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 (Figure 3.2). The oldest fossils known from the Colorado Plateau region are stromatolites—layered domes of carbonate sediment formed by mats of bacteria known as cyanobacteria. These are found in a layer called the Bass Limestone, exposed near the bottom of the Grand Canyon (Figure 3.3). These 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, thought by many paleontologists to be a resting state of single-celled algae (Figure 3.4). Several levels within the Chuar Group also contain stromatolites.

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 (see Figure 3.43), of which more than 50 species have been reported. Brachiopods are also frequently found in these rocks (Figure 3.5), and, more rarely, sponges and echinoderms similar to those found in the Cambrian of western Utah (see Figure 3.44).

Figure 3.2: Major Proterozoic and Paleozoic stratigraphic units of the Grand Canyon and Colorado Plateau.

Figure 3.3: Stromatolites from the mid-Proterozoic Bass Limestone of the Grand Canyon.

Stromatolites

Stromatolites are regularly banded accumulations of sediment created by the trapping and cementation of sediment grains in bacterial mats (especially photosynthetic cyanobacteria). Cyanobacteria secrete a sticky mucus that binds settling sediment; their photosynthesis creates a chemical environment that leads to the precipitation of calcium carbonate. The calcium carbonate then hardens the underlying layers of bacterial mats, while the living bacteria move upward so that they are not buried. Over time, this cycle of growth combined with sediment capture creates a rounded structure filled with banded layers.

Stromatolites peaked in abundance around 1.25 billion years ago, and likely declined due to predation by grazing organisms. Today, stromatolites exist in only a few locations worldwide, such as Shark Bay, Australia. Modern stromatolites form thick layers only in stressful environments, such as very salty water, that exclude animal grazers.

Figure 3.4: Microfossils from the late Proterozoic Chuar Group. A) A "vase-shaped microfossil" (VSM), thought to be the shell of an amoeba-like, single-celled organism (protist). B) Chuaria, a common acritarch fossil of uncertain affinities, most likely the resting stage of a single-celled eukaryote (protist).

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 (Figure 3.6) have been recovered from Devonian rocks in northern and central Arizona, and also in central Colorado, northwest of Denver (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.

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 (Figures 3.7 and 3.8; see also Figure 3.50). In at least one Redwall locality, straight nautiloids more than 60 centimeters (24 inches) long have been found. Some Pennsylvanian layers in the Colorado Plateau also accumulated in terrestrial environments, including swamps, rivers, lakes, and floodplains. Some of these sediments include significant coal deposits.

Figure 3.5. Cambrian brachiopods from the Colorado Plateau, each approximately 1 centimeter (0.4 inches) across. A) Acrothele subsidua. B) Obolella chromatica. C) Lingulella ella.

Conodonts

Conodonts are tiny, tooth-shaped microfossils (0.2–5 millimeters long), found in Cambrian- through Triassic-aged marine rocks. They have long been among the most important index fossils in these rocks, allowing the latter to be dated through biostratigraphy. For many years, paleontologists variously thought they were fragments of arthropods, fish teeth, mollusks, or even plants. In 1983 the discovery of a whole conodont animal in Scotland revealed that they belonged to small, fish-like animals that were distant relatives of bony fish.

Isolated conodont elements (Silurian).

Restoration of a live conodont animal. Length 2–4 centimeters (1–2 inches).

Figure 3.6: Placoderm fish from Devonian rocks near Payson, in northern Arizona. A) Textured dermal plates. B) Ventral median dermal plate. C) Life restoration of a placoderm, Bothriolepis, approximately 30 centimeters (12 inches) long.

Figure 3.7: Mississippian marine fossils from the Redwall and Surprise Canyon formations. A) Solitary rugose coral, Zaphrentis, about 2.5 centimeters (1 inch) tall. B) Colonial rugose coral, Michelinia, about 4 centimeters (1.6 inches) wide. C) Brachiopod, Schizophoria, about 2.5 centimeters (1 inch) wide. D) Brachiopod, Punctospirifer, about 4 centimeters (1.6 inches) wide. E) Gastropod, Straparolus, about 1 centimeter (0.4 inches) wide. F) Brachiopod, Buxtonia, about 4 centimeters (2 inches) wide.

Figure 3.8: Fenestellid bryozoan from the Redwall Limestone, Grand Canyon; 5 centimeters (2 inches) wide.

