The Wingate Sandstone erg was the first of two

large dune fields to develop in the early Jurassic. Along the southwest margin of this erg, fluvial and lake sedi- ments of the Moenave Formation were deposited at the same time as the Wingate Sandstone. Reconstruction from Ronald Blakey/Colorado Plateau Geosystems, Inc. Used with permission.

(Miller and others 1987; Hintze 1988; Miller and Hoisch 1995). To the south the ergs were limited by chains of volcanic mountains in south-central Ari- zona, from which a broad slope descended to the north into Utah’s interior basin (Riggs and Blakey 1993). East and southeast of the sand seas, in Col- orado and southeast of the Four Corners region, low hills and benches served as barriers to the dune fields. To the north, beyond the open end of the interior basin, the great sand sheets extended to the shoreline of the early Jurassic seas, located in what is now Wyoming, Montana, and Idaho. The sandy beaches of this coastline must have been the source for much of the sand deposited in the early Juras- sic ergs. The eolian deposits that dominate the Glen Canyon Group signify the burial of the old Chinle basin under sheets and dunes of wind-driven sand derived from the north and northwest. The move- ment of such massive amounts of sand requires a dry and windy climate. The relative verdant, well- watered basin of the late Triassic became a searing desert in the early Jurassic. In the context of such an acute shift in the regional environment, the Tri- assic-Jurassic biotic transition certainly comes as no surprise.

Though the Glen Canyon Group is dominated by eolian sandstone, which documents an enor- mous and persistent desert, it also contains other types of deposits that are extremely important as sources of information on the animals that inhab- ited the erg and nearby regions. All large ergs have interdune areas where the infrequent rainfall may become ponded in ephemeral or playa lakes. Such lakes are short-lived, usually disappearing in a mat- ter of days in the blistering climate of the desert. In addition, groundwater occasionally rises to the surface in places like the modern Sahara to form spring-fed oases that represent the only semiperma- nent bodies of water in the otherwise barren sandy landscape. As large dunes migrate downwind, the dry sand is blown over and past the more cohesive and moist sediment below so that the land surface is lowered as the dunes pass. The process of lowering

the surface by blowing away the dry sand and silt is known as deflation.

Deflation basins between the crests of advancing dunes can temporarily capture rainwater or allow groundwater to seep to the surface to form a moist substrate. The momentary abundance of water in these deflation basins may allow plants to germi- nate, giving rise to a small pocket of lush vegetation in the midst of a biological wasteland. The deflation basins, ephemeral lakes, and oases of sandy deserts act like biotic magnets, attracting great numbers of animals to drink, feed on whatever plants might be growing at the water’s edge, or lie in wait for prey to approach. In the early Jurassic sandstones of Utah such interdune deposits are represented by thin and laterally discontinuous lenses of mudstone and limestone that are interbedded with the much thicker and more extensive eolian sandstones. The deflation surfaces appear as prominent horizontal planes, known as “first-order bounding surfaces,” among the complex pattern of sweeping cross-beds that typifies eolian sandstones. Many deflation sur- faces and lenses of interdune deposits occur in the massive Navajo Sandstone, and some are known from the Wingate. Both fossils and footprints prin- cipally occur in these zones in these two eolian for- mations (Winkler and others 1991). In addition, the early Jurassic ergs were laced by dry washes that channeled runoff into the temporary lake basins after the rare cloudbursts. Sand would periodically drift into the washes, only to be scoured away dur- ing a subsequent desert flash flood. Some of the sandstone layers in the Navajo and Wingate rep- resent reworked eolian sand spread out across the floor of the dry washes during flood events.

In the case of the Wingate Sandstone the rela- tively small erg covered only the eastern half of Utah (fig. 3.5). On its southwest margin a complex river system separated the erg from the foothills of the Mesocordilleran High to the west. This river system transported water and sediment to the northwest, toward the sandy beach beyond the open end of the interior basin. The wind would then blow the sand

back into the erg. Some of the sand and silt carried by the river system was deposited along its course and over the adjacent floodplain. These river-depos- ited sediments flanking the eolian sandstones of the Wingate compose the major portion of the Moenave Formation. The Moenave is restricted to southwest Utah and northwest Arizona and is equivalent to at least the upper part of the Wingate Sandstone, into which the fluvial sediments pass as they are traced east. The Moenave fluvial system drained the slope descending from the volcanic highlands of south- central Arizona and carried water around the Wing- ate erg. The river-plain habitat, with more plentiful water, probably supported a richer biota than the sand sea to the northeast during Wingate-Moenave time. In the area around St. George the sediments preserved in the Moenave Formation document the presence of a large freshwater lake named Lake Dixie by scientists (Kirkland and Milner 2006). The sediments deposited in and around this lake pre- serve a spectacular array of footprints, trackways, and fossils that provide important clues to the dino- saur fauna that inhabited early Jurassic Utah.

