Agate house, a structure built around 1050 to 1300 out of petrified wood in Arizona.

Fossils of the Colorado Plateau

Simple map showing the area of the Colorado Plateau province of the southwestern United States.

Page snapshot: Introduction to the fossils of the Colorado Plateau region of the southwestern United States.


Topics covered on this page: Precambrian fossils; Paleozoic fossils; Cambrian fossils; Ordovician to Devonian fossils; Carboniferous fossils; Permian fossils; Triassic fossils; Chinle Formation (Triassic); Jurassic fossils; Early Jurassic fossils; Late Jurassic fossils (Morrison Formation); Dinosaur National Monument; Cleveland-Lloyd Dinosaur Quarry; Cretaceous fossils; Early Cretaceous fossils; Late Cretaceous fossils; Cenozoic fossils; Paleocene fossils; Eocene fossils; Quaternary fossils; Resources.

Credits: Most of the text on this page comes from "Fossils of the Southwestern US" by Warren D. Allmon and Richard A. Kissel, chapter 3 in the The Teacher-Friendly Guide to the Earth Science of the Southwestern USedited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published in 2016 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2022. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.

Updates: Page last updated April 8, 2022.

Image above: Agate house, a structure made from petrified wood, at Petrified Forest National Park, Arizona. This house dates to sometime between 1050 to 1300. Source: Petrified Forest National Park, National Park Service (public domain).

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.


Diagram showing the stratigraphy of the Grand Canyon.

Major Proterozoic and Paleozoic stratigraphic units of the Grand Canyon and Colorado Plateau. Modified from a diagram by Wade Greenberg-Brand originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.


Photo of stromatolites in the Precambrian Bass Limestone, Grand Canyon.. The photo shows an outcrop of reddish rock in which lighter-colored, sinuous layers can be seen oriented diagonally from the bottom left of the image to the top right.

Precambrian stromatolites in the Bass Limestone, Grand Canyon National Park, Arizona. NPS photo by Carl Bowman (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, image resized).

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.


2-panel figure showing photos of trace fossils from Grand Canyon National Park. Panel 1: Cruziana, a linear trace with a central depression often attributed to trilobites. Panel 2: A slab with overlapping traces, including worm burrows and Cruziana.

Trace fossils from Grand Canyon National Park, Arizona. Left: Cruziana, a type of trace fossil attributed to trilobites. Formation not identified, but probably Cambrian Tonto Group. (Length about 20 centimeters or 8 inches). Photo by Cassi Knight, Paleontology Guest Scientist (National Park Service, public domain)Right: Trace fossils (burrows and Cruziana) from the Cambrian Bright Angel Shale, Tonto Group. Photo by Cassi Knight, Paleontology Guest Scientist (National Park Service, public domain)


2-panel figure showing photographs of trilobites from the Cambrian Bright Angel Shale, Grand Canyon. Panel 1: specimen with a single whole trilobite and several trilobite fragments; the whole trilobite has no spines or ornamentation. Panel 2: Specimen of a single whole trilobite; this trilobite looks similar to the complete trilobite in the first panel.

Cambrian trilobites from the Bright Angel Shale (Tonto Group), Grand Canyon National Park, Arizona. Left: Trilobites identified as Dolichometoppus productus and Alokistocare altheaRight: Dolichometoppus productus. Left photo and right photo by NPS/Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


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.


Drawing of a placoderm, a type of primitive fish from the Devonian of Arizona. The fish looks like it has bony plates on the front half of its body, small eyes on the front of its head, and thin fins on each side of its body.
Reconstruction of the Devonian placoderm fish Bothriolepis, from the Devonian of northern Arizona. Drawing copyright Christi Sobel, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.

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.


2-Panel figure showing Mississippian corals from the Redwall Limestone, Grand Canyon. Panel 1: Photo of a coral that appears to be made up of a group of parallel tubes. Panel 2: A more typical tabulate coral made up of what appear to be a group of angular columns.

Tabulate corals from the Mississippian Redwall Limestone, Grand Canyon National Park, Arizona. Left photo and right photo NPS photos by Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


2-panel figure of Mississippian marine invertebrate fossils from the Redwall Limestone, Grand Canyon. Panel 1: Photo of a bryozoan. The fossil looks like a flat, netted impression in beige rock. Panel 2: Photo of a brachiopod. The photo shows a single striate valve preserved in gray rock.

Marine invertebrates from the Mississippian Redwall Limestone, Grand Canyon National Park, Arizona; on the left is a bryozoan, on the right a brachiopod. Left photo and right photo NPS photos by Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


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.


Photograph of the tooth plates of Deltodus, an ancient relative the the chimaera, from the Mississippian of Arizona. One of the plates is dull white in color with tiny pits covering its surface. The other is shiny and gray, showing similar pits to the first.

Tooth of Deltodus, an ancient relative of the chimaera (a type of cartilaginous fish related to sharks), from the Mississippian Surprise Canyon Formation, Supai Group, Arizona. Photo of specimen USNM PAL 412170 by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain).


2-panel figure from a published paper showing a Pennsylvanian amniote trackway from the Grand Canyon. Panel 1: Set of tracks in a diagonal path across a block of reddish rock; a hand holds a scale bar (scale in decimeters). Panel 2: Drawing of the same rock with trackway, pointing out a second trackway intersecting the main set of tracks. Scale is 50 centimeters.