The Surprise Canyon (which extends into the early Pennsylvanian) also contains layers with terrestrial plant fossils, including lycopods, seed ferns, and horsetails (Figure 3.9).

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. A number of beds also contain abundant and diverse shelly fossils, including brachiopods (Figure 3.10) and foraminifera.

Figure 3.9: Restorations of Pennsylvanian coal swamp plants. A) Lepidodendron, a lycopod (club moss), reached 30 meters (100 feet) tall. B) Close-ups of leaf scars on a Lepidodendron trunk. C) Medullosa, a tree fern, reached 10 meters (35 feet) tall. D) Calamites, a sphenopsid (horsetail), reached 20 meters (65 feet) tall. E) Cordaites, a gymnosperm seed plant, reached 10 meters (35 feet) tall.

The Permian-aged 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 (Figure 3.11), 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 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 (Figure 3.12). 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.

Figure 3.10: Pennsylvanian brachiopods from northern Arizona. A) Anthracospirifer, 2 centimeters (0.8 inches) wide. B) and C) Composita, 1.5 centimeters (0.7 inches) long. D) Pulchratia, 2 centimeters (0.8 inches) wide. E) Orthotetes, 4 centimeters (1.6 inches) wide.

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 (Figure 3.13). Ammonoid cephalopods are especially important for biostratigraphy in this and other Permian marine layers.

Figure 3.11: Permian dune deposits. A) Tetrapod tracks from the Coconino Sandstone. B) Life restoration of the track-maker, a small reptile.

Figure 3.12: Permian terrestrial reptiles. A) Dimetrodon, 3 meters (9 feet long), skull and restoration. B) Diadectes, 2 meters (6.5 feet) long, skull and restoration. C) Eryops, 1.5–2 meters (5–6.5 feet) long, skeleton and restoration.

Figure 3.13: Permian marine invertebrates from the Permian Kaibab Limestone. A) Brachiopod, Dictyoclostus, approximately 7.5 centimeters (3 inches) wide, with encrusting bryozoan. B) Brachiopod, Productus ivesi, approximately 5 centimeters (2 inches) wide. C) Brachiopod, Productus bassi, approximately 5 centimeters (2 inches) wide. D) Snail-like mollusk, Bellerophon, approximately 3.5 centimeters (1.5 inches) wide. E) Gastropod, Euomphalus, approximately 1.5 centimeters (0.6 inches) wide. F) Coiled nautiloid, approximately 10 centimeters (4 inches) wide. G) Bivalve, Allorisma, approximately 10 centimeters (4 inches) wide. H) Nautiloid, Tainoceras duttoni, approximately 10 centimeters (4 inches) wide. I) Nautiloid, Stearoceras sanandresense, approximately 10 centimeters (3 inches) wide.

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 (Figure 3.14).

Figure 3.14: Tetrapod fossils of the Moenkopi Formation. A) The trace fossil Cheirotherium barthii, likely an archosaur footprint. B) Arizonasaurus, restoration, approximately 3 meters (10 feet) long.

The late Triassic Chinle Formation (or Group) is exposed across much of the Southwest (Figure 3.15). This formation accumulated in a shifting set of habitats, from forests to rivers and lakes. Fossils of freshwater fish, clams, and crustacean burrows are common in parts of the Chinle, suggesting abundant water, but other evidence indicates that rainfall was probably highly seasonal, and the environment was frequently dry (Figure 3.16). Fossil fishes found here include the coelacanth Chinlea, the shark Xenacanthus, the lungfish Ceratodus, and Australosomus—a relative of today’s goldfish and tuna (Figure 3.17). Land-living reptiles include a parade of forms (Figures 3.18 and 3.19), including the bizarre armored group known as aetosaurs (e.g., Desmatosuchus, Stagnolepis), the dicynodont 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. The Chinle is also famous for its fossil trees, spectacularly exposed in Arizona’s Petrified Forest National Park (Figure 3.20). Forests of these conifers, most belonging to the species Araucarioxylon arizonicum, were also filled with ferns, cycads, horsetails, and ginkgoes.

Figure 3.15: Location of Chinle Formation outcrops across the Southwestern US.

Figure 3.16: Reconstruction of a late Triassic depositional environment, similar to that of the Chinle Formation.

