Stratigraphy of the Cedar Mountain

Formation in east-central Utah, with dis- tribution of some of the newly discovered dinosaurs.

Historically the Cedar Mountain Formation has not been the source of very much information on dinosaurs or other prehistoric creatures. Fos- sils of any kind are definitely less common in the Cedar Mountain Formation than they are in the underlying Morrison mudstones. Over the past two decades, however, numerous fossil discoveries have been made in the Cedar Mountain Formation, clearly demonstrating that it is not as “unfossilifer- ous” as it was often portrayed in the geological lit- erature of the 1950s and 1960s. It seems that one reason why this formation has been overlooked for so long is that many paleontologists were so dazzled by the abundance of dinosaur fossils in the Morri- son Formation that they spent little time prospect- ing Cedar Mountain outcrops for fossils. Beginning in the 1980s, however, scientists from the Utah Museum of National History, Utah Geological Sur- vey, College of Eastern Utah, University of Okla- homa, and other institutions have systematically searched for fossils in the Cedar Mountain Forma- tion. Collectively this work has demonstrated that the formation is in fact rich in fossils. The newly discovered material represents some exciting new perspectives on the history of dinosaurs in Utah and adjacent regions of North America. At the risk of overstating things slightly, the Cedar Mountain For- mation of Utah has become one of the most excit- ing horizons in contemporary dinosaur research. It reveals a menagerie of dinosaurs quite unlike the Morrison fauna and provides evidence of some pro- found changes in the land and life of Utah during the early part of the Cretaceous period.

Dinosaur faunas of the Cedar Mountain formation

The first formal description of a Cedar Mountain dinosaur was published by N. M. Bodily (1969), who tentatively identified Hoplitosaurus(?), a quadru- pedal and armored herbivore, from a site about 20 miles north of Moab. The Dalton Well Quarry in the same region was developed by scientists at Brigham Young University, after bones were discovered near

the west side of Arches National Park in the 1970s (Galton and Jensen 1979). This quarry is situated a few meters below the horizon that produced Hopli- tosaurus(?), near the base of the Cedar Mountain Formation. The bones excavated from the Dalton Well Quarry are still under study, and many more fossils could potentially be quarried from the site in the future. James Jensen described reptilian egg- shell fragments from the Cedar Mountain Forma- tion that were discovered near Castle Dale, Utah, on the west side of the San Rafael Swell (Jensen 1970). In the mid-1980s numerous small fossils were reported from what became known as the Rough Road Quarry, located in the northwest San Rafael Swell region east of Castle Dale (fig. 6.3). The Rough Road Quarry and several other sites nearby have produced an amazing variety of small fossils of both terrestrial and aquatic vertebrates. Small dinosaur teeth, crocodilian and turtle fossils, lizard and fish remains, and the teeth of several primitive mam- mals have all been recovered from this quarry (Nel- son and others 1984; Nelson and Crooks 1987; Eaton and Nelson 1991).

The current surge of interest in the dinosaurs of the Cedar Mountain Formation began in earnest during the late 1980s, when researchers from the Utah Museum of Natural History discovered a con- centration of bone in Emery County now known as the Long Walk Quarry (fig. 6.3). This site, which produced mostly fragmentary fossil material, is situ- ated in the lower Ruby Ranch Member of the Cedar Mountain Formation, about 45 feet above the top of the Morrison Formation. The Long Walk Quarry is thus intermediate in age between the younger (and higher) Rough Road Quarry and the older (and lower) Dalton Well Quarry. Utah state paleontolo- gist James Kirkland, Don Burge, and Ken Carpenter (College of Eastern Utah), and their many collabo- rators and volunteers have made many subsequent discoveries in east-central Utah including the Gas- ton Quarry, the Crystal Geyser Quarries, and the Price River Quarries (fig. 6.3). In addition, Richard Cifelli of the Oklahoma Museum of Natural History

has reported dinosaur and other reptile remains accompanying the fossils of small mammals that he found in the upper Cedar Mountain Formation at Mussentuchit Wash in southern Emery County (Cifelli 1993). J. G. Eaton and M. E. Nelson (1991) have reported additional small mammal remains from the Cedar Mountain Formation along the west side of the San Rafael Swell. The “bone rush” to the Cedar Mountain beds over the past few years has resulted in the discovery of abundant fossils from several different localities, documenting the pres- ence of many new species of dinosaurs and illumi- nating what was heretofore a poorly known period in the evolution of Utah’s dinosaurs (Kirkland and others 2005; Kirkland and Madsen 2007).

