Theropod remains

from the upper Cedar Mountain Formation: A. a claw from the foot of Dei-

nonychus, very similar

to those from the Ruby Ranch member; scale bar = 2 inches (5 cm); B. large theropod tooth from the upper Cedar Moun- tain Formation as it was found in the rock matrix; the total tooth length nearly 4 inches.

Ranch Member has also produced fossil teeth and isolated bones that clearly indicate the presence of much larger and completely different types of thero- pods (DeCourten 1991; Kirkland and Parrish 1995; fig. 6.27B). Large, bladelike teeth up to 4 inches (10 cm) long, with prominent serrations on the front and back edges, probably came from a fear- some predator perhaps similar to Acrocanthosaurus, known from early Cretaceous deposits in Texas and Oklahoma (Stovall and Langston 1950). Some of the teeth known from the upper Cedar Mountain strata are larger than those of an average Acrocanthosau- rus, however, indicating that the dinosaurs that pos- sessed them may have been as large as Allosaurus of the late Jurassic. Because it is rarely possible to make a precise identification of early Cretaceous thero- pods from the teeth alone, we can’t be sure exactly what genus and species is represented by these large teeth from the Ruby Ranch Member. We do know, however, that they were big! All of the dromaeosaurs would have been dwarfed by the multiton carni- vores that left the huge daggerlike teeth of the upper Cedar Mountain Formation.

Some of the nondinosaur fossils from the upper parts of the Cedar Mountain Formation shed addi- tional light on the changes in land and life that occurred during early Cretaceous time in Utah. The Mussentuchit is the youngest member of the Cedar Mountain Formation, and much of it is composed of gray carbon-rich mudstones that have produced many intriguing fossils. The carbonaceous matter in the Mussentuchit Member consists mostly of small bits of carbonized wood and microscopic masses of organic residues. This material, mixed with clay and carbonate minerals in the mudstones, creates a gen- erally darker gray color in the upper beds (fig. 6.28), as opposed to the pastel purples and lighter grays that typify the lower portions of the formation. Accompanying this increase in organic material is a rather sudden increase in the abundance of plant fossils of all sizes in the uppermost Cedar Mountain Formation. Studies of the pollen recovered from the Mussentuchit Member (Tschudy and others 1984)

have revealed the presence of many different types of conifers, cycads, ferns, and the flowering plants (angiosperms).

The advent of the angiosperms in the early Cre- taceous was a major event in the history of the ter- restrial biota because these plants would soon dominate the global flora, a distinction they hold to the modern day. The highly efficient mode of repro- duction used by the angiosperms, coupled with the variety of tissues each possesses, created new food sources for herbivorous animals of all types. After the early Cretaceous plant-eating vertebrates could take advantage of new nutritional resources, such as fruit and flowers, that only the angiosperms 6.28. A fossil quarry in the Mussentuchit Member of

the Cedar Mountain Formation in the San Rafael Swell region. The gray carbonaceous mudstones of this quarry have produced a great variety of small fossils. Photo by Frank DeCourten.

produce. The evolution of every group of herbiv- orous creatures, from the dinosaurs to the insects, was strongly affected by the development of the flowering plants.

Even though the angiosperms became estab- lished in central Utah during the time when the Mussentuchit Member of the Cedar Mountain For- mation was being deposited, they were still small and by no means the dominant plants of the time. In Utah the fossils of more primitive coniferous trees are abundant at many outcrops of the upper Cedar Mountain beds (Thayn and others 1983; Thayn and Tidwell 1984). Perhaps the most char- acteristic element of the Mussentuchit flora, how- ever, was the giant fern Tempskya (Tidwell and Hebbert 1976). The distinctive dark-colored and fibrous wood of this tree-sized fern is so common in the uppermost Cedar Mountain Formation that petrified fragments of it sometimes form a loose

pavement that litters the surface. In places large trunks of Tempskya are found still in vertical growth position: a genuine petrified forest of giant ferns! The dramatic increase in the abundance of plant fossils suggests that the forested areas were expand- ing and the vegetation was becoming more dense during late Cedar Mountain time. The Mussen- tuchit Member even contains a few thin coal seams that indicate profuse vegetation (Tidwell and oth- ers 1983). This proliferation of trees and shrubs may signify a greater supply of water and gener- ally less arid conditions across the central Utah in late Cedar Mountain time. Recall that Tempskya is a fern, albeit an unusually large one, and there- fore probably required a moist and humid habitat. Its great abundance in the upper Cedar Mountain Formation is a further indication of damp environ- ments. Such a notable shift in the patterns of climate 6.29. The winding ribbon of sandstone (middle distance) in the upper Cedar Mountain Formation in Emery County rep-

resents an ancient stream channel that became filled with sand and gravel. Such “paleochannels” allow geologists to reconstruct the size, direction, and speed of early Cretaceous streams of central Utah. Photo by Frank DeCourten.

and vegetation would be interesting by itself, but the story has an even more intriguing aspect.