Pennsylvanian vertebrate tracks from the Manakacha Formation, Supai Group, Grand Canyon National Park, Arizona. Photo of the tracks (A) and drawing of the same specimen (B). Scale is in decimeters (1 decimeter = 10 centimeters = about 3.9 inches). Figures 2A and 2B from S. M. Rowland, M. V. Caputo, and Z. A. Jensen (2020) PLoS ONE 15(8): e0237636 (Creative Commons Attribution 4.0 International license, image cropped).


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.


2-panel figure showing photos of unidentified seed fern fronds from the Permian Hermit Formation, Grand Canyon. Panel 1: Image of a large part of a frond with a fragment of a second frond to its left; the fronds are pinnately compound. Panel 2: Portions of two pinnately compound fronds.

Unidentified seed fern fronds (leaves) from the Permian Hermit Shale, Grand Canyon National Park, Arizona. Left photo and right photo by Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


Photograph of a fossil conifer branch from the Permian Hermit Shale of Arizona. The fossil is preserved in a medium-orange rock on which the impression of a branch with lateral branches given off on its right side are preserved. Impressions of scale or needle leaves are also present.

Branches and leaves of an ancient conifer (Walchia dawsonii), Permian Hermit Shale, Arizona. Photo of USNM P 38052 by Frederic Cochard (Smithsonian National Museum of Natural History, public domain).


Photograph of a light brown rock slab with a vertebrate trackway. Each track is wider than long and has pointed or clawed feet.

Vertebrate tracks in the Permian Coconino Sandstone, Grand Canyon National Park, Arizona. Photo by Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


Photograph of Chelichnus tracks, the five-toes prints of a mammal-like reptile, in the Permian Coconino Sandstone at Grand Canyon National Park.

Vertebrate tracks (Ichnotherium), Permian Coconino Sandstone, Grand Canyon National Park, Arizona. Photo by Cassi Knight, Paleontology Guest Scientist (public domain).


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.


2-panel figure showing photos of fossil marine invertebrates from the Mississippian Kaibab Formation, Grand Canyon. Panel 1: Photo fo a brachiopod; one striated valve (shell) can be seen. Panel 2: Portions of crinoid stalks; each partial stalk is made up of several ring-shaped segments stacked on top of each other.

Marine invertebrate fossils (a brachiopod on the left, segments of crinoid stalks on the right) from the Permian Kaibab Formation, Grand Canyon National Park, Arizona. Left photo and right photo by Michael Quinn/NPS (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


Fossil nautiloid (Tainoceras schellbachi) from the Permian Kaibab Limestone, Grand Canyon National Park, Arizona. Model by National Park Service Geologic Resources Division (Creative Commons Attribution-NonCommercial 4.0 International license).

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.


Map of the southwestern U.S. showing state and county boundaries. Mesozoic fossil tracksites are mapped. Triassic sites are marked by blue squares, Jurassic sites are marked by red circles, and Cretaceous sites are marked by black triangles. Most sites are in Colorado, Utah, northeastern Arizona, and northern New Mexico.

Major localities in which Mesozoic footprints (mostly dinosaurs) have been found. Map modified from a map by Alana McGillis that was compiled from multiple sources. Originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.


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.


Map of the southwestern area of the United States showing the extent of the Triassic Chinle Formation. The formation is found sporadically throughout Utah, in western Colorado, in northern Arizona, in southern Nevada, in southeastern Wyoming, and in New Mexico.
Location of Chinle Formation outcrops across the southwestern U.S. Map by Wade Greenberg-Brand, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.

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.


Photo of a lungfish toothplate from Petrified Forest National Park. The fossil is blue, gray, and white. The shape is triangular, and the surface appears folded or ribs, with teeth on the highest part of each fold or rib.

Fossil lungfish toothplate, Petrified Forest National Park, Arizona. Formation and age not specified, but probably Triassic Chinle Formation. Photo by U.S. National Park Service (NPS, public domain).


2-Panel figures showing photos of two fossil fish from the Triassic of Utah. Panel 1: Hemicalypterus, a fish with a deep body. The fossil showing the head with slightly projected lower jaw, the front half of the body with rows of rectangular scales, and a tall dorsal (back) fin. The tail end of the fish is not as well preserved. Panel 2: Portion of the fish Lepidotus showing scales covering the side and back of the body near the tail. Some spines for the dorsal fin can be seen.

Fossil ray-finned fish, Triassic, Utah. Left: Hemicalypterus weiri, Chinle Formation, San Juan County. Photo of USNM V23425 (Smithsonian National Museum of Natural History, public domain)Right: Lepidotus walcotii, Kanab Valley. Photo of USNM V8119 (Smithsonian National Museum of Natural History, public domain).


Photograph of Triassic clam fossils from the Chinle Formation, Arizona. The fossils are in situ (in place where they were found). Each is one valve (half) of a clam shell. The ground that are sitting on is littered with small stones.

Fossil clams, Triassic Chinle Formation, Petrified Forest National Park, Arizona. Photo by NPS (U.S. National Park Service, public domain).


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.


Photograph of a display of casts of vertebrate skeletons on display at Petrified Forest National Park. In the display are the bipedal Postosuchus, a carnivore; the four-legged, armored Desmatosuchus; and a small pterosaur (flying reptile) on the back of the Desmatosuchus.

