DINOSAURS OF UTAH


Tyrannosaurus at Utah Museum of Natural History

Museums throughout the United States and around the world display mounted skeletons of the meat-eating dinosaur Allosaurus cast from specimens collected at the Cleveland-Lloyd Dinosaur Quarry in east-central Utah. Most are replicas of the original fossils, but a few include some original fossil bone. Excavations at the Cleveland-Lloyd Quarry have yielded the remains of over 70 individual dinosaurs, two-thirds of which are carnivores, mostly of the genus Allosaurus. In 1988 the Allosaurus was named Utah's official state fossil.

The Cleveland-Lloyd Dinosaur Quarry is only one of many important dinosaur quarries in a 150-million-year-old rock unit known as the Morrison Formation, which is exposed throughout the intermountain west. Probably the most famous and spectacular of these is the quarry at Dinosaur National Monument in northeastern Utah, where 2,000 bones of dinosaurs are exposed in place on the cliff face that makes up one wall of the quarry visitor center. Utah is also the site of the earliest Morrison dinosaur discovery, Dystrophaeus viaemalae, a sauropod dinosaur discovered on the 1859 Macomb Expedition to southeastern Utah.

Although Utah is most famous for its Morrison Formation dinosaur fauna, Utah has a prolific fossil record that spans the entire "Age of Dinosaurs." The dinosaurs thrived for over 150 million years. A brief summary of the geologic time scale will help to put their history into perspective. The Earth is approximately 4.5 billion years old. Although the earliest life forms appeared perhaps 3.5 billion years ago, the fossil record becomes abundant only in the last 570 million years.

Geologic time is classified mainly on the basis of existing life forms, so the first 3.4 billion years of the earth's history is commonly referred to as simply the Precambrian ("before the Cambrian" - the first period of the Paleozoic). The last 570 million years is divided into three Eras: the Paleozoic ("old life"), Mesozoic ("middle life") and Cenozoic ("recent life"). Dinosaurs lived only during the Mesozoic, which spans a period of time from approximately 245 million years to 65 million years ago. The Mesozoic Era is commonly called the Age of Dinosaurs, although dinosaurs did not originate until near the end of the Triassic. The Mesozoic is divided into three time periods: the Triassic, Jurassic, and Cretaceous. Utah's fossil record spans the entire Mesozoic, yielding the fossil bones or the tracks of dinosaurs, and many other plants and animals. All of Utah's dinosaur-bearing strata are found in eastern and southern Utah. A series of mountain-building episodes that occurred across western Utah, excluding the southwestern corner, created the highlands that were the source area for most of the Mesozoic deposits of southern and eastern Utah, where dinosaurs and other animals lived.

Dinosaurs first appear at the end of the Middle Triassic or in early Late Triassic time, approximately 230 million years ago. The primitive dinosaurs of the Triassic were neither as abundant nor as varied as they would become in Jurassic and Cretaceous times. The dinosaurs are a large group of reptiles belonging to the Archosauria ("ruling reptiles"), which also includes the pterosaurs, or "winged lizards"; crocodilians, the only surviving archosaurs; and the thecodonts, primitive "socket tooth" archosaurs, who were the ancestors of all other archosaurs. Dinosaurs are classified into two major orders, the Saurischia ("lizard-hipped") and the Ornithischia ("bird-hipped"). The saurischians include of the herbivorous Sauropodomorpha (sauropods and prosauropods) and the carnivorous, bipedal Theropoda. The ornithischians are a more varied group of herbivores, including the stegosaurs, ornithopods, ankylosaurs, and ceratopsians. Modern birds are descended from the theropod dinosaurs.

These major groups were just developing in Late Triassic time. The Herrerasauria, a Triassic group of primitive bipedal predators, may be ancestral to all dinosaurs, because they had characteristics too primitive to be classified as either saurischian or ornithischian. The saurischian dinosaurs of the Triassic are represented by the Coelophysids, a small and primitive family of the theropods, and by the Prosauropods, smaller ancestors to the quadrupedal sauropods. The ornithischian dinosaurs are represented by the unclassified Fabrosaurs and a family of very primitive ornithopod known as Heterodontosaurids. Recent discoveries in South America are refining our knowledge of the earliest dinosaurs.