The carbonaceous nature of the sediments in the upper Cedar Mountain Formation seems to sug- gest more sluggish rivers and a more stagnant drain- age system. If the rivers were flowing swiftly, much of the accumulated plant litter would have been washed downstream or would have been decom- posed in the well-oxygenated water of ponds fed by the streams. Studies of the ribbons of sandstone left by rivers (the “paleochannels”) in the upper Cedar Mountain Formation (Harris 1980) suggest low-gra- dient channels with winding patterns that are typ- ical of sluggish streams in the modern world (fig. 6.29). At first glance, however, the deduction of lan- guid rivers seems to be at odds with our interpre- tation of wetter climatic conditions. More plentiful water would seem to increase the velocity of stream- flow rather than reduce it. The resolution of this interesting paradox involves an oceanic event that was occurring on a global scale near the end of the early Cretaceous. Very good geological evidence indicates that at this time sea level began to rise everywhere on earth. This was just the beginning of a process that would continue in even more dra- matic fashion into the Late Cretaceous. As sea level rose, the ancient Gulf of Mexico crept north into central North America while the ancestral Arctic ocean penetrated south, submerging the lowlands of modern Manitoba and Saskatchewan. Eventually these two encroaching arms of the sea would meet to form the Western Interior Seaway of the Late Cretaceous, splitting North America into two island continents.

Near the end of the early Cretaceous this oceanic advance or transgression was just beginning. One way to slow a river down is to raise its base level, the elevation of its mouth. The ultimate base level for nearly all the world’s rivers is sea level. If the seas rise, then the ultimate base level rises as well. This in turn decreases the elevation drop between the headwaters and the mouth of the river, causing it to flow with less energy. The transgression of the

early Cretaceous seas did just that; as the advanc- ing sea crept ever closer to Utah from the northeast and south, the rivers began to decelerate in response to the rising base level. The encroaching sea never reached central Utah during the early Cretaceous, but it came close enough to cause a reduction in the velocity of stream flow (fig. 6.3). With less vigor- ous rivers, less plant litter was flushed downstream and more organic matter began to accumulate in the sediments deposited in central Utah. The proximity of the advancing ocean may also be responsible for the increased moisture that is insinuated by the con- spicuous increase in the abundance of plant fossils in the carbonaceous sediments. Coastal regions are generally wetter than regions farther inland because the air moving onshore is more heavily laden with moisture evaporated from the surface of the nearby sea. Central Utah was becoming more “coastal” near the end of early Cretaceous time as the sea advanced to the west from the interior lowland of North America.

Thus by the time the sediments of the Mussen- tuchit Member were deposited it appears that the landscape of central Utah had experienced a sig- nificant change. Water was more plentiful in the swampy lowland, and plant growth was much more luxuriant than had been the case on the semi- arid plains during the earlier stages of the Creta- ceous. Optimal dinosaur habitats developed under these “improved” conditions. This verdant terrain also sustained large populations of other terres- trial and semiaquatic vertebrates. A rich assemblage of lizards, semiaquatic reptiles such as croco- diles and turtles, amphibians, and several differ- ent types of primitive mammals is documented by small fossils found in the Mussentuchit Mem- ber. The Mussentuchit mammals are a very inter- esting assemblage (fig. 6.30), including the world’s oldest marsupial (Cifelli 1993; Kirkland and Mad- sen 2007). The nondinosaur reptiles are dominated by semiaquatic crocodiles and turtles (Nelson and Crooks 1987) that thrived in the numerous streams and ponds in the swampy forests. Amphibians

such as salamanders were plentiful (Gardner 1995); because they cannot survive or reproduce outside of moist habitats, they provide further evidence of marshy conditions in the uppermost Cedar Moun- tain Formation. A diverse assemblage of lizards also prowled through the undergrowth of the lush Cedar Mountain forests, including a large active preda- tor that was probably close to 3 feet (1 meter) long (Cifelli and Nydam 1995; Nydam 1995). With so many egg-laying reptiles around, it is little wonder that many eggshell fragments have also been found

in the upper Cedar Mountain Formation (Jensen 1970). Some of these shell fragments may be from dinosaur nests, but we can only speculate about what creatures laid the eggs until someone finds an embryo preserved inside—and there are many good candidates in the upper Cedar Mountain fauna.