Display of vertebrate skeleton casts (reproductions) at Petrified Forest National Park, including the carnivorous rauisuchid Postosuchus (background/upper), the aetosaur Desmatosuchus (foreground/lower) and a pterosaur (on the back of the Desmatosuchus). Photo by Marine 69-71 (Wikimedia Commons, Creative Commons Attribution-Share Alike 3.0 Unported license, image cropped and resized).


Photograph of a cast of the skeleton of Placerias on display at Petrified Forest National Park. The animal is stout with four legs and a bizarre skull. The skull features two large, tooth-like projections in the maxilla (upper jaw), one on each side of the face.

The synapsid (mammal-like reptile) Placerias from the Triassic Chinle Formation, cast (reproduction skeleton) on display at Petrified Forest National Park, Arizona. Photo by NPS/Hallie Larson (National Park Service, public domain).


Photograph of the upper part of a skull of the phytosaur Smilosuchus. The skull is elongated and flat with pointed teeth and looks crocodile-like.

Upper part of the skull of a phytosaur (Smilosuchus gregorii), Triassic Chinle Formation, Apache County, Arizona. Photo of specimen USNM V18313 (Smithsonian Institution National Museum of Natural History, public domain).


Photograph of a mounted skeleton of Coelophysis baueri. The dinosaur is bipedal (walks on two legs), has shorter arms, a long neck, and a slightly elongated skull with small, pointed teeth. The tail extends horizontally from the body and does not rest on the ground. The feet have three large toes.

Coelophysis baueri, a small theoropod dinosaur from the Triassic Chinle Formation, Ghost Ranch, New Mexico. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of jumbled bones of the small predatory dinosaur Coelophysis preserved in a block.

Block of Coelophysis specimens collected in 1948 at Ghost Ranch, New Mexico, on display at the Cleveland Museum of Natural History. Photo by Paleeoguy (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license).


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.


Photograph of a pile of red-colored petrified logs at the base of gray-and-red layered badlands in Petrified National Forest, Arizona.

A pile of large logs from the Triassic Chinle Formation, Petrified Forest National Park, Arizona. Most fossil wood at the park is assigned to a species of conifer (Araucarioxylon arizonicum). Photo by T. Scott Williams (National Park Service, public domain).


A cross section of a petrified conifer tree trunk. The periphery of the trunk is reddish-orange whereas the interior is grayish. Concentric growth rings are clearly visible.

Cross section of a conifer (Araucarioxylon arizonicum) trunk, Triassic Chinle Formation, Petrified Forest National Park, Arizona. Photo of specimen YPM PB 045161 by William K. Sacco, 2003 (Yale Peabody Museum of Natural History/YPM, CC0 1.0 Universal/Public Domain Dedication, image accessed via GBIF.org).


Photograph of a piece of rock from the Chinle Formation of Arizona with two fern fronds on its surface. The larger frond on the left is palmately compound with pinnately compound divisions. The frond on the right is smaller and two- to three-times pinnately compound.

Fronds of leptosporangiate ferns (left, Phlebopteris smithii; right, Wingatea plumosa) from the Triassic Chinle Formation, Apache County, Arizona. Photo of specimens UCMP 1544 and UCMP 1548 by Diane Erwin (University of California Museum of Paleontology, Creative Commons Attribution 3.0 Unported license, via GBIF.org; image cropped and resized).


Photograph of a piece of rock preserving twice-pinnately compound fern fronds. The pinnules and pinnatifid (pinnately incised).

Fronds of a fern (left, Cladophlebis daughertyi) from the Triassic Chinle Formation, Petrified Forest National Park, Arizona. Photo source: National Park Service (public domain).

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.


2-panel image of Dilophosaurus, a predatory dinosaur from the Jurassic. Panel 1: Photo of a skeleton partially embedded in a matrix. The skeleton's neck is arched backward, the back legs and splayed outward, and the tail curls around the top of the skeleton. The skull has a short crest. Panel 2: Illustration of the dinosaur during life; the crest is colored red in this recreation, and the animal stands on two legs.

The predatory dinosaur Dilophosaurus wetherilli from the Early Jurassic Kayenta Formation, Coconino County, Navajo Nation, Arizona. Left: Reconstruction of the holotype (name-bearing) specimen, on display at the Royal Ontario Museum (Toronto, Canada). Photo by Eduard Solà (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped and resized). Right: Reconstruction of the animal by Leandra Walters. Note that it is has no frill, which was an addition made for the movies. Drawing by Leander Walters in P. Senter and J. H. Robins (2015) PLoS ONE 10(12): e0144036 (Creative Commons Attribution 4.0 International license). 


2-panel image showing photos of a fossil of the Jurassic mammal-like reptile Kayentatherium from Arizona. Panel 1: Photo of the skull showing long, rodent-like front teeth. Panel 2: Photo of the full skeleton partially embedded in rock matrix showing four legs and a medium-length tail.

Kayentatherium wellesi from the Early Jurassic Kayenta Formation of Arizona. Left: Skull. Right: A complete skeleton. The ruler is about 15 centimeters (6 inches) in length. Photos of specimen USNM PAL 31702 by Michael Brett Surman (Smithsonian National Museum of Natural History, Department of Paleobiology, public domain).


Photograph of a trail of three-toed dinosaur tracks on a slab in the Navajo Sandstone of Glen Canyon National Recreation Area, Arizona and Utah.

