Fossils of the Great Plains

Paleozoic fossils

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. The Capitan Reef stood about 600 meters (2000 feet) above the adjacent deep sea basin.

Like modern reefs, this reef complex was built from the shells of marine invertebrates. Most of the shells were made of calcium carbonate (CaCO₃), which has the same composition as limestone. After the reef was buried by sediment, much of its calcium carbonate was replaced by other minerals dissolved in groundwater. The most important of these minerals was silica (SiO₂), which has the same composition as glass. The resulting “glass” fossils can be freed from their surrounding limestone by treating the rock with acid, which dissolves the rock but leaves the delicate fossils behind. The late Permian rocks of westernmost Texas and southeastern-most New Mexico contain almost 1000 species of brachiopods, sponges, gastropods, foraminifera, algae, and other marine organisms.

For more images of fossils from the Permian reef complex in the southeastern New Mexico-western Texas region, see the Earth Science of the South-Central United States: Fossils of the Basin and Range page, which shows a selection of fossils from western Texas.

Mesozoic fossils

Triassic fossils

Triassic fossils are known from the Bull Canyon and Redonda formations (Chinle Group or Dockum Group) of east-central New Mexico. Fossils from eastern New Mexico are from groups of animals that also occur in the Triassic Chinle Formation of the Colorado Plateau and the Triassic (Chinle Group or Dockum Group) of the Texas Panhandle region. These include ray-finned and lobe-finned fishes, amphibians, and various types of reptiles, like armored aetosaurs and crocodile-like phytosaurs.

Jurassic fossils

Dinosaurs are also well represented across much of the Southwest through fossil footprints. The Purgatoire River tracksite, also called the Picketwire Canyonlands tracksite, is one of the largest dinosaur tracksites in North America. The tracks occur in the widespread Morrison Formation. The site is located in the Comanche National Grassland along the Purgatoire River south of La Junta, Otero County, Colorado.

Cretaceous fossils

Cretaceous fossils of the Western Interior Seaway

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. 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.

For more images of fossils from the Wester Interior Seaway in the southeastern eastern Colorado-western Kansas region, see the Earth Science of the South-Central United States: Fossils of the Great Plains page, which shows a selection of fossils from western Kansas.

Latest Cretaceous terrestrial fossils

The Raton and Vermejo formations of northern New Mexico and southern Colorado preserve an abundant fossil record from the latest Cretaceous period. The Raton Formation is the source of what is widely accepted as the only known footprint of Tyrannosaurus rex. The Raton Basin is also an area where the K-Pg boundary is preserved. One notable place where it can be seen is at Trinidad Lake near Trinidad, Colorado.

Cenozoic fossils

Paleogene fossils

Paleocene Castle Rock flora

At Castle Rock, on the margin of the Denver Basin, is a very high-diversity assemblage of fossil leaves. The Castle Rock plant fossils appear to represent the oldest known tropical rainforest, in existence just 1.4 million years after the end-Cretaceous mass extinction.

Eocene to Oligocene fossils

In the northeastern part of Colorado, exposures of the White River Group, which is probably best known from the White River Badlands of South Dakota, yield some fossil vertebrates that are late Eocene to Oligocene in age. These include rhinoceroses, entelodonts ("hell pigs"), and birds.

Quaternary fossils

Like other places across the United States, the Great Plains region of the southwestern U.S. yields fossils of Pleistocene ("Ice Age") animals like mammoths. Several well-known late Pleistocene to early Holocene archaeological sites in North America that include large mammal remains are in eastern New Mexico. (Archaeology differs from paleontology in that archaeologists study evidence of ancient humans and their cultures, whereas paleontologists study remains and traces of ancient organisms.)

The Folsom site was first discovered in 1908 by George McJunkin, a cowboy and amateur naturalist. McJunkin recognized that bones eroding out of a bank were those of an extinct bison, which was later identified as Bison antiquus. In the 1920s, a team from the natural history museum in Denver, Colorado, traveled to the Folsom Site to retrieve the bison bones for an exhibit. While excavating, they discovered stone spear points among the bones.

The Blackwater Draw Site is located near Clovis, New Mexico. It was first discovered and excavated in the late 1920s to early 1930s. During excavation, spear points were found among mammoth bones. Similar spear points were also found with mammoth bones at the Dent Site in Weld County, Colorado, at about the same time (for a photo of the mammoth skeleton from the Dent Site, see the header image on this page).

When they were discovered, these sites indicate that humans had inhabited North America for far longer than previously thought. Folsom and Clovis are now names for the styles of spear point found at the Folsom and Blackwater Draw (Clovis) sites, respectively. These types of spear points have also been discovered elsewhere in North America. Clovis points were made in the late Pleistocene (13,500 to 12,900 years ago), whereas Folsom points are slightly younger. The Blackwater Draw Site is a National Historic Landmark.