Imagine what central Utah must have been like around 100 million years ago when the upper lay- ers of the Cedar Mountain were deposited. Herds of several different types of ornithopods ambled slowly through the undergrowth of the shadowy wood- lands, grazing on the leaves of ferns, angiosperms, and other types of plants. They would occasion- ally have passed low-browsing armored nodo- saurs in the underbrush, who probably took little notice of their presence. Meanwhile the leafy veg- etation of the higher forest canopy was nipped by migrant sauropods moving across the landscape in rhythm with the seasons. Packs of stealthy raptors were a constant threat to the herbivorous dinosaurs and probably congregated along migration routes or bodies of water, awaiting unwary prey. Large men- acing theropods, fewer in number than the rap- tors, would move about as solitary individuals or in small groups, waiting for the opportunity to subdue weak, isolated, or incapacitated sauropods and orni- thopods. The insects, reveling in the arrival of the angiosperms, would have buzzed through the heavy moist air searching for nectar among the world’s earliest flowers. The swampy thickets would liter- ally have fluttered with the movement of smaller creatures such as lizards and birds, while primitive crocodiles cruised the streams crossing the region. Much of central Utah during the time represented by the upper Cedar Mountain Formation proba- bly looked, sounded, and felt a bit like the modern Everglades of Florida.

Periodically, under the erratic climate of the early Cretaceous, droughts would sweep across the land- scape. The rivers would wither, and the lush forests would turn brown. Plant-eaters of all types would be attracted to the dwindling bodies of water; in their weakened condition, they became easy targets for 6.30. Mammal fossils from the upper Cedar Mountain

Formation of Utah: A. a jaw fragment from Kokopelia

juddi, the world’s oldest marsupial; B. a multituberculate

tooth; C. a mammal tooth with three cusps. Scale bar = 0.04 inch (1 mm) in all sketches. A: redrawn from Cife- lli 1993; B: based on photograph by Eaton and Nelson 1991; C: adapted from Nelson and Crooks 1987.

predators. Even if they avoided the raptors and large theropods, the dry conditions limited the growth of their food. Death by starvation was always a threat during prolonged droughts. In such instances the skeletons of several individuals from a herd might be preserved as fossils in close proximity to each other. At other times, during wet climate cycles or as hurricanes and tropical storms moved inland, the seawater was too abundant. Floods may have raged across the region, transporting sand, gravel, and mud that would eventually settle out as layers of sediment in the lowlands. As the interlaced rivers rose and breached their banks, many dinosaurs and other creatures were swept to their death, their remains concentrated where the floodwaters pon- ded. The result would have been the several bone beds in the upper Cedar Mountain Formation that produce the disarticulated fossils of many different kinds of animals.

the early Cretaceous faunal transition

While all the repetitive cycles in seasons, climate, life, and death were transpiring across the central Utah landscape, changes in global geography were underway in the early Cretaceous that had profound influences on North American dinosaur faunas. As we have seen, the dinosaur assemblages known from the upper and lower portions of the Cedar Mountain Formation are notably different. These differences are summarized in figure 6.5.

The lower Yellow Cat Member and Poison Strip Sandstone produce an assemblage dominated by a diverse array of large and relatively primitive orni- thopods (such as Planicoxa and Iguanodocolossus), a few nodosaurs (Gastonia), large raptors (Utahrap- tor), some small predators (Nedcolbertia), therizi- nosaurs (Falcarius), and several different types of sauropods (Cedarosaurus, Venenosaurus). Not all of these dinosaurs lived at exactly the same time, but collectively they demonstrate the overall charac- ter of Utah’s dinosaur communities about 120 mil- lion years ago. As our knowledge of the lower Cedar

Mountain dinosaur fauna has improved over the past two decades, paleontologists have noted an overall similarity between this array and early Cre- taceous dinosaur communities that lived in Europe at the same time (Kirkland, Carpenter, and oth- ers 1998; Carpenter and others 2002; Kirkland and Madsen 2007). Reconstructions of global paleo- geography (fig. 6.31) suggest that North America and Europe were joined at this time, allowing free migration of dinosaurs between the two continents. This faunal interchange may be the basis for the