Fossils of the Colorado Plateau

Precambrian fossils

The rocks of the Colorado Plateau represent a diversity of environmental conditions over hundreds of millions of years. These rocks are well exposed in the many canyons that occur across this region, most spectacularly in the Grand Canyon in northern Arizona. The oldest fossils known from the Colorado Plateau region are stromatolites, which are layered domes of carbonate sediment formed by mats of photosynthetic bacteria known as cyanobacteria. These are found in a layer called the Bass Limestone, exposed near the bottom of the Grand Canyon. They stromatolites date to the mid-Proterozoic, around 1.2 billion years ago.

Late Proterozoic rocks known as the Chuar Group (around 740 million years old) contain a variety of microfossils. Most are referred to as acritarchs, which are spherical objects 1–5 millimeters in diameter. Acritarchs are thought by many paleontologists to be the resting form of single-celled algae. Several levels within the Chuar Group also contain stromatolites. Stromatolites are layered structures formed by photosynthetic bacteria called cyanobacteria.

Paleozoic fossils

Cambrian fossils

The earliest Phanerozoic sediments recorded in the Grand Canyon area are known as the Tonto Group. The fossils in these rocks are usually not very well preserved, but they come in great variety, dominated by trilobites, of which more than 50 species have been reported. Brachiopods are also frequently found in these rocks, and, more rarely, sponges and echinoderms similar to those found in the Cambrian of western Utah.

Ordovician to Devonian fossils

Ordovician and Silurian rocks are absent in the southern part of the Colorado Plateau. Devonian rocks are present in the area, but are only moderately fossiliferous. The Devonian Temple Butte Formation, exposed in the Grand Canyon, contains poorly preserved brachiopods, corals, crinoids, and also occasionally the remains of placoderms—an extinct group of fishes that dominated the waters of the Devonian. Their name means “plated skin,” and they were characterized by bony armor covering the front part of the body and pectoral fins.

Fragments of the bottom-dwelling placoderm Bothriolepis have been recovered from Devonian rocks in northern and central Arizona, and in central Colorado, northwest of Denver (in Eagle and Garfield counties). Placoderms became extinct during a mass extinction event near the end of the Devonian period. Conodonts—small, primitive, eel-like vertebrates—are also known from Devonian and later rocks throughout the region.

Carboniferous fossils

Mississippian fossils

Mississippian rocks exposed in the Grand Canyon, including the Redwall Limestone and Surprise Canyon Formation, contain abundant brachiopods, corals, bryozoans, gastropods, bivalves, nautiloid cephalopods, crinoids, and shark teeth. In at least one Redwall locality, straight-shelled nautiloids more than 60 centimeters (24 inches) long have been found.

Pennsylvanian fossils

Some Pennsylvanian layers in the Colorado Plateau also accumulated in terrestrial environments, including swamps, rivers, lakes, and floodplains. The Mississippian to Pennsylvanian Surprise Canyon Formation contains layers with terrestrial plant fossils, including lycophytes, calamites (an extinct group related to horsetails), marattioid ferns, and seed ferns. 

Pennsylvanian to Permian marine rocks in the southern Colorado Plateau include the Supai Group in northern Arizona and the Honaker Trail Formation in southern Utah. In the Grand Canyon and the surrounding area, the Supai Group contains trace fossils (burrows, trackways, and other marks) in almost all layers. In 2020, the oldest tracks of amniotes (the group of animals including reptiles, birds, and mammals) in the Grand Canyon were described from the Pennsylvanian Manakacha Formation of the Supai Group. A number of beds also contain abundant and diverse shelly fossils, including brachiopods and foraminifera.

Permian fossils

Permian terrestrial fossils

Permian rocks of the Grand Canyon region contain terrestrial fossils. The Hermit Formation yields fossil plants. The overlying Coconino Sandstone is a 90-meter-thick (300-foot-thick) terrestrial deposit, with cross-bedded layers that are characteristic of sand dunes. More than 20 types of vertebrate footprints have been identified within this unit, made by a variety of amphibians and reptiles that trekked through damp sand some 280 million years ago. A variety of other tracks are attributed to arthropods, including spiders, scorpions, beetles, and millipedes.

One of the most important deposits of early Permian vertebrate fossils in North America is known from the El Cobre Canyon Formation (Culter Group) in the Arroyo del Agua area, west of Abiquiu, Rio Arriba County, in northern New Mexico. This site has produced the skeletal remains of many large terrestrial amphibians and reptiles, including Sphenacodon, Eryops, Ophiacodon, and Diadectes. Eryops and the sail-backed predator Dimetrodon are also abundant in the Permian rocks of southern Utah, including Monument Valley and the Valley of the Gods. The small reptile Seymouria is found in Permian rocks south of Moab, Utah.