The Jurassic Kayenta Formation overlies the Chinle and is exposed in northern Arizona (Painted Desert and vicinity). It contains fossils of early mammal relatives as well as dinosaurs, including the larger theropod Dilophosaurus (Figures 3.21 and 3.22). Although it was once desert dune sand, the overlying Navajo Sandstone also contains dinosaurs (Figure 3.23). 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 (Figure 3.24). 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 (Figure 3.25), including carnivorous theropods such as Ceratosaurus and Allosaurus, herbivores such as Dryosaurus, Stegosaurus, and Camptosaurus, and gigantic long-necked sauropods, such as Barosaurus, Diplodocus, Apatosaurus, Brachiosaurus, and Camarasaurus. One giant sauropod from the Morrison, originally named Seismosaurus, was initially thought to be the world’s largest dinosaur. That honor may instead go to another Morrison titan, Amphicoelias, which may have reached more than 50 meters (150 feet) long and weighed more than 120,000 kilograms (160 tons). 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.

Figure 3.17: Fish of the Chinle Formation. A) Skeleton (top) and restoration (bottom) of the freshwater shark Xenacanthus; approximately 30 centimeters (1 foot) long. B) Restoration of the ray-finned fish Australosomus; approximately 6 centimeters (1.5 inches) long. C) Restoration of the coelacanth Chinlea; approximately 1.5 meters (5 feet) long. D) Restoration of the lungfish Ceratodus; approximately 1.2 meters (48 inches) long. E) Ceratodus tooth; approximately 5 centimeters (2 inches) wide.

Figure 3.18: Skull and restoration of the Triassic theropod dinosaur Coelophysis, approximately 2.5 meters (8 feet) long.

Figure 3.19: Non-dinosaur tetrapods from the Chinle Formation. A) Skeleton and restoration of the armored aetosaur reptile Desmatosuchus, approximately 4.5 meters (15 feet) long. B) Skeleton and restoration of the synapsid reptile Placerias, approximately 3.5 meters (11.5 feet) long. C) Skull and restoration of the carnivorous reptile Postosuchus, approximately 4–5 meters (13–16 feet) long. D) Tooth and restoration of the phytosaur Smilosuchus, approximately 4 meters (13 feet) long.

Figure 3.20: Fossil log from Petrified Forest National Park, northern Arizona.

Figure 3.21: Skeleton and restoration of the theropod dinosaur Dilophosaurus, approximately 6 meters (20 feet) long.

Figure 3.22. Partial lower jaw of a small early relative of mammals, Dinnetherium nezorum, from the early Jurassic Kayenta Formation, near Tuba City, Arizona.

Fossil Mammals: It's (almost) all about the teeth

Mammals have evolved into an amazing variety of shapes and sizes, and much of this diversity and success is due to their teeth! Mammals are "warm-blooded," meaning they can regulate their own body temperature. This requires a high metabolism, the derivation of energy from food. Mammals meet their heavy food requirements with the help of a distinctive chewing system, starting with their teeth. Unlike reptiles, most mammals—including humans—have several different kinds of teeth in their mouths. Also unlike reptiles, some of these teeth are highly complex, with many bumps and grooves on the chewing surfaces. This range of tooth forms allows mammals to efficiently eat many different kinds of food. It also allows different kinds of mammals to eat different foods. This means that different mammals usually have very different teeth, and that you can often identify a mammal species using only its teeth. This is extremely important for studying fossils, because mammal teeth are frequently found as fossils. Mammal paleontology is therefore largely the study of fossil teeth.

A) Upper molar of peccary (Tagassu), deer (Odocoileus), and camel (Poebrotherium). B) Upper right-side dentition of Hyanodon, a dog-like carnivore. C) Incisors and canines of the entelodont Archaeotherium.

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. Over several years, crews from the Museum excavated thousands of fossil bones and shipped them back to Pittsburgh for study and display. The site was declared a national monument in 1915, but new discoveries continue to this day—in late 2015, one of the oldest known pterosaurs was found in the monument’s Triassic rocks. The famous "Wall of Bones" (Figure 3.26), 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 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.

Figure 3.23: The early sauropod dinosaur Seitaad ruessi from the Navajo Sandstone of Utah, approximately 4 meters (12 feet) long. This skeleton is reconstructed, and only the black bones are known from fossils.

Figure 3.24: Geographic extent of the late Jurassic Morrison Formation.