Ornithopod-type tracks, Powell Fossil Track Block Tracksite, Jurassic Navajo Sandstone, Glen Canyon National Recreation Area, Arizona and Utah. Photo source: National Park Service (public domain).


2-panel figure of the Early Jurassic prosauropod Seitaad reussi from New Mexico. Panel 1: Photograph of one side of the fossil specimen of the dinosaur, which clearly shows two feet and part of an arm. Panel 2: Illustration showing a reconstruction of the skeleton in a silhouette of the living dinosaur. The dinosaur is bipedal (walks on two legs) with a long neck, small head, and long tail.

A prosauropod (Seitaad ruessi, a relative of sauropods) from the Early Jurassic Navajo Sandstone, San Juan County, Utah. Left: Portion of the skeleton showing two feet. Scale bar = 10 centimeters (3.6 inches). Figures 1C and 3A from J. J. W. Sertich and M. A. Lowewen (2010) PLoS ONE 5(3): e9789 (Creative Commons Attribution license, images cropped and reconfigured). Right: Reconstruction of the skeleton and silhouette of the animal; bones in white were actually discovered, bones in dark gray are inferred. Scale bar = 1 meter (39.4 inches).


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.


Map showing the distribution of the Jurassic Morrison Formation in the United States and Canada. The Morrison Formation is found in most of Colorado and Wyoming; the western parts of North Dakota, South Dakota, and Nebraska; the northwestern corner of Kansas; northern New Mexico; northeastern Arizona; eastern Utah; a little of eastern Idaho; most of Montana; and southern Saskatchewan and Manitoba in Canada.

Geographic extent of the Jurassic Morrison Formation. Map modified from a map by Wade Greenberg-Brand, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.


Photograph of a skeleton of Ceratosaurs on display at a museum. Ceratosaurus is a predatory dinosaur that walked on two legs. Its skull has a distinctive short nasal horn.

Ceratosaurus cast (reproduction skeleton) on display at the Natural History Museum of Utah (Salt Lake City). Original from the Cleveland-Lloyd Dinosaur Quarry, Jurassic Morrison Formation, Utah. Photo by Jens Lallensack (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).


Photograph of the skeleton of Stegosaurus ungulatus on display in a museum. Stegosaurus is a four-legged, plant-eating dinosaur. It has a small head. Its most distinctive feature is two rows of large, bony plates that stick up from its back. Stegosaurus also typically as tail spikes, although they appear to be absent in this specimen.

Stegosaurus ungulatus from the Jurassic Morrison Formation, Dinosaur National Monument, Utah and Colorado. Specimen on display at the Carnegie Museum of Natural History (Pittsburgh, Pennsylvania). Photo by Perry Quan (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.0 Generic license, image resized).


Photograph of Steggy the Stegosaurus model, which was once on display at the Smithsonian and is now at the Museum of the Earth in Ithaca, New York.

Life-size paper mache model of Stegosaurus that was built for the 1904 St. Louis World's fair. The model, which depicts antiquated ideas about the posture of this dinosaur, was displayed at the Smithsonian Institution's National Museum of Natural History for nearly a century. It is now on permanent exhibit at the Paleontological Research Institution's Museum of the Earth. Learn more here. Photograph by Jonathan R. Hendricks.


Photograph of a skeleton of Apatosaurus on display at the Carnegie Museum. Apatosaurus is a sauropod, a long-necked, herbivorous dinosaur that walks on all fours.

Skeleton of Apatosaurus louisae, a type of sauropod, from the Carnegie Quarry, Jurassic Morrison Formation, Dinosaur National Monument, Utah and Colorado. On display at the Carnegie Museum in Pittsburgh, Pennsylvania. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).


Photograph of a petrified log from Escalante Petrified Forest in the Jurassic Morrison Formation of Utah. The log looks like a weathered pieces of wood. It is light brown or beige in color.

Petrified log at Escalante Petrified Forest State Park, Jurassic Morrison Formation, Garfield County, Utah. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).


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.


Black-and-white photograph of six horses pulling a wooden wagon . Men sit on the two rear horses. The wagon has wooden crates carrying dinosaur bones on it. A second wagon appears to be attached behind the first.

Excavated bones being transported away from Carnegie Quarry by a team of horses and a wagon. Date not specified. Photo by NPS/Earl Douglass Diaries, edited by Evan Hall (National park Service, public domain)


Two-panel figure showing the "Wall of Bones" at Dinosaur National Monument. The wall is a slanted rock wall with a jumble of dinosaur bones that have been exposed and left in place, where they can be viewed by visitors. Panel 1: A woman sits on the wall and appears to be taking notes. Panel 2: A woman sits on the wall with a camera strap around her neck; she is holding a large camera in her right hand..

Wall of bones at Dinosaur National Monument, Utah and Colorado. Left: A geoscientists-in-the-Parks paleontology assistant inventorying fossils, 2019. Photo source: NPS (National Park Service, public domain). Right: A geoscientist photographing bones, 2012. NPS photo by Ashley Dineen, GIP (National Park Service, public domain).


Photograph of a juvenile sauropod skeleton in a death pose partially embedded in rock. The sauropod has a long neck, a small head and is quadrapedal (walks on all fours). The tail is arched upward.

Skeleton of a juvenile Camarosaurs lentus, a type of sauropod, from the Carnegie Quarry, Jurassic Morrison Formation, Dinosaur National Monument, Utah and Colorado. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).


