Global distribution of dino-

saur clades in the late Triassic. Graphic from Parker and others (2005), reproduced by permission of the Royal Society.

early Dinosaurs in north america

The first appearance of dinosaurs in North Amer- ica is difficult to determine with precision because the rocks that formed during the time of their ori- gin have not produced abundant fossils of terres- trial creatures. Enough is known from Triassic rock sequences, however, to indicate that the dinosaurs arrived soon after the Pisanosaurus-Herrerasaurus- Eoraptor assemblage emerged in Argentina. Sed- iments of the Chinle Formation and Moenkopi Formation of the Colorado Plateau region, the Doc- kum Formation (or Group) of Texas, and the New- ark Supergroup (Cumnock, Pekin, and New Oxford Formations) of New Jersey have all produced dino- saur bones. These strata are all about the same age as or perhaps slightly younger than the dino- saur-bearing rocks of South America. Even though the fossil record of the first North American dino- saurs is very sketchy, it appears that they achieved a worldwide distribution almost immediately after their origin, wherever that event may initially have occurred. Such rapid dispersal is reasonable given the great mobility of dinosaurs and the unified nature of the world’s land masses during the later part of the Triassic. With a skeleton designed for swift and efficient movement over land, and with few oceanic barriers to their migration, the world’s first dinosaurs spread to every corner of Laurasia and Gondwana with remarkable swiftness.

utah in the early triassic

The Triassic was a time of great change in the land and life of ancient Utah and adjacent regions. The changes that occurred in Utah during the Triassic set the stage for events and landscapes that would follow in later portions of the Mesozoic era. Utah landscapes and environments in early Triassic time are much different from those that existed in the late Triassic. It is important to review these profound events carefully, because they had an extremely important consequence: collectively the geologic events of the middle Triassic created enormous

tracts of dinosaur habitat. Utah would never have become populated by such a wondrous array of dinosaurs had the stage not been set by the geologi- cal upheaval during Triassic time.

When the Triassic period began 250 million years ago, nearly all of western Utah was covered by a broad, shallow sea (fig. 2.5). The shoreline of this sea extended north-south through central Utah from roughly the southern border of the state to the eastern end of the modern Uinta Mountains, which were not yet elevated. Offshore from the coastline, mud and sand were washed from the exposed land to the east, settling on the ocean bottom to form layers of shale, siltstone, and sandstone. Layers of silty limestone accumulated elsewhere on the sea- floor, where calcium carbonate was precipitated from ocean water. Today these marine sediments of western Utah are represented by rock units such as the Thanyes, Dinwoody, and Woodside Forma- tions. Fossils of marine invertebrate animals such as sea urchins, clams, brachiopods, and cephalopods clearly indicate the oceanic nature of these depos- its. Sedimentary rocks of this general type and age extend to the west across western Utah and Nevada, suggesting that the open ocean stretched a consid- erable distance in that direction. In central Nevada, at the Berlin-Ichthyosaur State Park in northern Nye County, muddy limestones of early Triassic age have produced spectacular fossils of these gigantic marine reptiles. So far we haven’t found any Trias- sic marine reptiles in western Utah, but they almost certainly existed in the shallow water that covered that part of the state some 240 million years ago.

East of the early Triassic coastline was a low plain that gradually ascended to highlands farther east. Rivers draining more distant elevated areas flowed sluggishly to the west and northwest (fig. 2.5), across the nearly flat coastal plain. The rivers slowed as they approached the sea, dropping much of their load of suspended silt and mud and building up broad mud flats in the process. The mudflats and the river floodplains leading to them extended south- east from the coastline across all of eastern Utah,

even covering portions of modern-day Colorado, Arizona, New Mexico, and Wyoming. Up to 1,500 feet of fine-grained silt, locally mixed with sand and clay, accumulated on these expansive mudflats and floodplains. These sediments are now known as the Moenkopi Formation in southern Utah (fig. 2.6) and the Four Corners area, the Ankareh Formation in the Wasatch Mountain region, and the Popo Agie Formation of Wyoming and Idaho. Emulating Trias- sic rocks elsewhere in the world, the Moenkopi and contemporaneous rocks are thoroughly stained red by the oxidation (“rusting”) of iron-bearing mineral grains. The Triassic “age of red beds” begins with the deposition of the Moenkopi Formation and time- equivalent units west of the early Triassic shoreline.

The low relief of the central Utah coastline, cou- pled with strong climatic shifts that induced tem- porary changes in the level of the sea, caused the early Triassic shoreline to migrate east and west sev- eral times. Each time the sea penetrated into eastern

Utah, it submerged the mudflats and floodplains, covering them with thin layers of limestone or other marine sediment. When the marine advance (known as a transgression) was over, the sea with- drew to the west. The withdrawal of the ocean from eastern Utah (known as a regression) allowed river- deposited mud to bury the thin limestone deposited earlier. This cycle of coastal oscillation occurred sev- eral times, giving rise to a belt of complex interfin- gering between marine and nonmarine sediments in the early Triassic rock record of central Utah (fig. 2.6). Around Kanab, for example, the Moen- kopi Formation consists mostly of mudflat and floodplain deposits but also contains three portions deposited either under ocean water or close to the edge of the sea. These marine or marginal marine portions of the Moenkopi in this area are known as the Timpoweap, Virgin Limestone, and Shnabkaib Members.

