods, ornithiscians, and sauropods, though the precise species are impossible to know. Some of the theropod footprints may have been made by Acrocanthosaurus, one of the largest known carnivorous dinosaurs. Skeletal remains of Acrocanthosaurus have been found in Lower Cretaceous rocks in Oklahoma, Texas, and Wyoming (see a picture of an Acrocanthosaurus skeleton on the South-central U.S.: Fossils of the Coastal Plain page).

Cenozoic fossils

In the Late Cretaceous, the Western Interior Seaway retreated and the Rocky Mountains rose to the west; streams carried gravel, sand, and silt that eroded from these newly formed mountains. The sediments filled wide, shallow valleys to create a broad, gently dipping plain that contains a variety of terrestrial and freshwater fossils, including gastropods, clams, algae, and the occasional plant.

By the middle Eocene, the environment became wetter and cooler. Mammals were the dominant animals on land, including a mix of extinct, extirpated, and extant animals that would look out of place in North America today. The Miocene-Pliocene Ogallala Group of Kansas and Oklahoma has produced abundant remains of rodents, horses, and land tortoises, among others.

Pleistocene deposits of central Texas have yielded the bones of sloths and glyptodonts, among other forms.