Permian marine fossils

Above the Coconino Sandstone is another Permian marine unit, the Kaibab Limestone, which contains abundant fossils from marine vertebrates and invertebrates, including brachiopods, sponges, corals, crinoids, echinoids, gastropods, bivalves, nautiloid and ammonoid cephalopods, conodonts, and shark teeth. Ammonoid cephalopods are especially important for biostratigraphy in this and other Permian marine layers.

Triassic fossils

Early and middle Mesozoic rocks (Triassic and Jurassic periods) of the Colorado Plateau are just as fossil rich as Paleozoic-aged rocks. The Triassic rocks of northern Arizona are famous for their fossils of terrestrial (land-dwelling) and freshwater vertebrates. The middle Triassic Moenkopi Formation contains one of the best vertebrate fossil assemblages of this age anywhere in the world, including abundant bones and trackways of both large amphibians and reptiles. Skeletal remains in the Moenkopi include freshwater sharks, coelacanths, and lungfish, as well as amphibians, rhynchosaur reptiles, and the non-dinosaur archosaur Arizonasaurus.

Chinle Formation

The late Triassic Chinle Formation (or Group) is exposed across much of the Southwest. This formation accumulated in a shifting set of habitats, from forests to rivers and lakes.

Fossils of freshwater aquatic animals

Fossils of freshwater fish, clams, and crustacean burrows are common in parts of the Chinle Formation, suggesting abundant water, but other evidence indicates that rainfall was probably highly seasonal, and the environment was frequently dry. Fossil fishes found here include the coelacanth Chinlea, the shark Xenacanthus, the lungfish Ceratodus, and Australosomus, a relative of today’s goldfish and tuna.

Terrestrial animal fossils

Terrestrial animals include a parade of forms, such as the bizarre armored reptiles known as aetosaurs (for example, Desmatosuchus), the rauisuchid Postosuchus, the synapsid (mammal-like reptile) Placerias, crocodile-like phytosaurs (the most common vertebrate in the formation), and the small theropod dinosaur Coelophysis.

In the 1940s, hundreds of Coelophysis skeletons were discovered at Ghost Ranch in Rio Arriba County, north-central New Mexico. This fossil treasure trove may have formed when the dinosaurs died around a shrinking water source during the dry season, and then were swept up and deposited by a flash flood. The study of fossils from this deposit helped to make Coelophysis one of the world’s best-known dinosaurs.

Terrestrial plant fossils

The Chinle is also famous for its fossil trees, spectacularly exposed in Arizona’s Petrified Forest National Park and also abundant in the surrounding region. Most wood preserved in this region belongs to the conifer species Araucarioxylon arizonicum. Other plants include lycophytes, ferns, cycads, cycadeoids, other conifers, and ginkgoes.

Jurassic fossils

Early Jurassic fossils

The Jurassic Kayenta Formation overlies the Chinle Formation and is exposed in the Painted Desert and vicinity in norther Arizona. It contains fossils of early mammal relatives as well as dinosaurs, including the larger theropod Dilophosaurus. One type of synapsid, or mammal-like reptile, discovered in the Kayenta Formation Kayentatherium wellesi, an animal reminiscent of a large woodchuck. In 2000, remains of an adult Kayentatherium was found with skulls of 38 juveniles; in 2018, a report interpreted these remains as a mother with her young.

Although it was once desert dune sand, the overlying Navajo Sandstone also contains dinosaurs and trackways.

Late Jurassic fossils

As the Jurassic continued, dinosaurs continued to expand in diversity and size. Their dominance is abundantly demonstrated in the Jurassic Morrison Formation, an extensive layer of rock that crops out in all four states of the Southwest, as well as in the Dakotas, Montana, Nebraska, Kansas, Oklahoma, Texas, and Idaho. Although the Morrison Formation contains more than 90 known species of fossil vertebrates, including fish, turtles, amphibians, and early mammals, it is most famous for its abundance and diversity of dinosaurs. Dinosaurs from the Morrison include carnivorous theropods such as Ceratosaurus and Allosaurus, herbivores such as Dryosaurus, Stegosaurus, and Camptosaurus, and gigantic long-necked sauropods, such as Amphicoelias, Apatosaurus, Barosaurus, Brachiosaurus, Camarasaurus, and Diplodocus.