All of this new information allows us to formu- late a much more complete interpretation of Utah’s early Cretaceous dinosaur faunas than was possi- ble only a few decades ago. The story of Early Cre- taceous dinosaurs in Utah is still unfolding, of course, and many new types certainly await dis- covery in the years to come. Nonetheless, we are beginning to recognize several different faunas that seem to be restricted to certain horizons within the Cedar Mountain Formation. Kirkland and sev- eral colleagues have suggested that at least two, per- haps more, distinct faunas can be recognized among the dinosaur remains excavated from this forma- tion in Utah (Carpenter and others 2002; Kirk- land 2005). Let’s review the newly discovered fossils from the Cedar Mountain Formation from oldest to

youngest, emphasizing how dinosaur communities changed in Utah from about 125 million years ago to 98 million years ago.

the yellow Cat fauna: the Basal assemblage

The lower portion of the Cedar Mountain Forma- tion in eastern Utah is known as the Yellow Cat Member, a sequence of mudstone layers interbed- ded with limestone and sandstone (fig. 6.5). Over- lying the Yellow Cat Member in eastern Utah is a thin pebbly sandstone sequence known as the Poi- son Strip Sandstone. The strata of these two mem- bers contain the fossils of the earliest Cretaceous dinosaur fauna in the Colorado Plateau region. This fauna is probably around 120–125 million years old, some 20 or so million years younger than the dino- saurs that occur in the Morrison Formation imme- diately below. Elements of the basal fauna have thus far been found primarily in Grand County in places such as the Gaston Quarry, the Dalton Well Quarry, and other localities near Green River. This oldest assemblage of dinosaurs from the Cedar Mountain Formation has been referred to as the Yellow Cat 6.6. Falcarius utahensis: A. skeleton; and B. skull. This

odd therizinosaur is the oldest of the new dinosaurs discovered recently in the Cedar Mountain Formation. Scale bar for skeleton = 39 inches (1 m), total length of skull is about 12 inches. Skeletal reconstruction based on Kirkland and others 2005.

B

A

fauna (Kirkland, Britt, and others 1997; Kirkland, Cifelli, and others 1997), contains at least eight dif- ferent dinosaur genera, and may represent two dis- tinct subassemblages (Kirkand and Madsen 2007).

The remains of the oldest dinosaur from the Yel- low Cat Member were found in western Grand County and named Falcarius utahensis by Kirk- land, Zanno, and others (2005). Falcarius belongs to a group of odd bipedal dinosaurs known as the therizinosaurs. They are classified as maniraptoran theropods (typified by the “raptors”) but were prob- ably not strict carnivores, unlike other members of that group. The leaflike teeth of the therizino- saurs were small and quite unlike the sharp dag- gerlike teeth typical of other theropods. With tiny denticles along the tooth margins, Falcarius teeth would have been equally useful for shredding plant material or for consuming small prey animals. Fur- thermore, the pubis of Falcarius did not project strongly forward as in most other thropods but was positioned almost vertically (fig. 6.6). This unique pelvic architecture would have accommodated a greater digestive mass, giving the 12-foot-long (4-meter) Falcarius a somewhat ungainly pot- bellied appearance.

Falcarius also had extremely long fingers, tipped with curving claws 4–5 inches (10–12 centimeters)

long. The forelimbs of Falcarius appear to have been extremely flexible and capable of a wide range of sophisticated movements (Zanno 2006). Most pale- ontologists agree that these unique skeletal features suggest that Falcarius was probably an omnivore that acquired plant material or animals selectively through the use of its flexible arms and long fingers and claws. Another intriguing aspect of Falcarius is that its remains were discovered in a bone bed that includes fossils of perhaps a dozen or more individ- uals of various sizes. The unexcavated portion of the bone bed may contain bones belonging to hundreds of individual therizinosaurs that died in the same place at the same time. Thus it appears that Falcarius foraged over the Cedar Mountain basin in herds or at least gathered in such groups temporarily at spe- cific places. No one knows exactly why so many Fal- carius individuals were entombed together, but the deposits that contain their fossils could have accu- mulated around a dwindling pond during the dry season. If so, thirst and starvation were the likely causes of the mass mortality of so many Falcarius individuals.

Also present in the strata that contain the Fal- carius bone bed is another small predator named Geminiraptor suarezarum by P. Senter and oth- ers (2010). Geminiraptor is based on a 4-inch