Photograph of a mammal footprint showing three toes and a thumb-like fourth toe from the Jurassic of Dinosaur National Monument. A dime is next to the track for scale (the track is about the same size as the dime.)

Jurassic track of a mammal estimated to be 190 million years old. This track was discovered at Dinosaur National Monument in 2009. Photo source: U.S. National Park Service (public domain).


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.


Photograph of the quarry at Clevand-Lloyd Quarry in the Jurassic of Utah. Sandbags are stacked on on side of the quarry pit and bones occur on the quarry floor. Tools are scatter around. A dinosaur egg in a clear plastic display case sits next to a sign that says "Dinosaur egg found here Sept 1987."

Exhibit at the Cleveland-Lloyd Dinosaur Quarry, Utah, showing bones and equipment as they would have existed during excavation. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of a mounted Allosaurus skeleton at Cleveland-Lloyd quarry. The dinosaur is a predatory theropod. It has a relatively large head with pointed teeth. The arms are robust and the hands have large claws. The dinosaur is bipedal, and the feet also have large claws. The tail is extended in back of the body and held off the ground.

Allosaurus fragilis on display at the Cleveland-Lloyd Dinosaur Quarry, Utah. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of a display at the Cleveland-Lloyd quarry in Utah. The display shows a line of Allosaurus femurs lined up from smallest to largest, illustrating the growth and development of the animal.

Growth series of Allosaurus femurs on display at the Cleveland-Lloyd Dinosaur Quarry, Utah. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).

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.


Diagram showing color-coded silhouettes of dinosaurs from the Early Cretaceous Yellow Cat Member of the Cedar Mountain Formation scaled to size. Dinosaurs shown: Cedarosaurus weiskopfae, Length = 15 meters. Cedrorestes crichtoni, Length = 6 meters. Falcarius utahensis, Length = 5 meters. Gastonia burgei, Length = 5 meters. Geminiraptor suarezarum, Length = 1.5 meters. Hippodraco scutodens, Length = 4.5 meters. Iguanacolossus fortis, Length = 9 meters. Martharaptor greenriverensis. Mierasaurus bobyoungi, Length = ∼9 meters. Moabosaurus utahensis, Length = 9.75 meters. Nedcolbertia justinhofmanni, Length = ∼3 meters. Utahraptor ostrommaysi, Length = 5.5 meters. Yurgovuchia doellingi, Length = 2.5 meters.

Silhouettes showing the diversity and relative sizes of dinosaurs from the Early Cretaceous Yellow Cat Member, Cedar Mountain Formation, Utah. These dinosaurs are about 125 to 120 million years old. Diagram by PaleoNeolithic (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).


2-Panel image of Utahraptor, a predatory dinosaur from the Early Cretaceous of Utah. Panel 1: Photo of a reconstructed skeleton on display in a museum showing rear feet, each with a large retracted claw on the inner toe. Panel 2: Illustration of the animal during life. It is covered with feathers, has a long tail, and forearms that long like short wings. The head has a mount with pointed teeth. Panel 3: Drawing showing the silhouettes of Utahraptor and an adult man. Utahraptor is slightly taller than the person.

Utahraptor, a theropod dinosaur from the Early Cretaceous Yellow Cat Member of the Cedar Mountain Formation, Grand County, Utah. Left: Skeleton (cast or repoduction) on display at the Bean Life Science Museum, Brigham Young University (Provo, Utah). Photo illustration by Jaren Wilkey/BYU (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized). Upper right: Reconstruction of living animal by Emily Willoughby (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped and resized). Lower right: Silhouette of the living animal (about 7 meters or 23 feet long) compared to the size of an adult man (about 1.8 meters or 6 feet tall). Drawing by PaleoNeolitic (Wikimedia Commons, Creative Commons CC0 1.0 Universal/Public Domain Dedication). 


2-Panel figure showing illustrations of the hadrosuar (duck-billed dinosaur) Iguanacolossus from the Early Cretaceous of Utah. Panel 1: Silhouette of the animal with real bones superimposed in their life positions. Few bones were found, including some from the skull, the hind leg, the spine, the pelvis, the tail, and the shoulder. Panel 2: Reconstruction of the living animal. The animal is shown walking on four legs with tail extended straight backward off the ground. The neck is relatively long, the head is slightly elongated, and the mount is somewhat beak-like. The front feet has "thumbs" that look like spikes.

Iguanacolossus fortis, a hadrosaur from the Yellow Cat Member, Cedar Mountain Formation, Grand County, Utah. Top: Silhouette of the animal with bones that were actually found overlain. Bottom: Reconstruction of the living animal by Lukas Panzarin. Source: Figure 3 from McDonald et al. (2010) PLoS ONE 5(11): e14075 (public domain, image slightly modified from original).


Photograph of the skull of Abydosaurus from the Cretaceous of Utah shown in side view. The skull is tall at the back (near the neck) and much shorter near the mount. The teeth are relatively short and appear pointed. The skull has three large openings (fenestrae) on its side.

Skull of the sauropod Abydosaurus mcintoshi from the Cretaceous Mussentuchit Member, Cedar Mountain Formation, about 98 million years old. Specimen on display at Dinosaur National Monument, Utah. Photo by James St. John (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.0 Generic license, image resized).


