Fossils of the Rocky Mountains

Precambrian fossils

Paleoproterozoic rocks are exposed in the Medicine Bow Mountains of Wyoming. These sediments contain stromatolites, layered domes or mounds of sediment formed by mats of photosynthetic cyanobacteria (sometimes also called blue-green algae). The stromatolites are more than 2 billion years old. 

Late Precambrian rocks are exposed in northwestern Montana (especially in Glacier National Park) and northern Idaho. They include limestones formed from carbonate sediments deposited on a warm, shallow sea floor. These rocks also contain stromatolites.

Paleozoic fossils

Shallow marine waters continued to cover most of this area through the early part of the Paleozoic (Cambrian to Silurian), supporting a great diversity of life including trilobites, graptolites, brachiopods, and cephalopods. The sea retreated briefly during the middle Devonian, exposing the earlier rocks to erosion and resulting in unconformities in the geological record. Sea level rose again in the late Devonian, covering nearly all of Montana, Wyoming, and part of Idaho. These late Paleozoic seas were filled with diverse and abundant fusulinid foraminifera as well as crinoids, conodonts, mollusks, sponges, brachiopods, graptolites, and fish, including sharks.

Mesozoic fossils

Triassic fossils

In southeastern Idaho, Triassic deposits are largely composed of marine sedimentary rocks with sparse fossils, except for the Thaynes Formation, which contains fossils of fish, crinoids, bivalves, gastropods, ammonoids, crustaceans, and shark teeth. The Columbia Plateau has Triassic red beds and thin deposits of coal, both of which indicate some terrestrial deposition.

Jurassic Morrison Formation

The Jurassic Morrison Formation is a layer of late Jurassic-aged rock exposed across a wide swath of the Rocky Mountains and Great Plains. The silty sediments of the Morrison were deposited by eastward-flowing rivers sweeping across broad, swampy floodplains, and contain extraordinary accumulations of dinosaur bones, as well as fossils of land plants including horsetails, ferns, seed ferns, conifers, cycadeoids (an extinct group of cycad-like plants), and ginkgoes, as well as fish, frogs, lizards, crocodiles, turtles, and small mammals. The Morrison Formation dinosaurs include some of the most widely recognized in North America.

Como Bluff, a region in the Morrison Formation, is historically important as one of the sites of the "Bone Wars," a famous period in nineteenth-century paleontology when Edward Drinker Cope (Academy of Natural Sciences, Philadelphia) and Othniel Charles Marsh (Yale) engaged in a cutthroat competition to name as many dinosaurs as possible. Dinosaur bones were first discovered at Como Bluff by railroad workers in 1877. Cope and Marsh, who were already adversaries, each dispatched their own collectors to find dinosaur bones in the fossil-rich Morrison deposits in the region. Treachery, destruction of fossils, and even rock-throwing ensued among the rival collectors.

Marsh ultimately bested Cope in the Bone Wars. More of Marsh's names for major Morrison Formation dinosaurs are widely recognized today, including the predator Allosaurus, the spike-tailed Stegosaurus, and the sauropods (long-necked dinosaurs) Apatosaurus, Brontosaurus, and Diplodocus. Cope named the sauropod Camarasaurus. Publicity around the Bone Wars and the feud between Cope and Marsh, which lasted until Cope's death in 1897, eventually had a negative impact on the careers and finances of both men.

Cretaceous fossils

Cretaceous marine fossils

Cretaceous marine rocks are well exposed in many parts of Wyoming, particularly around the edges of the Bighorn Basin. Notable fossils here include flat bivalves, such as Inoceramus, some of which reached enormous sizes, and heteromorph ammonoids including Didymoceras. 

Cretaceous terrestrial fossils

Fossils from the Early Cretaceous include those of fish, turtles, crocodilians, gastropods, bivalves, and plants.  The enigmatic tree fern Tempskya has been found in eastern Idaho and in Wyoming, along with other plant fossils like palms. 

A variety of dinosaur fossils (bone, teeth, and eggshell fragments) have been found from ceratopsians (horned dinosaurs), Ankylosaurus (an armored dinosaur with a clubbed tail), and theropods (predatory dinosaurs) are found in the Late Cretaceous. Late Cretaceous deposits of Idaho contain coal, leaves, and freshwater clams. Big Cedar Ridge Fossil Plant Area of Critical Environmental Concern, a site maintained by the Bureau of Land Management, preserves an important Late Cretaceous flora of the Meeteetse Formation in Wyoming.