The Morrison Formation also contains abundant plant fossils, including horsetails, ferns, seed ferns (seed plants with fern-like leaves), cycadeoids, ginkophytes, and conifers. These fossils suggest that the Morrison ecosystem was a mosaic of river, lake, and floodplain environments developed on an enormous alluvial plain covered by sediment eroding from the ancestral Rocky Mountains.

Dinosaur National Monument

Two spectacular places to see these dinosaur fossils still in place are Dinosaur National Monument, on the border between Utah and Colorado, and the Cleveland-Lloyd Dinosaur Quarry in Utah. The dinosaur fossil beds in Dinosaur National Monument were discovered in 1909 by a paleontologist working for the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. Over several years, crews from the museum excavated thousands of fossil bones and shipped them back to Pittsburgh for study and display. The famous "Wall of Bones", located within the monument’s main building, consists of a steeply tilted rock layer containing hundreds of dinosaur bones. The layer was tilted by the Laramide Orogeny millions of years after the dinosaurs lived, died, and were buried.

The site was declared a national monument in 1915, but new discoveries continue to this day. In 2009, the tracks of tiny Jurassic mammals were found. In late 2015, one of the oldest known pterosaurs was found in the monument’s Triassic rocks.

Cleveland-Lloyd Dinosaur Quarry

The Cleveland-Lloyd Dinosaur Quarry, located in northern Emery County, Utah, contains the densest concentration of Jurassic-aged dinosaur bones ever found. Over 10,000 bones belonging to at least 74 individual dinosaurs have been excavated at the quarry. Curiously, more than 70% of these bones come from carnivores, primarily Allosaurus fragilis. With more than 46 individual specimens of Allosaurus, scientists have been able to both deduce how Allosaurus aged and compare individuals to better understand variation within the species.

Cretaceous fossils

Early Cretaceous to earliest Late Cretaceous terrestrial animals

Fossils from the Cretaceous Cedar Mountain Formation of Utah include a wide variety of dinosaurs. The fossil-bearing rocks of the formation span more than 20 million years of time, from at least about 125 to 120 million years ago to 98 million years ago. Thus, fossils from much of the formation (Yellow Cat, Poison Strip, and Ruby Ranch members) are Early Cretaceous in age, whereas the youngest fossils, which are from the Mussentuchit Member, may be late Early Cretaceous to earliest Late Cretaceous in age.

Dinosaurs from the Cedar Mountain Formation include herbivores (plant-eaters) like ankylosaurs (armored dinosaurs), hadrosaurs (duck-billed dinosaurs), ornithopods, and sauropods (long-necked dinosaurs). There are also a variety of predatory theropods, including the large and ferocious-looking Utahraptor. Utahraptor is a type of dromaeosaur, a group of dinosaurs closely related to birds. Dromaeosaurs had a sickle-like claw on the inner toe of each foot. Scientists think that they used this claw to attack their prey. Utahraptor is the largest known dromaeosaur. It grew to as much as 6 to 7 meters (20 to 23 feet) long. In contrast, Velociraptor, a dromaeosaur from Mongolia, reached only about 1.8 meters (6 feet) in length.

The Mill Canyon Dinosaur Tracksite in Moab, Utah, preserves dinosaur tracks from the Cedar Mountain Formation. (Unfortunately, this tracksite sustained damage during a project to replace a walkway in 2022.) Fish, amphibians, reptiles, birds, and mammals have been found in the Mussentuchit Member of the Cedar Mountain Formation.

Late Cretaceous fossils

Late Cretaceous terrestrial animals

Late Cretaceous rocks are widely exposed across southern Utah, and contain a great abundance and diversity of dinosaurs. Late Cretaceous dinosaurs are also known form northern New Mexico. Herbivores are represented by hadrosaurs (duck-billed dinosaurs), ankylosaurs (armored dinosaurs), and several kinds of ceratopsians (horned dinosaurs).

Theropods (three-toed, carnivorous dinosaurs) are well represented in the southern Colorado Plateau. In the Late Cretaceous, most large teeth and bones found in the region were originally thought to be from close relatives of Tyrannosaurus rex, such as the Albertosaurus, which are well known farther north in Wyoming and Montana. In 2010, however, some of these fossils were identified as a new genus of theropod named Bistahieversor. This predator is estimated to have been around 9 meters (30 feet) long, weighing at least a ton. 