Photograph of a mounted skeleton of the Early Cretaceous ankylosaur Animantarx on display in a museum. The animal was a plant-eater that walked on four legs. In had spikes sticking out sideways over its shoulders, on the side of its neck, and on the back of its head. The photograph shows the front has of the animal best; the tail can barely be seen in the image.

The ankylosaur Animantarx ramaljonesi from the Late Cretaceous Mussentuchit Member, Cedar Mountain Formation, about 98 million years old. Specimen on display at the Utah State University Eastern Prehistoric Museum (Price, Utah). Photo by Jens Lallensack (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).



Image showing four photographs of a fossil marsupial (pounched mammal) tooth in four orientations, each side, top, and bottom. The tooth is black in color, conical and slightly curved in shape, with a slightly rounded point at the tip.

Four views of a marsupial (pouched mammal) tooth from the Late Cretaceous Mussentuchit Member, Cedar Mountain Formation, about 98 million years old. Specimen on display at the Utah State University Eastern Prehistoric Museum (Price, Utah). Source: Fig. 14D, E, F, G from Avrahami et al. (2018) PeerJ 6: e5883 (Creative Commons Attribution 4.0 International license, image cropped, resized, and labels modified).


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.


2-panel image of a juvenile Parasaurolophus (a hadrosaur or duck-billed dinosaur) from Utah. Panel 1: Specimen partially embedded in rock matrix with the skull, front leg, back leg, and tail labeled. Panel 2: Drawing of the same specimen with bones color-coded to indicate preservation as bone, bone impressions, weathered bone, and skin.

A juvenile Parasaurolophus (a type of duck-billed dinosaur) from the Late Cretaceous Kaiparowits Formation, Grand Staircase-Escalante National Monument, Utah. Left: Whole specimen. Right: Drawing of the same specimens with bones (orange), bone impressions (green), weathered bone (blue), and a skin impression (pink) color-coded. The letters indicate the names of the bones. Illustrations from figure 3 in A. A. Farke et al. (2014) Peer J 1: e182 (Creative Commons Attribution 3.0 Unported license, image cropped, reconfigured, and labeled).


2-panel figure of the Cretaceous ankylosaur (armored dinosaur) Akainacephalus. Panel 1: Photograph of a reconstructed skeleton on display in a museum. This dinosaur is herbivorous, walks on all fours, has a tail with a bony club at the end, and large bony spikes on its back. Panel 2: Drawing illustrating the skeleton in top and side views. Bones that were actually found are colored orange, whereas the bones colored white are inferred.

Akainacephalus johnsoni, an ankylosaur, from the Late Cretaceous Kaiparowits Formation, Grand Staircase-Escalante National Monument, Utah. Left: Reconstructed skeleton on display at the Utah Museum of National History. Photo by Keratopusuyuuta (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized). Right: Top and side views of the reconstructed skeletons; shaded bones are those that were discovered. Illustrations from figure 28 in J. P. Wiersma and R. B. Irmis (2018) Peer J 6: e5016 (Creative Commons Attribution 4.0 International license, image cropped and reconfigured).


2-panel figure of Titanoceratops from the Cretaceous of New Mexico. Panel 1: Photograph of a mounted skeleton of Titanoceratops on display at a museum.Panel 2: Drawing reconstructing the animals in life. The animals are four-gelled with stout bodies and relatively short tails and necks. The head has a large frill, a long horn over each eye, and a small nose horn. The mouth is beak-like.

Titanoceratops ouranos, a ceratopsian (horned dinosaur) from the Late Cretaceous Fruitland or Kirtland Formation, San Juan Basin, New Mexico. Left: Skeleton (partially reconstructed) on display at the Sam Noble Museum in Norman, Oklahoma. This specimen holds the Guinness World Record for largest land animal skull, granted in 2013. Photo by Kurt McKee (flickr, Creative Commons Attribution-ShareAlike 2.0 Generic license, image cropped and resized). Right: Reconstruction of living animal by LadyofHats (Wikimedia Commons, public domain)


Photograph of the skull of Bistahieversor. The top of the skull is preserved along with one side of the jaw and a vertebra. The skull is from a predatory dinosaur with pointed teeth.

Skull of Bistahieversor sealeyi, Late Cretaceous Kirtland Formation, San Juan Basin, New Mexico. On display at the New Mexico Museum of Natural History and Science, Albuquerque, New Mexico. Photo by Lee Ruk (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.0 Generic license, image resized).


2-panel figure about pterosaurs from the Late Cretaceous of western North America. Panel 1: Map of western North America showing locations of four pterosaur finds. In the southwest, Navajodactylus was found in northern New Mexico and a larger, unnamed pterosaur was found in Utah. Panel 2: Photos of a bone from a Cretaceous pterosaur found in Utah; it might be an ulna, one of the forearm bones.

Left: Map showing similarly-aged pterosaur discoveries from the Late Cretaceous of the western United States and Canada. In the southwest, Navajodactylus is from the Kirtland Formation, San Juan Basin, New Mexico. The unnamed animal labeled RAM 22574 is from the Kaiparowits Formation, Grand Staircase-Escalante National Monument, Utah. Right: Views of each side of a pterosaur bone, possibly an ulna (forearm bone), from the Kaiparowits Formation, Utah. Images from Figures 1 and 2 in A. A. Farke (2021) PeerJ 9: e01766 (Creative Commons Attribution 4.0 International license, images cropped and resized).


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.