Cenozoic fossils

Paleogene fossils

The early and mid-Cenozoic was a time of significant tectonic activity in the northwest-central States. During the Paleogene, the Western Interior Seaway advanced across the continent for the final time before tectonic uplift caused it to drain away. Nevertheless, extensive sedimentary deposits represent lakes, rivers, and floodplains. The uplift of mountain ranges to the west was a source of sediment that was transported and deposited by rivers in basins throughout the Great Plains and Rocky Mountain regions. 

Cenozoic rocks preserve a fossilized view of ecosystems dramatically different from those of the Cretaceous. The dinosaurs disappeared, and mammals replaced them as the dominant large vertebrates on land. The warm, humid climate allowed the growth of lush forests. Plants that grew in these forests, including magnolia, ginkgo, sequoia and cypress, were preserved as the coal that is mined today in places such as Wyoming. Lakes were widespread, and were sites of deposition for thick, organic-rich sediments. 

Green River Formation

The Eocene Green River Formation is made up of thick layers of brown to cream-colored shale 600–2000 meters (1970–6560 feet) thick, with occasional layers of chert, limestone, and evaporites. It outcrops across a large area of southwest Wyoming, northwest Colorado, and northwest Utah, and composes the largest known accumulation of lacustrine sedimentary rock in the world.

The sediments of the Green River Formation accumulated in a system of large lakes during the Eocene, between 58 and 40 million years ago. In Colorado and Utah, sediments of the Eocene Green River Formation accumulated in ancient Lake Uinta and are preserved in the Piceance and Uinta Basins. A second lake, called Fossil Lake, occurred on the border of northeastern Utah and southwestern Wyoming; its sediments are preserved in the region of Fossil Butte National Monument in Wyoming. Sediments of a third lake, Lake Gosiute, are preserved in the Green River Basin in Wyoming. The Green River Formation lakes existed for differing amounts of time, with Fossil Lake existing for the shortest time and Lake Uinta the longest.

The Green River Formation is famous for the great number of well-preserved fossils found in its lake sediments, especially aquatic organisms, including stromatolites, mollusks, crayfish, turtles, alligators, and numerous fish. Well-preserved specimens of the fish Knightia are commonly found in the Green River Formation, and it is one of the most abundant vertebrate fossils in the world. A member of the herring family, the average Knightia is 7–12 centimeters (3–5 inches) long. Knightia is thought to have fed on algae, tiny crustaceans, and insects; in turn, it was a major source of food for many of the larger fish in these Eocene lakes. They are commonly found in mass mortality or “death bed” layers because they swam in schools.

Many terrestrial animals are also preserved, including insects, birds, and terrestrial mammals. Green River plants include horsetails, water ferns, terrestrial ferns, conifers, and angiosperms, perhaps most notably spectacular palm leaves (Sabalites powellii). Green River Formation specimens are on display at many museums in the United States. Many specimens from Fossil Lake in particular are on display at the Field Museum of Natural History in Chicago, Illinois.

Yellowstone Petrified Forest

The Eocene was a time of extensive volcanism in the Rocky Mountain region. This is reflected in the occurrence of silica-rich layers in the Green River Formation, which formed from weathered volcanic ash, as well as the famous Yellowstone Petrified Forest. This extraordinary assemblage of multiple layers of volcanic ash contains numerous upright-standing, petrified tree trunks and abundant transported logs and stumps. It formed when ash was repeatedly eroded off of volcanoes and re-deposited in braided streams and rivers.

Neogene fossils

Clarkia Fossil Beds

Abundant Miocene plant remains can also be found in northern Idaho (Shoshone and surrounding counties), where an ancient lake (approximately 15 million years old) provided ideal conditions for the fossilization of soft plant parts. Fossils in the Miocene Clarkia Fossil Beds are so well preserved that some leaves even retain their original color; most are yellow, orange, and brown since they were shed during fall months, although they rapidly oxidize and turn black when exposed to air. The lake formed when a basin was dammed by basalt flows on the Columbia River Plateau.

Although best known for its plants, the Clarkia Beds also contain well-preserved fossil fish, snails, insects, and a rare mammal. Like most lagerstätten (exceptionally well preserved fossil beds), the Clarkia Beds probably formed in a low-oxygen sedimentary environment, which slowed decay of the organic remains.