As noted, the development of ergs in the early Jurassic occurred in two phases: the relatively small Wingate erg and the much larger and later Navajo erg (fig. 3.4). Between these two episodes of sandy deserts, a large river system similar to the Moenave fluvial complex evidently covered this entire portion of eastern Utah. Thus the Kayenta Formation, which separates the Navajo and Wingate Sandstones, con- sists almost entirely of river deposited sand, silt, and mud. For some reason the amount of sand blowing into the interior basin diminished during Kayenta time. The rivers draining to the northwest began to penetrate into the older Wingate erg, picking up great amounts of loose sand. The rivers developed braided patterns as huge sandbars choked the chan- nels and forced the streams to fork and split as the water they carried passed around the sandy obstruc- tions. As the braided pattern developed, a broad plain laced by numerous criss-crossing watercourses evolved on top of the buried Wingate erg. Such a

broad surface of sediment transported by a braided stream system is known as a braid plain. The Kay- enta Formation represents a braid plain interlude between two episodes of erg development. The cli- mate in Kayenta time was probably still desertlike, but dune fields were only a restricted element of the scene during that brief phase of a river-dominated arid landscape. In the northeast corner of Utah, around Dinosaur National Monument, the Glen Canyon Group consists of about 700 feet of eolian sandstone without the fluvial Kayenta component. Evidently the braid plain constructed by the Kay- enta Fluvial system did not extend into this corner of Utah: the dune fields persisted there throughout the entire early Jurassic.

Paleontology of the Glen Canyon Group: a new array of Dinosaurs

Until very recently our knowledge of the life that existed in and around the great ergs and braid plains of the early Jurassic was extremely limited due to the rarity of fossils from rocks of this age. This scar- city of body fossils resulted from at least three fac- tors. First and foremost, deserts are generally regions of minimal biological productivity. The lack of water in the early Jurassic would have restricted the growth of plants, which in turn meant lim- ited food resources for herbivores and few prey ani- mals for carnivores. Modern deserts are infamous for their stark and lifeless visage, a consequence of the minimal presence of life in such hostile envi- ronments. The desert landscapes of the early Juras- sic throughout the Colorado Plateau region were certainly more barren than in the preceding late Triassic (Chinle time). Second, the constant shift- ing of sand as large dunes migrated across the erg would have resulted in the alternate burial and reex- posure of the organic remains. Such events do not favor preservation of fossils. Finally, the cliff-form- ing nature of the eolian sandstones that domi- nate in the Glen Canyon Group makes it difficult for scientists to find the few fossils that might be

preserved in these rocks. The vertical walls of rock are for the most part impossible to survey for fos- sils in any detailed or comprehensive manner. Most of the fossils that have been found in these forma- tions were located either in blocks that have fallen from the cliffs or in the few areas where the strata are exposed along the level ground surface.

We do have some fossils and recent discoveries, however, such as the spectacularly abundant foot- prints at the St. George Dinosaur Discovery Site (SGDS). These are beginning to reveal some inter- esting details about a heretofore poorly documented fauna that thrived among the dunes and along the watercourses of early Jurassic Utah. Moreover, the dinosaurs and other terrestrial vertebrates known from these rocks indicate a much different commu- nity from that of the underlying Chinle Formation. The Triassic-Jurassic transition is clearly evident when we compare to the two fossil assemblages.

Dinosaur footprints from the Glen Canyon Group

The footprints of dinosaurs and other vertebrates are fairly common in many horizons in the Wingate, Moenave, Kayenta, and Navajo Formations. Foot- prints tend to be more common in the finer-grained mud and silt deposited by rivers that ran through or beside the erg and in the silty limestone and mud that accumulated in oasislike interdune ponds. The water at such sites would have attracted great num- bers of animals, and the soft, tacky mud at the water’s edge would have served as a good medium for the preservation of footprints and trackways, recording the comings and goings of life drawn in from the surrounding desert. Some footprints have been discovered in the dune sands as well; many of them appear to have been made when the sand was moistened by dew or rain and therefore more cohe- sive than it normally was. Many studies of dinosaur footprints from early Jurassic strata of Utah have been completed over the past two decades (e.g., Stokes 1978; Stokes and Madsen 1979; Baird 1980; Lockley 1991b; Lockley and Hunt 1995, among many

other studies). The discovery of the extensive track- bearing horizons in the Moenave Formation near St. George in 2000 stimulated a unprecedented surge of interest among paleontologists (see summary in Milner and others 2006b).