One geologic rock unit from which these earliest dinosaurs are well known is the Chinle Formation, which is exposed over wide areas of northern Arizona, northern New Mexico, and southern Utah. The Chinle Formation is most famous for its abundant remains of colorful and well preserved petrified wood, found in places like Petrified Forest National Park of northern Arizona. The theropod Coelophysis is the most common dinosaur of the Chinle, and is known from Chinle deposits throughout New Mexico and Arizona, including a mass burial of hundreds of individuals, all Coelophysis, at Ghost Ranch, New Mexico. The only other dinosaurs known from the Chinle are the Fabrosaurs, primitive ornithischians of uncertain affinities. Skeletal remains of dinosaurs have yet to be found in any Chinle deposits in Utah. The thecodonts were still very common, but died out by the end of the Triassic. Their fossil remains have been found in Utah, especially the crocodilian-like phytosaurs. The lack of fossilized skeletal remains of dinosaurs does not reflect their absence in Utah, because the tracks of Triassic dinosaurs, as well as those of other reptiles, are found in the Chinle deposits of southern Utah and the equivalent Popo Agie Formation of northeastern Utah. Most of the Triassic tracks are of reptiles that were not dinosaurs, but also include a number of dinosaur "ichnogenera". Atreipus was originally thought to be the track a small theropod, although recent evidence suggests that it could belong to an ornithopod. Agialopus, from the Popo Agie Formation of northeastern Utah, is a small three-toed track that was probably made by a coelophysid-like theropod.

Lower and Middle Jurassic deposits in Utah also yield abundant, and sometimes very large, track sites that attest to the presence of dinosaurs in Utah, although their skeletal remains are again absent. In contrast to the Late Triassic track assemblages, dinosaur tracks outnumber the tracks of other reptiles by early Jurassic time. The formations of the Lower and Middle Jurassic are predominantly sandstones, indicating an arid environment which may not be favorable for the preservation of fossils. In fact, many of these track sites, particularly those in the Navajo Sandstone, are interpreted to be "oases", or playa lakes, on the basis of associated plant fossils and sedimentological indicators of depositional environment. However, skeletal remains of dinosaurs and other reptiles are known from exposures of these same formations in Arizona, including the Kayenta and Navajo formations. At two sites in southwestern Utah, Washington City and Warner Valley, the Moenave Formation has yielded three distinct types of trackways. They are all tridactyl, or three-toed, tracks which resemble "ichnogenera" known from Late Triassic and Early Jurassic deposits in the eastern United States and other part of the world.

The Moenave is equivalent to the Wingate Sandstone, the lowest formation of the Glen Canyon Group. The Glen Canyon Group also includes the Kayenta Formation and the Navajo Sandstone, which makes up the spectacular monoliths of Zion National Park. It is exposed throughout the Colorado Plateau, including southern and eastern Utah. The Wingate Formation has yielded numerous dinosaur track sites, especially in the vicinity of Moab, Utah and Grand Junction, Colorado. Many of these Navajo tracksites in the Moab area of southeastern Utah, which also include tracks of mammal-like reptiles, lizards, and invertebrates, represent desert oasis deposits. Exposure of the Navajo Sandstone (also known as the Nugget Sandstone in northeastern Utah) along the shore of Red Fleet Reservoir in northeastern Utah has yielded a large tracksite consisting of over 400 tracks of at least two types of dinosaurs.

Following the desert-like conditions of the Lower Jurassic, the Middle Jurassic is marked by a series of invasions from the north of an interior seaway. The marine sequences have contributed mostly marine invertebrates to Utah's fossil record, but in the Red Fleet Reservoir area of northeastern Utah, bones of giant marine reptiles (pliosaurs and ichthyosaurs) have been, as have also dinosaur tracks. Between these marine invasions, deposition of the terrestrial Entrada Sandstone formed the arches of Arches National Park and the goblins of Goblin Valley State Park. Again lacking fossilized skeletal remains, the Entrada Sandstone has yielded numerous dinosaur tracksites, including a "megatracksite" along the flanks of the Salt Valley Anticline north of Moab that extends for over ten miles.

The fluvial (stream-deposited) sediments of the Morrison Formation dominated the Upper Jurassic landscape of eastern Utah. Originating approximately 150 million years ago as floodplain deposits, the Morrison Formation is exposed throughout the Colorado Plateau, including Colorado, Wyoming, eastern Utah, northern New Mexico, parts of Montana and South Dakota, and the panhandle of Oklahoma.

