Hand of Ornit-

holestes. Note the highly

elongated fingers and the reversed form of digit I, the “thumb,” which could have been oppos- able to the other two fin- gers. Scale bar = 2 inches (5 cm). Redrawn from Osborn 1916.

chance to steal a hatchling as a succulent supple- ment to its normal diet. If so, the adaptations for speed make perfect sense. After all, if an animal is going steal babies from the nest of dinosaurs such as Allosaurus, it had better be able to run! Imagine the pursuit that might have followed the theft of such a hatchling by Ornitholestes. Clutching the shriek- ing baby in its hands, the “bird-robber” would race away while the mother allosaur followed, franti- cally trying to save her offspring. If, with her longer stride, she managed to gain ground on the fleeing Ornitholestes, the thief would probably veer sharply to one side while the less agile allosaur stumbled to redirect its pursuit. After a time the allosaur would become exhausted from the chase while Ornit- holestes disappeared over the horizon to find a safe place to consume its stolen meal.

Ornitholestes was first discovered in Wyoming in 1900, but little well-preserved and complete mate- rial belonging to this dinosaur has surfaced since that time. Fragmentary remains of Ornitholestes are now known from several sites in Colorado and possibly from the Cleveland-Lloyd Quarry (Stokes 1985) in Utah. But we should recall that the fossils of small dinosaurs are much less likely to be dis- covered than are the remains of larger creatures. In addition, the bones of Ornitholestes and other coe- lurosaurs were much more delicate than those of

the carnosaurs. The relative fragility of coelurosau- rian material strongly reduces the probability of its fossilization, compounding the problem of nondis- covery related to the small size. It is likely that Orni- tholestes was more common during the late Jurassic than the relatively rare fossils preserved in the Mor- rison Formation might suggest. Russell (1989) has estimated that Ornitholestes might have constituted about 6 percent of the dinosaurs present during the late Jurassic in North America, even though its remains represent only 0.6 percent of the fossils col- lected from rocks of this age. These estimates make some sense because many more small animals than large animals are usually present in modern terres- trial vertebrate communities. Ornitholestes, along with its other coelurosaurian relatives, were proba- bly fairly common in Utah during the late Jurassic.

Coelurus

Coelurus was a close relative of Ornitholestes. In fact, ever since the first discovery of Coelurus remains from Wyoming in the late 1870s, paleontol- ogists have been debating whether or not Coelurus and Ornitholestes represent the same genus. More recent studies of the available fossil material of these two genera by J. H. Ostrom (1980) and Carpenter and others (2005) have concluded that they are not the same dinosaur but quite similar. This historical debate reflects both the fragmentary nature of the 5.14. Reconstruction of the skeletons of Ornitholestes

(top) and Coelurus (bottom), two similar small theropods from the Morrison Formation. Based on reconstructions of Carpenter and others 2005.

fossils used to define the two genera and their gen- eral similarity to each other.

Coelurus was a small theropod about 8 feet (2.5 meters) long, 2 feet (0.6 meter) tall at the hip, and weighing around 30 pounds. These dimensions are very close to those of Ornitholestes. Coelurus was evidently a better runner, however, with lon- ger hind limbs and a lighter, more graceful neck (fig. 5.14). The cervical vertebrae are very lightly con- structed, with ribs (or rib facets) that are fused to the body of the neck bones in the form of a more or less cylindrical collar (fig. 5.15). The caudal ver- tebrae are unusual in that they have large internal air spaces. These hollow tail bones provide the basis of the name of the genus: Coelurus means “hol- low tail.” The skull of Coelurus is not well known, but the fragmentary bones available suggest that its head was more slender and delicate than the skull of Ornitholestes. Fossils of Coelurus are extremely rare in the Morrison Formation, and what has been recovered is fragmentary. Thus the classification of Coelurus at the family level remains in doubt and paleontologists are still unsure about its relation- ships with other coelurosaurs. In addition to the original Wyoming discovery, it has been identified in Colorado (Gilmore 1920; Britt 1991) and in Utah

(Norman 1990). Until more complete material is discovered, we can only regard Coelurus as a small Onitholestes-like theropod that might have been seen occasionally on the late Jurassic plains of west- ern North America.