Diagram showing Cretaceous coal swamps in which Cretaceous coals accumulated. The cutaway view shows the Mancos Shale, The Dakota Sandstone, and the Mesaverde Group making up the subsurface. On the surface, mountains are on the left and a shallow sea on the right, with an alluvial plain between them. Coal swamps occur near the edge of the sea. A delta is shown extending into the sea.
Reconstruction of the paleoenvironment in with Utah's Cretaceous coal deposits formed. When trees and other vegetation fall into the stagnant water of a coastal swamp, deterioration of the organic material is delayed, and a thick layer of peat is formed. Over time, the peat is compressed into coal. Diagram modified from a diagram by Wade Greenberg-Brand, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.

4-panel image showing photos of a selection of plant fossils from the Late Cretaceous of Utah. Panel 1: A small cover branch with needed-like leaves. Panel 2: An angiosperm leaf with a broad blade, a stalk, and a toothed margin. Panel 3: Detail of a small conifer branch with needle-like leaves. Panel 4: A round leaf with dark veins that are difficult to see.

Late Cretaceous plants from the Ferron Sandstone Member (Mancos Shale), Utah. Left to right: A) A conifer shoot (Elatides curvifolia). B) An angiosperm (flowering plant) leaf. C) A conifer shoot (Elatides curvifolia), same specimen as shown in A. D) Fern pinnule (leaf segment). Scale bars = 5 mm (A, B, C), 3 mm (D). Photos by N.A. Jud, from Fig. 3 in Jud et al. (2018) Science Advances 4: eaar8568 (Creative Commons Attribution-NonCommercial 4.0 International license, image reconfigured, Figs. A and B resized).


2-panel figure showing photos of Cretaceous plant fossils from the San Juan Basin of northern New Mexico. Panel 1: A petrified tree stump. Panel 2: Impression of a palm leaf, showing plicate structure. In other words, the leaf looks like it is folded like a folding fan.

Fossil plants, Late Cretaceous Fruitland Formation, San Juan Basin, New Mexico. Left: A petrified stump. Right: Sabalites, a palm leaf. Left image and right image by NickLongrich (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, images cropped and resized).


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.


Photograph of fossil oysters from the Cretaceous Cliffhouse Sandstone, New Mexico. The photo shows some specimens with shells and some that are just rounded internal molds. A mechanical pencil is used for scale (shells are about as long as the pencil or shorter).

Shells and internal molds of the oyster Inoceramus, Cretaceous Cliffhouse Sandstone, Chaco Culture National Historical Park, New Mexico. Photo source: National Park Service (public domain).


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.


Photographs showing two views of an ammonite shell from the Cretaceous of New Mexico. The shell is spiraling in one plane. It is showing in side (left) and end (right) views. The sinuous suture pattern can be seen. The shell is light gray.

An ammonite (Spathites puercoensis), Late Cretaceous (likely Mancos Shale), Sandoval County, New Mexico. Photo of specimen PRI 70328 (Paleontological Research Institution, via Cretaceous Atlas of Ancient Life, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license).


Photograph of the shell of an ammonite from the Cretaceous of New Mexico shown in side view. The shell is medium brown in color and spiraling in one plane. The sinuous ammonitic sutures can be seen.

An ammonite (Metoicoceras swallovi), Late Cretaceous Semilla Sandstone Member, Mancos Shale, Sandoval County, New Mexico. Photo of specimen KUMIP 150805 (University of Kansas Natural History Museum, via Cretaceous Atlas of Ancient Life, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license).

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.


2-Panel image fo the multituberate mammal Taeniolabis from the Paleocene of New Mexico. Panel 1: Photograph of the jaw mounted on a wall. The front of the jaw curves upward into a point. The teeth are flat and rectangular with multiple cusps. Panel 2: Reconstruction of the living animal showing is as a woodchuck or beaver-like creature with a thin, pointed tail.

Taeniolabis, a multituberculate mammal, Paleocene Nacimiento Formation, San Juan Basin, New Mexico. Left: Lower jaw with characteristic teeth. Photo of USNM PAL 372278 by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain)Right: Reconstruction of Taeniolabis taoensis by Nobu Tamura (Wikimedia Commons, Creative Commons Attribution ShareAlike 4.0 International license, image cropped).


2-Panel image of the Paleocene fossil mammal Periptychus (a condylarth) from New Mexico. Panel 1: Photos showing two views of the skull, one from the top and one from the bottom. The jaw is not attached. The skull has one large fenestra (hole) on each side. The teeth appear blunt. Panel 2: Drawing showing a reconstruction of the complete skeleton. The animal walked on four legs and had a long tail.

Periptychus carinidens, an archaic condylarth mammal from the Paleocene Nacimiento Formation, New Mexico. Left: Upper part of skull shown from above and below. Right: Reconstruction of complete skeleton. Source: Figures 3B, C and Figure 47 from Shelley et al. (2018) PLoS ONE 13(7): e0200132 (Creative Commons Attribution 4.0 International license, images cropped, resized, and labels changed).


2-Panel image fo the mammal Wortmania from the Paleocene of New Mexico. Panel 1: Photo of the top of the skull from the side; large teeth can be seen. Panel 2: Reconstruction of the living animal showing a stout, 4-legged creature with long teeth, small ears, a strangely square jaw, and a long, hairless tail. The feet have long claws.

