Region 4: The Great Plains

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.

Mesozoic

Significant fossil-bearing rocks dating to the Triassic can be found in the valleys of north central Texas, but they are better exposed in the Central Lowland region and are therefore discussed in the preceding section. Similarly, the Jurassic is not well-represented in this region, but 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 (Figure 3.44).

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 swamps. These swamps 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 (Figure 3.45). The flat floor of this inland sea provided the basis for the modern topography of the Interior Plains.

Figure 3.45: The Western Interior Seaway.

The Western Interior Seaway was home to numerous open-water animals, which are represented by the abundant and beautifully preserved fossils found in the Niobrara Chalk of Kansas. These animals included mosasaurs (Figure 3.46), plesiosaurs (Figure 3.47), giant turtles, sharks, and other fishes, including the enormous Xiphactinus (Figure 3.48). The diverse invertebrate fauna includes ammonoids, crinoids, echinoids, bivalves (especially inoceramids and rudists), and gastropods (Figures 3.49 - 3.51). Vertebrates in the Cretaceous deposits of Kansas also include pterosaurs and birds (Figures 3.52 and 3.53). Interestingly, coprolites—fossilized feces (Figure 3.54)—are also fairly common.

Figure 3.54: Coprolites from the Cretaceous Smoky Hill Chalk of Kansas. About 2 centimeters (0.8 inch) in diameter.

Most Cretaceous fossils in western Kansas are found in chalk, a carbonate rock made up primarily of the fossils of microscopic marine algae, called coccolithophores (Figure 3.55). Today, such sediments accumulate mainly in the deep sea, but during the Cretaceous, when sea levels were much higher than today, chalk accumulated in shallow (100 - 300 meter [328 - 984 foot]) inland seas in both North America and Europe. 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 Smoky Hill Chalk Member of the Niobrara Chalk Formation is famous for its spectacularly preserved marine vertebrates, including mosasaurs, plesiosaurs, fish, pterosaurs, and birds. The few benthic organisms that were able to tolerate the low oxygen levels include the stalkless crinoid Uintacrinus (Figure 3.56) as well as rudist and inoceramid bivalves (Figures 3.57 and 3.58).

In Texas, extensive Cretaceous outcrops are preserved in the Edwards Plateau. There, limestone units produce a host of marine invertebrate fossils. 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 (Figure 3.59 and 3.60). 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 (Figure 3.61), one of the largest known carnivorous dinosaurs. Skeletal remains of Acrocanthosaurus have been found in early Cretaceous rocks in Oklahoma and Texas, and also Wyoming.

Figure 3.55: Microscopic view of chalk, showing that it is composed almost completely of the shells of protists called coccolithophores. Scale bar = 4 nanometers (4 x 10-9 meters; about 0.0000001575 inches).

Figure 3.58: Rudists were unusual cone- or cylinder-shaped bivalves that clustered together in reef-like structures and went extinct at the end of the Mesozoic era. They ranged in size from a few centimeters to more than 50 centimeters (1.5 feet) tall.

Figure 3.59: Sites in Texas where Cretaceous dinosaur footprints are known.

Cenozoic

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 (Figure 3.62). (Interestingly, it was Charles Darwin who first realized that horses had originally evolved in the Americas, spread throughout the world, became extinct in the New World, and were finally reintroduced by the Spanish.) Pleistocene deposits of central Texas have yielded the bones of sloths and glyptodonts, among other forms.