The first dinosaur to be found in Utah was discovered during an 1859 U.S. Army Corps of Topographical Engineers Expedition led by Captain John N. Macomb to explore southeastern Utah and locate the confluence of the Green and Grand (Colorado) Rivers. A number of bones from the front leg were collected by the expedition's scientist Dr. John S. Newberry, but much material remained imbedded in hard sandstone on the steep cliff where the site is located. The specimen, a sauropod dinosaur, was named Dystrophaeus viaemalae by paleontologist E. D. Cope. Since it is based on limited material, its taxonomic position is uncertain. The exact location of this site had been lost since its original discovery and was only relocated in 1987. The well-known Morrison dinosaur fauna includes Utah's official state fossil, the meat-eating theropod Allosaurus; other theropods, including Ceratosaurus, Stokesosaurus, and Marshosaurus; the sauropod dinosaurs Apatosaurus (commonly known as Brontosaurus), Camarasaurus, and Diplodocus; and the ornithischians Camptosaurus, Dryosaurus, and Stegosaurus. Many Morrison quarries have yielded the remains of these and other dinosaur species, but two quarries located in Utah are probably the most famous because they are developed sites open to the public: the Cleveland-Lloyd Dinosaur Quarry, located 30 miles south of Price in Emery County, and the quarry at Dinosaur National Monument. Above the Morrison Formation and following a break in the rock record, the Lower Cretaceous Cedar Mountain Formation is similar in appearance to the Morrison Formation but contains an entirely different dinosaur fauna. New genera of sauropods and large theropods replace the old. The earliest dromeosaurs ("running lizards") appear, including a new specimen of one of these small, fierce predators that has been given the name "Utahraptor". Ornithischian dinosaurs begin to dominate and include the first iguanodons (Iguanodon and Tenontosaurus) and some of the earliest nodosaurs (the armored dinosaurs). Dinosaur eggshell material has also been found in the Cedar Mountain Formation. New discoveries from this formation will increase our knowledge of this poorly known period of dinosaur history.

A series of invasions by the Cretaceous interior seaway that follows leaves another gap in the dinosaur record. Two specimens of plesiosaurs, a marine reptile that was not a dinosaur, have been reported in these sediments, one from the Tropic Shale of southwestern Utah and the other from the Mancos Shale just outside of Utah in western Colorado.

As the seas retreated for the last time near the end of the Cretaceous, the episodes of mountain building that formed the Rocky Mountains began. The dinosaur fauna of the Upper Cretaceous, which includes Tyrannosaurus, Triceratops, and Hadrosaurus, is best known from sites in Montana and Alberta, Canada. But these dinosaurs have been found in Utah as well. Coal-bearing rocks, including the Blackhawk Formation, were formed from extensive swamps along the edge of this retreating sea. In the coal mines of Carbon, Emery, and Grand counties the presence of dinosaurs is revealed by the tracks that criss-cross the ceilings of the mines. Fossil trees, upright in growth position, have been found surrounded by the tracks of dinosaurs that fed upon their leaves. Although few fossil bones have been reported, at least two hadrosaur (duckbill dinosaur) jawbones are known.

Above the coal beds, deposits of the Cretaceous-Tertiary North Horn Formation occur, mostly on the Wasatch Plateau. The lower part of the formation has yielded the remains of Upper Cretaceous dinosaurs, including the Cretaceous sauropod Alamosaurus, Arrhinoceratops and other ceratopsian dinosaurs, and hadrosaurs. Dinosaur eggshells as well as teeth and bones from tiny Mesozoic mammals are found as well. The upper part of the formation contains the fossil remain of larger Paleocene mammals. Further examination of this formation, which spans the Cretaceous-Tertiary boundary may help to answer questions about the extinction of the dinosaurs.

See: William Lee Stokes, Geology of Utah. Salt Lake City: Utah Museum of Natural History and Utah Geological Survey, 1986. Walter R. Averett (editor). Paleontology and Geology of the Dinosaur Triangle. Grand Junction Colorado: Museum of Western Colorado, 1987. Walter R. Averett (editor). Guidebook for Dinosaur Quarries and Tracksite Tour, Western Colorado and Eastern Utah. Grand Junction Colorado: Museum of Western Colorado, 1991

Martha Hayden