Wortmania otariidens, a eutherian mammal from the Paleocene Nacimiento Formation, New Mexico. Left: Skull. Right: Reconstruction of the living animal by Matt Celeskey (2012). Source: Figures 1A and 15 from T. E. Williamson and S. L. Brusette (2013) PLoS ONE 8(9): e75886 (Creative Commons Attribution license, images cropped, resized, and relabeled).


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.


Photograph of the skull of a fossil alligatorid. The photo shows the top part of the skulls, which is elongated-triangular with the nose pointed toward the right. The jaw, of which only side is preserved, is laid flat below the upper part of the skull. A 10-cm scale is at the bottom of the image.

Skull of a fossil alligatorid (Procaimanoidea utahensis), Eocene Uinta Formation, Uintah County, Utah. Photo of USNM V 15996_1 by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain).


Photograph of a reconstructed mammal skeleton on display in a museum. The mammal is a herbivore that has four legs, a large rectangular skull, a medium-length neck, and a relatively long tail. The feet have toes, not hooves.

Protoreodon, an artiodactyl (even-toed hoofed mammal) from the Eocene Uinta Foramtion, Uintah County, Utah. Specimen on display at the Utah Field House of Natural History State Park Museum, Vernal, Utah. Photo by J. R. Hendricks and E. J. Hermsen.


Mounted skeleton of a titanothere on display in a museum. The titanothere is an extinct mammal with a large, rhino-like head. It has bony projects on the top front of its face. It walked on four legs, had a short tail, and had toes (not hooves). It was a large animal that ate plants.

Pseudodiplacodon progressum, an titanothere from the Eocene Uinta Foramtion, Uintah County, Utah. Specimen on display at the Carnegie Museum of Natural History, Pittsburgh, Pennsylvania. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


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 evaporties. 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. 


Maps of Wyoming, Colorado, and Utah at four different times in the Paleocene and Eocene showing the evolution of the Green River Formation lakes. At 56 million years ago, a small Lake Uinta occurs in central Utah. At about 52 million years ago, Lake Uinta occurs in northeastern Utah and northwestern Colorado, Fossil Lake occurs on the Utah-Wyoming border, and Lake Gosiute occurs in southwestern Wyoming. At about 50 million years ago, Lake Uinta is still present in Utah and Colorado, Lake Gosiute has enlarge, and Fossil Lake is gone. At about 45 million years ago, only a small remnant of Lake Uinta occurs, mostly in northeastern Utah.

The size and location of various lakes in which the Green River Formation sediments were deposited during the Eocene epoch. Maps modified from maps by Wade Greenberg-Brand, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US, after figure 3 in L. Grande (2013) The Lost World of Fossil Lake.


Photograph of a fossil cricket from the Eocene Green River Formation of Colorado. The fossils shows the characteristic long rear legs with short spines sticking out from their sides.
Photograph of a slab of rock from the Eocene Green River Formation of Utah. The rock is beige or light brown in color with tracks of a bird (impressions of three-toed feet).
2-panel image showing photos of Green River plant fossils. Panel 1: Photograph of a display of plant fossils made up of a series of rectangular blocks of rock with fossils mounted on a wall. Right: Photograph of a fossil Macginitea leaf, a palmately lobed leaf with five lobes and a toothed margin.

Eocene Green River Formation (Parachute Creek Member) plant fossils at display at the Utah State Field House of Natural History in Vernal, Utah. Left: Wall of plant fossils. Right: Specimen of Macginitea, an extinct relative of the modern sycamore (Platanus). Photos by Jonathan R. Hendricks (2014).


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.


2-panel figure showing black-and-white photos of ground sloth fossils from Grand Canyon National Park, 1936. Panel 1: Skull of a ground sloth. Panel 2. Pieces of hide with attached hair.

Fossils of giant ground sloths (Nothrotherium) from Rampart Cave, Grand Canyon National Park, Arizona, 1936. Left: Fossil skull. Photo source: Grand Canyon National Park (flickr, Creative Commons Attribution 2.0 Generic license, image cropped). Right: Fossil hide and hair. Photo source: Grand Canyon National Park (flickr, Creative Commons Attribution 2.0 Generic license, image cropped).


2-panel figure with photos of cave excavations of ground sloth fossils. Panel 1: Black and white photo of men excavating fossil ground sloth bones in 1936. Panel 2: Color photo of a person next to a pile of ground sloth dung in a cave.

Cave excavations, Grand Canyon National Park, Arizona. Left: Men collecting ground sloth bones, Rampart Cave, 1936. Photo source: Grand Canyon National Park (flickr, Creative Commons Attribution 2.0 Generic license, image cropped). Right: Fossil dung in a cave; the dung is estimated to be 20,000 years old. Photo by Robyn Henderek (National Park Service, public domain).

Resources

Resources from the Paleontological Research Institution & partners

Cretaceous Atlas of Ancient Life, Western Interior Seaway (Colorado, Iowa, Kansas, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Utah, Wyoming): https://www.cretaceousatlas.org/geology/

Digital Atlas of Ancient Life Virtual Collection: https://www.digitalatlasofancientlife.org/vc/ (Virtual fossil collection featuring 3D models of fossil specimens sorted by group)

Digital Encyclopedia of Ancient Life: https://www.digitalatlasofancientlife.org/learn/

[email protected]: Quick guide to common fossils: https://earthathome.org/quick-faqs/quick-guide-common-fossils/


Go to the full list of resources about fossils in the southwestern U.S.

Go to the full list of general resources about fossils