Family Dromaeosauridae? Marshosaurus bicentesimus

The remains of Marshosaurus were first dis- covered at the Cleveland-Lloyd Quarry and were described by James H. Madsen Jr. in 1976 (Mad- sen 1976b). Britt (1991) has identified Marshosaurus fossils from the Morrison Formation at Dry Mesa in western Colorado and Chure and others (1997) reported it from Dinosaur National Monument. In spite of its broad distribution in the western United States, this theropod is known only from isolated bones, including the ilium, pubic, and pelvic bones and several tooth-bearing bones of the skull such as the premaxilla, maxilla, and dentary. Although it is difficult to assign Marshosaurus to any specific fam- ily of theropods with such limited knowledge of the skeleton, it does not appear to represent any of the groups that we have discussed thus far. Marshosau- rus appears to be most similar to various mem- bers of the family Dromaeosauridae, an assemblage that includes the vicious “raptors” that have become familiar in popular culture since the release of the movie Jurassic Park in 1993. Some uncertainty per- sists concerning the family-level classification of Marshosaurus, however, and some paleontologists (e.g., Molnar and others 1990; Benson 2010) assign it to other taxonomic categories or consider its rela- tionships uncertain (Chure and others 2006). If it is a “raptor,” Marshosaurus is the earliest known mem- ber of its lineage to appear in Utah. As we will see in the next chapter, the Dromaeosauridae become well established in Utah in the early Cretaceous, some 30 million years after the Morrison sediments were deposited.

Marshosaurus was a medium-sized thero- pod; with an average length between 15 and 20 feet (5–6.5 m), it was about twice as large as Coelurus 5.15. Cervical vertebra of Coelurus fragilis, in side view

(top) and front view (bottom). Scale bar = 0.75 inch (2 cm). Redrawn from original sketch by Marsh 1881.

or Ornitholestes. The pubic bone has a slight for- ward curve and small “boot” that is unlike that of other late Jurassic theropods (fig. 5.16). Marshosau- rus appears to have had more teeth than Allosaurus or Ceratosaurus, and the details of their serrations and placement in the jaw are different from those of the larger theropods (Madsen 1976b). The detailed morphology of the premaxilla and maxilla of Mar- shosaurus and the way they were joined to each other are similar in some respects to the “raptors” Deinonychus and Velociraptor, but the bones of Mar- shosaurus are larger. In addition, Deinonychus and Velociraptor are both Cretaceous theropods and are much younger than Marshosaurus, casting some doubt on the closeness of their relationship. Mar- shosaurus may not have been the only dromaeosaur inhabiting the Morrison plain during the late Juras- sic. Britt (1991) discovered several teeth that have dromaeosaur affinities at Dry Mesa in western Col- orado, but the precise identity of the tooth bearers cannot be established until more remains of them are found.

Family Ornithomimidae

The family Ornithomimidae (“bird-mimickers”) includes some very interesting coelurosaurs that probably looked much like large reptilian ostriches. The ornithomimids had very long hind limbs, with extremely elongated mid-foot bones (metatarsals). The great lengthening of the metatarsals lifted the ankle high above the ground, as in an ostrich or emu, and increased the stride of these swift preda- tors. The forelimbs of the ornithomimids are very long and slender, with elongated fingers. The long arms and hands would have been much more useful in grasping and manipulating objects than were the stubby forelimbs of the carnosaurs. Ornithomimid skulls are small and narrow but have a relatively large braincase and enlarged eye sockets. All orni- thomimids had a toothless beak that contributed to their general birdlike appearance. The edges of the beak appear to have been very sharp, capable of cut- ting through animal flesh just as hawks and eagles use their beaks to rip prey animals apart without the aid of teeth.

The ornithomimid theropods are most common in the late Cretaceous, but the earliest members of this group seem to have made their appearance in North America in the late Jurassic. Galton (1982) reported the occurrence of Elaphrosaurus, a prim- itive ornithomimid, in the Morrison Formation of Colorado. Only the upper arm bone (humerus) of Elaphrosaurus was found at the site, and very little material belonging to this genus has surfaced from the Morrison anywhere else. Our knowledge of Elaphrosaurus is derived mainly from African speci- mens and suggests that it was a graceful animal with long, slender limbs. The toes and fingers of Elaphro- saurus were also elongated. The size and shape of the ilium and humerus are different in Elaphrosau- rus than in other ornithomimids, however, leading some paleontologists (e.g., Barsbold and Osmól- ska 1990) to consider its family-level classification uncertain. Even though it has not yet been positively identified from Utah, the occurrence of Elaphrosau- rus in the Morrison Formation of the Rocky Moun- tain region is significant for two reasons. First, it 5.16. Pelvis of Marshosaurus bicentesimus, from the