The titanosaur (a type of large sauropod, or long-necked, herbivorous dinosaur) Alamosaurus sanjaunensis is known from the Late Cretaceous of New Mexico, Texas, and Utah. A large vertebra (part of the backbone) of this dinosaur that was discovered in the Ojo Alamo Formation of New Mexico in 2004 led researchers to conclude that it may be the largest dinosaur yet found in North America and nearly the largest known worldwide. (You can see a picture of an Alamosaurus skeleton from a display at the Perot Museum on the Earth Science of the South-central United States: Fossils of the Basin and Range page.)

Pterosaurs, including Navajodactylus, have been found in New Mexico and Utah. Fossils of mammals—mostly very small teeth—are also found in the Cretaceous rocks of southern Utah. These fossils represent marsupials (pouched mammals, like koalas and kangaroos), placentals (placental mammals, which include most living mammals), and several extinct groups that left no modern descendants.

Late Cretaceous terrestrial plants

Cretaceous plant fossils are abundant in Utah, especially in Emery and Carbon counties, where large quantities of land plants accumulated in coastal swamps and eventually formed significant deposits of coal. In addition to conifers and tree ferns, which were major contributors to these coal deposits, the region’s Cretaceous land plants include angiosperms (flowering plants) such as palms and magnolias. Currently, fossil evidence suggests that angiosperms originated in the earliest Cretaceous and had diversified and taken over many terrestrial ecosystems by the mid-Cretaceous.

Late Cretaceous marine fossils

As the Western Interior Seaway flooded North America during the Late Cretaceous, portions of the Colorado Plateau were submerged and covered in a series of marine layers. These rocks frequently contain abundant vertebrate and invertebrate fossils. Marine vertebrates include bony fish, sharks, ichthyosaurs, mosasaurs, and plesiosaurs, while invertebrates encompass a great diversity of mollusks, barnacles, and echinoderms.

Relatives of living oysters were diverse and abundant during the Cretaceous; they could cement themselves to surfaces and varied widely in shape and ornamentation. Inoceramus was a large, flat bivalve that could reach diameters of up to 1.2 meters (4 feet)! These mollusks were also important hard substrates for other organisms to attach to in soft sea-floor sediments.

The Late Cretaceous Mancos Shale preserves the abundant remains of marine snails, sharks, mosasaurs, crinoids, bivalves, and ammonoids. The Dakota Group includes abundant ammonites and bivalves, as well as the last known North American lungfish from just east of Arches National Park in southeastern Utah. The Tropic Shale and its equivalents are some of the most fossil-rich marine units in North America; ammonoids found here include especially valuable index fossils such as Sciponoceras and Collignoniceras.

Cenozoic fossils

Paleogene

Paleocene

The Cretaceous-Paleogene (K-Pg) boundary is visible in several areas of the Southwest, especially in the San Juan Basin of New Mexico. This boundary marks the mass extinction that wiped out non-avian dinosaurs, flying reptiles, swimming reptiles, and many other forms of life across the globe.

Rocks above the K-Pg boundary in the San Juan Basin are rich in fossil land mammals. These include a number of major groups, many of which became extinct within a few million years. Multituberculates were a group of mostly small, rodent-like mammals that originated in the Cretaceous and disappeared in the early Oligocene. The name “multituberculate” refers to their distinctive multi-cusped teeth.

Other mammals abundantly represented by fossils in the San Juan Basin are commonly referred to as "archaic," meaning that their features are primitive compared to those of later mammals. These archaic mammals include condylarths, an informal term for a number of early mammals that are not closely related to each other. Some of the earliest known primates were also present at this time. Cenozoic rocks in the San Juan Basin also contain abundant flowering plants.

Eocene

Uinta Formation

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, and primitive carnivores known as creodonts.

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, northwestern Colorado, and northeastern Utah. The Green River comprises 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 numerous fish. Many terrestrial animals are also preserved, including insects, birds, and mammals. Green River plants include horsetails, water ferns, terrestrial ferns, conifers, and angiosperms. Green River Formation specimens are on display at many museums in the United States. 

Quaternary fossils

Between 40,000 and 10,000 years ago during the Pleistocene epoch, the Shasta ground sloth (Nothrotherium shastense) inhabited Rampart Cave in the Grand Canyon. Dung samples from this and other caves are rich in well-preserved pollen and other plant material, which allows the diet of these extinct animals to be reconstructed. The preserved dung also contains sloth DNA. Like many other species of large mammals, these sloths became extinct abruptly at the end of the Pleistocene, probably due at least in part to human hunting.

Until 1976, Rampart Cave contained the thickest and least disturbed deposit of stratified Shasta ground sloth dung known to science. While the dung was partially excavated, much of the remaining deposit was destroyed by a human-caused fire in the 1970s.