Figure 3.25: Some common and familiar dinosaurs from the Morrison Formation. A) Apatosaurus (approximately 23 meters [75 feet] long), skeleton and restoration; B) Allosaurus (approximately 8.5 meters [28 feet] long), skeleton and restoration; C) Stegosaurus (approximately 9 meters [30 feet] long), skeleton and restoration.

Figure 3.26: The Wall of Bones at Dinosaur National Monument, Colorado and Utah.

Dinosaurs are also well represented across much of the Southwest through fossil footprints (Figure 3.27). The Purgatoire River tracksite, also called the Picketwire Canyonlands tracksite, is one of the largest dinosaur tracksites in North America. The site is located in the Comanche National Grassland along the Purgatoire River south of La Junta, in Otero County, Colorado.

The Morrison Formation also contains abundant plant fossils, including conifers and ferns—flowering plants had not yet evolved (Figures 3.28 and 3.29). 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.

Cretaceous rocks are widely exposed across southern Utah, and contain a great abundance and diversity of dinosaurs. Herbivores are represented by hadrosaurs, ankylosaurs, and several kinds of horned dinosaurs (ceratopsians) (Figures 3.30 and 3.31). Carnivorous dinosaurs include a variety of theropods, from the large and ferocious-looking Utahraptor to the even larger tyrannosaurs (Figures 3.32 and 3.33). Theropods are well represented in the southern Colorado Plateau, but not until recently have paleontologists been able to determine exactly which species are represented by the fossils found there. Most large teeth and bones found in the area were originally thought to be from close relatives of T. rex, such as the Albertosaurus, which are well known farther north in Wyoming and Montana. In 2010, however, some of these fossils were recognized as belonging to a previously unknown theropod, named Bistahieversor. This predator is estimated to have been around 9 meters (30 feet) long, weighing at least a ton. Although the Cretaceous was also a golden age for flying reptiles (pterosaurs), they are only rarely found in the Colorado Plateau’s Mesozoic rocks.

Figure 3.27: Major localities in which Mesozoic footprint assemblages (mostly dinosaurs) have been found.

Figure 3.28: Some of the most common plants from the late Jurassic Morrison Formation.

Figure 3.29: Reconstruction of the biological community represented by fossils in the Morrison Formation.

Figure 3.30: Cretaceous horned dinosaurs from the Colorado Plateau. A) Skull of Utahceratops, approximately 2.3 meters (7 feet) long. B) Skeletal reconstruction of Pentaceratops, approximately 6 meters (20 feet) long. C) Skull of Kosmoceratops, approximately 2 meters (6 feet) long.

Figure 3.31: The hadrosaur Parasaurolophus; approximately 10 meters (33 feet) long, skeleton and reconstruction.

Figure 3.32: Tyrannosaurs in New Mexico. A) Locations where tyrannosaur fossils have been found. B) Cast of the only footprint ever confidently assigned to Tyrannosaurus rex, found near the town of Cimarron, Colfax County, New Mexico. The print measures 86 centimeters (34 inches) long. This cast is on exhibit at the Smithsonian Institution’s National Zoo in Washington, DC.

Figure 3.32: Skeleton and reconstruction of the theropod dinosaur Utahraptor, approximately 6.5 meters (20 feet) long.

Fossils of mammals—mostly very small teeth—are also found in the Cretaceous rocks of southern Utah (Figure 3.34). These fossils represent marsupials, placentals, and several extinct groups that left no modern descendants.

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 (Figure 3.35). In addition to conifers and tree ferns (Figure 3.36), which were major contributors to these coal deposits, the region’s Cretaceous land plants include angiosperms (flowering plants) such as palms and magnolias. Angiosperms probably originated in the late Jurassic, but had diversified and taken over many terrestrial ecosystems by the mid-Cretaceous.

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 (Figure 3.37). These rocks frequently contain abundant vertebrate and invertebrate fossils. Marine vertebrates include bony fish, sharks, ichthyosaurs, mosasaurs, and plesiosaurs (Figure 3.38), 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 Mancos Shale preserves the abundant remains of marine snails, sharks, mosasaurs, crinoids, bivalves, and ammonoids (Figure 3.39). 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.

Figure 3.34: Restoration of Alphadon, a tiny marsupial mammal from the Cretaceous of southern Utah; up to 30 centimeters (12 inches) long.