Cleveland-Lloyd Quarry of Emery County, Utah. Scale bar = 2 inches (5 cm). Redrawn from Madsen 1976b.

adds support to the idea of a high degree of fau- nal interchange between North America and Africa during the late Jurassic. Second, it suggests that the ornithomimids might have originated in North America long before the late Cretaceous, when their remains became much more abundant.

Family Troodontidae Koparion douglassi

A relatively recent discovery of a new dino- saur predator from the Morrison Formation was announced in 1994 by paleontologist Daniel Chure. Koparion douglassi (Chure 1994) was named on the basis of teeth discovered in the Brushy Basin Mem- ber at Dinosaur National Monument. Though this genus and species is based on a single small tooth (fig. 5.17), it is clearly different from those of any other theropod known from the Morrison prior to Chure’s discovery. Koparion had tiny teeth, less than 0.1 inch tall. The tooth is recurved toward the rear and has small denticles lining the front and back edges. The largest denticle is positioned at the apex of the tooth and is aligned with the smaller denti- cles along the back edge of the tooth. The crown of the tooth has a slight constriction at its base (Chure 1994). These details of such a tiny tooth may seem trivial, but they are very important. They not only serve to distinguish this tooth from those of all other Morrison theropods but also document the presence of a new family of predators among the Morrison carnivores: the fascinating troodontids. The troodontids (Troodontidae) are a rare fam- ily of theropod dinosaurs formerly known only from the Cretaceous strata of North America and Asia (Osmólska and Barsbold 1990). The troodon- tids were small and highly specialized theropods with very long hind limbs, grasping forelimbs, and a very lightly constructed skull. All troodontids have many small teeth, with relatively large hooked den- ticles similar to those of Koparion. The brain case of the troodontids was very large relative to their body size. In fact these dinosaurs were the “braini- est” of all: they had the highest ratio of brain size to

body size (known as the EQ or encephalization quo- tient) of any known group of dinosaurs. The large orbits (eye sockets) of the troodontids faced more directly forward than did those of any other group of dinosaurs, suggesting that they had excellent ste- reoscopic vision and could see very well in dimly lit habitats such as the shadowy floor of Mesozoic for- ests. In such environments the good depth percep- tion afforded by the placement of their eyes would also have been advantageous in finding and seizing prey animals that might have scampered through the tangle of shrubs and limbs.

These specialized characteristics of the troodon- tids seem to suggest an “intelligent” theropod, a selective predator that hunted small animals in for- ested settings where good visual acuity, agility, and cunning would have been necessary to catch prey. Was Koparion an “intelligent” dinosaur? Until more of its skull and other parts of the skeleton are found, we can’t be sure. If the rest of its skeleton is as simi- lar to the troodontids as are its teeth, however, then Koparion probably could easily have outwitted any of its contemporaries of the late Jurassic. Kopar- ion might have lurked motionless in the shadowy forests on the Morrison plain of Utah, its glisten- ing reptilian eyes watching for movement among the leaves. No motion would have gone unde- tected: it would have responded to the slightest 5.17. A tiny tooth of Koparion douglassi from the Mor-

rison Formation of Dinosaur National Monument. Scale bar = 0.04 inch (1 mm). Based on photograph in Chure 1994.

rustling. Moving silently forward, its head bobbing and weaving to keep the small prey in sight, Kopar- ion might have deliberately approached its unsus- pecting target from a direction calculated to conceal its approach. At just the right instant it might have lunged toward the animal, grasped it in its dex- trous hands, and held the wriggling creature before its large eyes. Koparion might have paused momen- tarily as it rolled the prey around in its hands. Was it studying the animal, deciding whether to eat it or release it? Was it “thinking” at all? Maybe.

