Fossils of the Rocky Mountains

Paleozoic fossils

Ordovician fossils

Shallow marine waters continued to cover most of this area through the early part of the Paleozoic (Cambrian–Silurian), supporting a great diversity of life including trilobites, graptolites, brachiopods, and cephalopods. The Ordovician Harding Sandstone of central Colorado is famous for containing some of the oldest known fossil bone in the world, which belonged to fishes called pteraspidomorphs. These animals lacked jaws, but much of their bodies were covered with a bony shell made of numerous small plates. The Harding Sandstone’s depositional environment was once thought to be fresh or brackish water, but now most geologists believe it was part of a shallow marine setting.

Carboniferous to Permian fossils

The late Paleozoic saw the seas retreat, and some Pennsylvanian layers in central Colorado, including the area around Vail and Miturn in Eagle County, accumulated in terrestrial environments such as rivers, lakes, swamps, and floodplains. Some of these sediments include significant coal deposits.

The Permian Lyons Sandstone, deposited as a series of desert sand dunes, contains abundant fossil insects and the footprints of various reptiles and amphibians.

Mesozoic fossils

The Jurassic Morrison Formation is exposed across much of central Colorado, including at a site known as Dinosaur Ridge. This National Natural Landmark is located in Morrison, Colorado, just west of Denver. Dinosaur Ridge includes tilted exposures of the Morrison Formation (on the west side of the ridge), from which many dinosaur skeletons were collected in the late nineteenth century.

The rocks on the east side of Dinosaur Ridge are part of the Cretaceous Dakota Formation and contain hundreds of dinosaur footprints. The Dakota Sandstone, which is also exposed at Red Rock Canyon in Colorado Springs, overlies both the Purgatoire and Morrison formations and stretches into the Colorado Plateau. The Dakota Sandstone is well known for its terrestrial and marine fossils. The formation contains abundant terrestrial plants, occasional dinosaur footprints (attributed to Iguanodon and Ankylosaurus) and skeletal remains, and abundant marine invertebrates, especially mollusks and fish. The layers above the Dakota are also rich in marine fossils.

The Fort Hayes Limestone and Codell Sandstone, exposed in Red Rock Canyon and many other areas, contain abundant ammonites, bivalves, and shark teeth.

Exposures of the Lamarie Formation near Golden, Colorado, on the border between the Rocky Mountain and Great Plains provinces, preserved dinosaur trackways and ancient palm fronds. Fossil mammals tracks have also been recorded.

The K-Pg boundary is visible on South Table Mountain south of Denver. 

Cenozoic fossils

Paleogene

Florissant Fossil Beds

One of the most amazing fossil occurrences in the Southwest is found at Florissant Fossil Beds National Monument in Teller County, central Colorado. There, 35-million-year-old lake sediments from the late Eocene epoch contain a huge diversity of extraordinarily preserved plant and animal fossils. Florissant's fossils were the result of massive volcanic eruptions nearby, which deposited a variety of volcanic layers including lahars, pyroclastic ash, and pumice. The layers dammed streams to form a lake that was then filled with additional volcanic deposits, as well as layers of diatoms, forming sediments known as diatomite. The extraordinary preservation of fossils in this lake may have been caused by an interaction of the volcanic ash with algal mats in the lake, inhibiting decomposition.

Most conspicuous among the Florissant fossils are numerous large petrified tree stumps similar to modern redwoods, which are among the largest-diameter fossilized trees in the world. Fossils of branches, leaves, fruits, seeds, cones, and flowers are also abundant. More than 1500 species of spiders and insects are preserved at Florissant, including grasshoppers, butterflies, moths, wasps, bees, ants, mosquitos, crickets, flies, and many other types of beautifully preserved arthropods. Vertebrate remains include fish, birds, and a few mammals.

Based on these fossil remains—the plants, in particular—scientists estimate that the climate of the Eocene was significantly warmer than it is today. The modern mean annual temperature at Florissant is around 4°C (39°F), while the estimated mean annual temperature during the Eocene was 13°C (55°F).

Quaternary fossils

In 2010, discoveries near Snowmass Village, Colorado revealed one of the most significant sites of large Pleistocene mammals ever found in North America. There, more than 4500 bones were recovered, representing more than 40 types of ice age animals. Larger mammals included mastodons and mammoths, bison, deer, horses, camels, and ground sloths. Smaller mammals included otters, beavers, chipmunks, rabbits, muskrats, and mice. Birds, snakes, and lizards were also discovered. This site provides an amazingly well-preserved snapshot into Colorado's ice age environment.