Late Precambrian rocks (approximately 1.8 billion years old) are exposed in northeastern Utah, where they contain what may be the oldest known fossils in the region: acritarchs. These unique fossils are probably the resting stages of single-celled eukaryotic plankton (see Figure 3.4B). Precambrian rocks of the Rocky Mountains also contain stromatolites, layered domes of sediment formed by mats of bacteria living in a shallow sea (see Figure 3.3).

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 (see Figures 3.43, 3.45, and 3.46). 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 (Figure 3.60). 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. The late Paleozoic saw these 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 (Figure 3.61). Some of these sediments include significant coal deposits (see Figures 3.9 and 3.35). The Permian Lyons Sandstone, deposited as a series of desert sand dunes, contains abundant fossil insects and the footprints of various reptiles and amphibians (see Figure 3.11).

The Jurassic Morrison Formation is exposed across much of central Colorado, including around Denver, at a site known as Dinosaur Ridge. This National Natural Landmark is located in Morrison, Colorado, just west of Denver. The site 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 (see Figures 3.24 and 3.25). The rocks on the east side of the Ridge are part of the Cretaceous Dakota Formation and contain hundreds of dinosaur footprints (see Figure 3.27). 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 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 (see Figure 3.39). 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.

Figure 3.60: The jawless fossil fish Astraspis from the Ordovician Harding Sandstone of Colorado. A) One of the bony scutes that covered the fish's articulated exoskeleton, approximately 1 centimeter (0.4 inches) long. B) Restoration, about 30 centimeters (1 foot) long.

Figure 3.61: The Pennsylvanian period in the ancestral Rockies. Artist's reconstruction of the landscape of central Colorado during the Pennsylvanian period, with large stands of giant club mosses (lycopods).

The Cretaceous Raton Formation of northern New Mexico preserves an abundant fossil record from the latest Cretaceous period. It is also the source of what is widely accepted as the only known footprint of Tyrannosaurus rex (see Figure 3.32B). The Raton Basin in northern New Mexico is also the only area of New Mexico where the K-Pg boundary is preserved—this boundary is also visible on South Table Mountain south of Denver.

Above the K-Pg boundary, Paleocene mammals (mostly represented by teeth) are found near Golden, Colorado. Nearby at Castle Rock, on the margin of the Denver Basin, is a very high-diversity assemblage of fossil leaves, which appears to represent the oldest known tropical rainforest (Figure 3.62), in existence just 1.4 million years after the end-Cretaceous extinction.

The Green River Formation is a layer (600–2000 meters [1970–6560 feet] thick) of brown to cream-colored shale, with occasional layers of chert and limestone, which outcrops across a large area of southwest Wyoming, northwest Colorado, and northwest Utah. The Green River comprises the largest known accumulation of lacustrine sedimentary rock in the world. Its sediments accumulated in a system of lakes that covered this area during the Eocene, between 58 and 40 million years ago (Figure 3.63). The Green River is famous for the great number of well-preserved fossils found in its lake and river sediments, especially aquatic organisms such as fish, gastropods, and algae, but also many terrestrial plants and animals (Figure 3.64), including insects, birds, and mammals. The formation also contains a large amount of oil shale.

The Uinta Formation, exposed in the Uinta Mountains of northeastern Utah (about 42–45 million years old), is famous for its middle Eocene mammals, including marsupials, insectivores, primates, rabbits, rodents, hoofed mammals, condylarths (see Figure 3.40), and primitive carnivores known as creodonts (Figure 3.65).

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 (Figure 3.66). 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 sequoias, which are among the largest-diameter fossilized trees in the world. Fossils of fruits, seeds, cones, and flowers are also abundant. Among the animals preserved at Florissant are more than 1500 species of spiders and insects, 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).

Figure 3.62: Fossils from an early Paleocene rainforest, at Castle Rock, alongside Interstate 25 near Denver, Colorado. Some of the fossil leaves are so well preserved that original leaf material is present. A) Undescribed fern leaves. This split slab of rock shows the abundance of fossil leaves at the site. B) An undescribed angiosperm species. This specimen shows insect damage. C) Allantodiopsis erosa. The elongated tip of this leaf is called a "drip tip," which helps the leaf shed water in very wet conditions. Drip tips are common in modern tropical rainforests. D) "Zizyphus" fibrillosus. This leaf may be related to the modern buckthorn.

Figure 3.63: The size and location of various lakes in which the Green River sediments were deposited at different times during the Eocene epoch.

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.

Figure 3.64: Fossil insects and plants from the Green River Formation of Utah and Colorado. A) Unidentified beetle, about 1 centimeter (0.4 inches) long. B) Beetle (weevil), 13 millimeters (0.5 inches) long. C) Scorpionfly, 2.5 centimeters (1 inch) long. D) Robber fly, 11 millimeters (0.4 inches) long. E) Maple leaf, Acer sp., 6.25 centimeters (2.5 inches) wide. F) Planetree leaf, Platanus wyomingensis, 14.5 centimeters (5.7 inches) wide. G) Poplar leaf, Populus willmattae, 7.3 centimeters (2.9 inches) long. H) Legume leaf, Leguminosites lesquereuxiana, 3 centimeters (1.2 inches) wide.

Figure 3.65: Uinta Formation mammals. A) Skull of the phenacodont Tetraclaenodon, approximately 23 centimeters (9 inches) long. B) Restoration of the creodont Hyaenodon, approximately 2 meters (6 feet) long. C) Skull of the artiodactyl Protoreodon, approximately 17 centimeters (6.7 inches) long. D) Restoration of Protoreodon, approximately 70 centimeters (28 inches) long.

Figure 3.66: Fossil plants and insects from the Florissant Fossil Beds of Colorado. A) Fossil redwood stump, approximately 2 meters (6 feet) in diameter. B) Fossil angiosperm leaf, slab approximately 5 centimeters (2 inches) across. C) Fossil wasp, Palaeovespa florissantia, approximately 2 centimeters (0.8 inches) long. D) Crane fly, approximately 2.5 centimeters (1 inch) long. E) Wasp and cypress leaves, approximately 5 centimeters (2 inches) long.