In contrast to the richly fossiliferous oceanic rocks of western Utah, the early Triassic nonma- rine strata of eastern Utah have produced far fewer fossils. Nonetheless, the scanty fossil evidence pre- served in the Moenkopi Formation suggests that the mudflats and swampy floodplains were popu- lated by a variety of terrestrial and semiaquatic ver- tebrates. Not surprisingly, given the abundance of water in these habitats, amphibians dominate the scanty Moenkopi vertebrate fauna. Several different types of flat-headed amphibians have been identi- fied, some of them fairly large. Fragmentary remains of terrestrial reptiles also have been discovered in the Moenkopi, but their abundance pales in com- parison with the much more common amphibians. One of the best-known reptiles is Arizonasau- rus, a 10-foot-long (3.3-meter) quadrupedal preda- tor with a large sail-like fin along its back (Nesbitt 2003). Arizonasaurus was not a dinosaur but was still well adapted for life on land and appears to have been an efficient predator of smaller reptiles and amphibians. Arizonasaurus probably represents a group of advanced pseudosuchian reptiles known as the Rauisuchia (fig. 2.2) and illustrates the strong 2.5. The Utah region at the end of the early Triassic,

about 245 million years ago. Reconstruction from Ronald Blakey/Colorado Plateau Geosystems, Inc. Used with permission.

evolutionary trend among Triassic reptiles toward efficient movement on land and a predatory diet. Numerous other pseudosuchians and related rep- tiles were competing with Arizonasaurus for success as land predators. We have no evidence that dino- saurs played a major role in the terrestrial ecosys- tem at the time when the Moenkopi sediments were accumulating in the Utah region.

Scant vertebrate fossils from the Moenkopi For- mation are supplemented by another source of information on the nature of land life in Utah dur- ing the early Triassic. Footprints and trackways of terrestrial animals are very common in the mud- stones and siltstones of the Moenkopi Formation. The sticky mud that was deposited along the low coastal plain evidently served as a perfect medium 2.6. The Moenkopi Formation near Goblin Valley. Red mudstone and tan sandstone in the fore-

ground represent the Moenkopi Formation. Several formations of the overlying Glen Canyon Group are exposed in the cliffs beyond. Courtesy John Telford.

for the preservation of footprints made by animals moving across the landscape. Some spectacular early Triassic trackway sites have been discovered in the Colorado Plateau region. These footprints have been intensively studied by paleontologists in recent years. While some are clearly reptilian in form, none of them can be confidently attributed to dinosaurs. The Moenkopi-age tracks in the Colorado Plateau seem to have been made by a variety of nondino- saurian reptiles such as rauisuchids, ornithosuchids, lizards, and therapsids.

Combining the evidence from both footprints and body fossils, it appears that dinosaurs were very rare if present at all in the Utah region dur- ing the early Triassic, although a diverse reptile and amphibian fauna existed. This may be because the swampy environment favored the amphibi- ans or perhaps because remains of whatever dino- saurs might have been present by chance were not preserved. Whatever the reason, the nonmarine vertebrate fossils and tracks from the Moenkopi Formation offer no evidence that the age of dino- saurs had begun in Utah in early Triassic time. No solid, indisputable evidence of dinosaurs has ever been discovered in rocks of this age.

the late triassic: a time of Change

During the middle portion of the Triassic the geo- logical setting of Utah began to change in ways that were extremely important in creating the Mesozoic wonderland in the eastern part of the state. Laura- sia, carrying North America with it, began to sepa- rate from the rest of Pangaea as rifting and volcanic activity intensified along what is now the north- ern Atlantic seaboard. The North American region was carried to the northwest as a rift valley opened within Laurasia that would eventually develop into the modern Atlantic Ocean basin. Although the western part of the embryonic North Ameri- can continent was not directly affected by the rift- ing, it did begin to yield to the compressive forces that were generated as the North American portion

of Laurasia started to move against and over the sea- floor to the west. The geological serenity that pre- vailed in western Northern America during the late stages of Pangaea was replaced by compressional forces that caused uplift of the earth’s crust through- out the Great Basin and Rocky Mountain regions. No major mountain ranges were elevated during the Triassic period, but many areas began to rise as the immense compressional forces started to warp the crust upward.

One such uplift was a gentle arch that emerged in eastern Nevada and western Utah, referred to by W. L. Stokes (1986) as the Mesocordilleran High. The lifting of the Mesocordilleran High raised the ancient seafloor of western Utah and caused the withdrawal of the early Triassic seas from most of the Great Basin, including western Utah (fig. 2.7). The west coast of North America stepped to the west as the early Triassic seafloor emerged in the