Figure 3.35: Reconstruction of the paleoenvironment in which 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.

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 (see Figure 3.1). 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 (Figure 3.40). Multituberculates were a group of mostly small, rodent-like mammals that originated in the Cretaceous and disappeared in the early Oligocene—their name refers to their distinctive multicusped teeth. Other mammals abundantly represented by fossils in the 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. Late Cretaceous and early Cenozoic rocks in the San Juan Basin also contain abundant flowering plants (Figure 3.41).

Figure 3.36: Cretaceous land plants of Utah. A) Cross-section of the tree fern Tempskya, approximately 30 centimeters (1 foot) wide. B) Reconstruction of the tree fern Tempskya. C) Fossil leaf of a redwood, Sequoia coneata, approximately 10 centimeters (4 inches) long, from eastern Utah. Sequoias were major contributors to Utah's coal beds.

Figure 3.37: Cretaceous stratigraphy of the Colorado Plateau.

Figure 3.38: Cretaceous marine vertebrates of the Southwest. A) Restoration of Elasmosaurus, a large plesiosaur from the Niobrara Chalk of Kansas and eastern Colorado, approximately 14 meters (46 feet) long. B) Mosasaur tooth, approximately 5 centimeters (2 inches) long. C) Restoration of the late Cretaceous mosasaur Tylosaurus, approximately 15 meters (50 feet) long.

Figure 3.39: Cretaceous marine mollusks of the Mancos Shale and Dakota Group. A) Bivalve, Inoceramus sp., 10 centimeters (4 inches) wide. B) Bivalve, Exogyra trigeri, 8 centimeters (3 inches) wide. C) Bivalve, Exogyra laevis, 4 centimeters (1.5 inches) wide. D) Bivalve, Gryphaea newberryi, 4 centimeters (1.5 inches) wide. E) Heteromorph ammonite, Didymoceras, 15 centimeters (6 inches) wide. F) Ammonite, Plesiacanthoceras, 15 centimeters (6 inches) in diameter. G) Ammonite, Paracompsoceras, 20 centimeters (8 inches) in diameter. H) Restoration of a living ammonite. I) Restoration of an orthocone cephalopod, Baculites, usually 3–4 centimeters (2 inches) in diameter and up to 60 centimeters (2 feet) long.

Figure 3.40: San Juan Basin Paleocene mammals. A) Skull and restoration of the multituberculate Ptilodus mediaevus; body 30–50 centimeters (12–20 inches) long, skull approximately 6 centimeters (2.4 inches) long. B) Skull and restoration of Taeniolabis, a member of a condylarth group known as taeniodonts, which had front teeth developed into tusks; body approximately 100 centimeters (39 inches) long, skull approximately 15 centimeters (6 inches) long. C) Skull and restoration of another taeniodont, Onychodectes; body 60 centimeters (24 inches) long, skull 10 centimeters (4 inches) long. D) Jaw and restoration of the condylarth Periptychus; jaw 10 centimeters (4 inches) long. E) Skeleton and reconstruction of Tetraclaenodon, a member of a condylarth group known as phenacodonts, which may have been closely related to the ancestors of the modern perissodactyls (including horses and rhinos); body 75 centimeters (30 inches) long, skull 11.5 centimeters (4.5 inches) long.

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 (Figure 3.42). 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.

Figure 3.41: Idealized illustrations of fossil angiosperm plants from the late Cretaceous and early Paleogene of the San Juan Basin of northern New Mexico. A). Osmunda, leaf size 20–150 centimeters (8–59 inches). B) Carya, leaflets 20–36 centimeters (8–14 inches). C) Allantodiopsis, leaf size 5–17 centimeters (6–7 inches). D) Sequoia, individual leaves approximately 0.65 centimeters (0.25 inches) long. E) Platanus, 20 centimeters (8 inches). F) Magnolia, 15–17 centimeters (6–7 inches). G and H) Ficus, approximately 7.5 centimeters (3 inches). I) Cissus, 20 centimeters (8 inches).

Figure 3.42: The Shasta ground sloth, Nothrotherium shastense. These sloths were 2.75 meters (9 feet) long as adults and weighed approximately 250 kilograms (550 pounds). A) Preserved sloth dung inside Rampart Cave, Arizona. B) Skeleton. C) Restoration.