the st. George Dinosaur Discover site:

a Glimpse into the early Jurassic Dinosaur fauna

Though early Jurassic dinosaur footprints from southwest Utah had been known for decades, the discovery of extensive track-bearing surfaces in the Moenave Formation by Sheldon Johnson in 2000 has provided a wealth of new information on the dinosaurs and environments that existed in the region approximately 195–198 million years ago. The principal track-bearing horizon is now par- tially exposed and preserved for public viewing at the St. George Dinosaur Discovery Site at Johnson Farm (SGDS). Since the initial discovery and devel- opment, more than twenty recent scientific studies have focused on the sediments (e.g., Kirkland and Milner 2006), footprints and trackways (e.g., Milner and others 2006b; Milner and others 2009), and fos- sils (e.g., Milner and Kirkland 2006) of the Moenave Formation at the SGDS. Collectively these studies have provided a wealth of new information that illu- minates a critical and heretofore somewhat obscure chapter in the story of Utah dinosaurs.

Well-preserved footprints and trackways are found in twenty-five different layers within the Whitmore Point Member of the Moenave Forma- tion at the SGDS (fig. 3.6). The Moenave Formation in this area consists of sandstone, mudstone, and shale deposited along the edge of Lake Dixie (Mil- ner and Kirkland 2006, 2007). This shallow saline lake was subject to significant fluctuation in size and depth over time. At its maximum size Lake Dixie completely submerged the area from St. George to Kanab and extended north beyond the present site of Zion National Park. Smaller fluctuations of lake level may have occurred on a seasonal basis, result- ing in the frequent shifting of the shoreline of Lake

Dixie across the low basin in southwest Utah. The sediments in the Moenave Formation at the SGDS are thus a mixture of fine-grained open-water deposits, sandy lakeshore sediments, and material washed into the lake by ephemeral streams. When the level of Lake Dixie fell, extensive mudflats would have been exposed in the wake of the receding water, only to be resubmerged when the lake later expanded. Mud cracks and diamond-shaped salt crystal casts are common in the Moenave Forma- tion at the SGDS and provide evidence for periodic exposure of fine-grained lake-bottom sediments.

Fish fossils found in the Moenave Formation at the SGDS indicate that Lake Dixie was popu- lated by a variety of primitive fish, including sharks,

lungfish, coelacanths (lobe-finned fish), and heavily armored relatives of the modern gar known as semi- onotids (fig. 3.7). These fish were relatively large, with some of them attaining maximum lengths approaching 2 meters (6 feet). Agal structures and fossils of conchostracan crustaceans (commonly known as “clam-shrimp”) are also preserved in the Moenave sediments and provide additional evi- dence of a thriving aquatic ecosystem in Lake Dixie. Fragmentary plant fossils in the lakeshore sediments suggest the presence of trees and shrubs along the shore of Lake Dixie and the streams that drained into it. In addition to the fossils of aquatic ani- mals and plants in the Moenave Formation, more than six thousand well-preserved footprints have been discovered in these strata at or near the SGDS. These clearly indicate that dinosaurs were frequent visitors to the shores of Lake Dixie.

The footprints at the SGDS are dominated by rel- atively small Grallator tracks (about 95 percent of the preserved tracks) and large Eubrontes tracks, both likely made by predatory and bipedal theropod dinosaurs. The larger Eubrontes tracks are gener- ally 30–45 cm (12–18 inches) long and have narrow marks made by sharp claws at the tips of the toes (fig. 3.8). Based on the size of the Eubrontes tracks, it appears that a theropod dinosaur much larger than Coelophysis of the late Triassic prowled the shores of Lake Dixie. A good candidate for the track-maker of the SGDS Eubrontes tracks is Dilophosaurus (described below) from the overlying Kayenta fauna or a theropod similar to it. Supporting this conclu- sion are a few fragmentary fossils (portions of ver- tebra) and dinosaur teeth in the track-bearing layers that could have belonged to Dilophosaurus or some very similar animal. The smaller but more abun- dant Grallator tracks (fig. 3.9) appear to be made by a Megapnosaurus-like theropod, also known from the Kayenta Formation (described in more detail in a later section). In addition, Batrachopus, a footprint thought to have been made by a primitive crocodil- ian reptile, also occurs in the Moenave Formation in southwest Utah.