The Great Plains of eastern Colorado, northern New Mexico, Oklahoma, Kansas, and northern Texas are dominated by Mesozoic- and Cenozoic-aged rocks. In the southeastern corner of New Mexico, however, is an extension of the Permian Basin. During the Permian period, this area contained a marine environment. Today, it is home to the largest, best preserved, most accessible, and most intensively studied Paleozoic reef in the world—the Capitan Reef (Figure 3.67)—which stood about 600 meters (2000 feet) above the adjacent deep sea basin. Many of the region’s reef fossils are silicified; that is, they were preserved through replacement of their original calcium carbonate (CaCO3) shells by silica (SiO2). The late Permian rocks of westernmost Texas and southeasternmost New Mexico contain almost 1000 species of brachiopods (Figure 3.68), sponges, gastropods, foraminifera, algae, and other marine organisms.
The Cretaceous rocks of Colorado produce an abundance and diversity of marine fossils from the end of the Mesozoic. Fossils of ammonoids, bivalves, fish, and marine reptiles document the life of the shallow sea (Figure 3.69; see also Figures 3.38 and 3.39). Along the sea’s margins to the west, dinosaur footprints and flowering plants are preserved in what were once forested and swampy coastal areas. To the east, the fossils of open-water animals, such as mosasaurs, plesiosaurs, turtles, sharks, and other fishes—including the massive Xiphactinus—are preserved. In addition to strictly aquatic animals, the Cretaceous deposits of Colorado also preserve the remains of pterosaurs and toothed birds. These fossils are generally preserved in deposits of chalk (which is itself formed mostly from the shells of single-celled plankton) that accumulated at the bottom of the sea 80 to 90 million years ago.
Figure 3.67: Permian reefs of southeastern New Mexico and western Texas. This map illustrates the paleogeographic relationship of the reefs to the topographic lows and highs of the shallow sea that covered the area during the Permian. The reef occupied the rim of the Delaware Basin, an extension of the Permian Basin.
Following the retreat of the Western Interior Seaway and the rise of the Rocky Mountains to the west, the Paleogene-era Great Plains saw deposition by streams carrying gravel, sand, and silt eroding off the Rockies to the west. Wide, shallow valleys were filled with sediment, creating a broad, gently dipping plain. Tropical forests (including the world’s oldest known tropical rainforest at Castle Rock, Colorado; see Figure 3.62) growing along the margins of the new Rockies were home to mammals, crocodiles, and turtles. Fossils of fishes, plants, and insects are preserved within the lake deposits. Later in the Cenozoic, during the Miocene to Pliocene epochs, the climate cooled and dried, and this led to major changes in the region’s fauna. For example, the Ogallala Formation of New Mexico contains abundant fossils of primitive horses and land tortoises (Figure 3.70), as well as mammal footprints.
Figure 3.68: Capitan reef fossil brachiopods. A) Prorichtofenia permiana, approximately 6 centimeters (2.5 inches) tall. B) Ametoria residua, 3.7 centimeters (1.5 inches) wide. C) Grandaurispina sp., 8.5 centimeters (3.3 inches) wide. D) Collematra gregaria, cluster of large pedicle valves, 15 centimeters (6 inches) long. E) Collematra elongata, pedicle valve, 6.6 centimeters (2.6 inches) long. F) Penicularis subcostata, 5.7 centimeters (2.25 inches) wide. G) Bathymyonia sp., 4.6 centimeters (1.8 inches) wide.
Tortoises are a group of turtles that live on land, and have short, strong legs used for support and digging burrows. In contrast, most turtles live in the water and have webbed feet to help them swim efficiently, but will venture onto land occasionally to lay eggs.
Figure 3.69: Skeleton of the giant Cretaceous fish Xiphactinus, about 5 meters (16 feet) long.
Figure 3.70: Tortoise, Geochelone sp., about 1 meter (3 feet) long.