Coelurosaur Cuisine: Small Animals of the Morrison Formation

Even though the small carnivorous coelurosaurs are not particularly common in the Morrison For- mation, there are enough indications of their pres- ence that we might wonder what specific animals they captured as food. None of these smaller the- ropods from the late Jurassic of Utah seem to have been capable of successfully attacking the large prey animals, even if they hunted in groups or packs. Instead the large herbivores such as Camptosau- rus, Stegosaurus, and the sauropods were probably the prime targets of the larger dinosaur predators. A stegosaur would probably have had little to fear from Koparion or Ornitholestes.

In addition to dinosaur fossils, the Morrison For- mation in Utah and nearby regions has produced the remains of many different small creatures that might have been pursued by the coelurosaurs. Tur- tles were evidently common in the ponds, lakes, and streams of the late Jurassic plain. For example, two different turtles, Glyptops plicatus and Dino- chelys whitei, are known from Dinosaur National Monument (Chure and Englemann 1989; West and Chure 1984). These turtles or close relatives are known from many other sites in the Morrison For- mation throughout Utah and Colorado. In addi- tion, the remains of lizards and snakes are also fairly common in the Morrison Formation (Callison 1987; Prothero and Estes 1980). Several different croco- diles are also present in the late Jurassic sediments

of Utah, but their remains are usually very fragmen- tary. The fossils of amphibians (such as frogs and salamanders) are very rare in the Morrison Forma- tion, but they do occur in places (Chure and Engle- mann 1989) along with fossils of snails and clams (Gorman and others 2008). Several different kinds of fish are known from the Morrison Formation, including lungfish, freshwater sharks, and heavily scaled bony fish (Kirkland 1987; Chure and Engle- mann 1989). Though none of the Morrison coelu- rosaurs exhibit adaptations for exclusive fish eating, they might occasionally have taken advantage of fish left stranded when ponds or lakes dried up. Also, trace and body fossils yield good evidence that many kinds of insects swarmed over the Morrison basin in Utah, Colorado, and Wyoming (Hasiotis 2004; Gorman and others 2008; Smith and others 2011). The insect fauna included soil bees, termites, crickets, and various aquatic insects.

No bird fossils have yet been positively identified from the Morrison Formation, but such fossils are exceedingly rare in rocks of any age owing to their typically delicate construction. Nevertheless, pale- ontologists suspect that birds were well established by the late Jurassic and that several different kinds of birds may have soared through the Utah skies in the company of flying reptiles known as ptero- saurs. At least seven different kinds of pterosaurs have been identified from fragmentary remains preserved in the Morrison Formation in the Utah region (Foster 2003; Chure and others 2006). These largest of these birdlike reptiles had a wing span of nearly 8 feet (2.5 m), but most were much smaller.

In addition to these small creatures, ratlike mam- mals appear to have been relatively common in the Morrison basin during the late Jurassic, though they are known primarily from tiny isolated teeth. Nonetheless, the assemblage of primitive mammals included at least twenty-one different genera (Foster 2009), such as the multituberculates, triconodon- tids, and symmetrodontids, among others (Cle- mens and others 1979; Prothero and Jensen 1983; Chure and Englemann 1989; Bakker and Carpenter

1990). As the names of these groups of mammals suggest, each is distinguished by the unique mor- phology of its teeth (e.g., fig. 5.18). None of the mam- mals known from the Morrison Formation appeared to have weighed more than about 6 ounces, the size of a large mouse (Foster 2009). Such small creatures might have been an important food source for the small and agile coelurosaurian theropods.

Thus it appears that the small coelurosaurs might have survived on a mixed diet of ratlike mammals, lizards, snakes, and perhaps even birds, pterosaurs, fish, young crocodiles, and insects. Catching such small prey would require specializations much differ- ent from those necessary for pursuing the great herds of sauropods or subduing a large stegosaur. The coe- lurosaurs of the Morrison Formation, in contrast to the large carnosaurs and ceratosaurs, had to be quick, agile, dexterous, and perhaps even “smart” to survive by catching and consuming such small, evasive, and easily concealed prey. As we will see in the coming chapters, this fundamental dichotomy between large and powerful carnivores and small and crafty preda- tors persisted until the end of the Mesozoic, but with different genera filling the two roles. In spite of their prominent differences, the hulking carnosaurs and the lithe coelurosaurs were collaborators in maintain- ing the ecological balance between predators and prey across the Morrison lowland of Utah. Together they limited the populations of the large and small herbi- vores to sustainable levels for at least 10 million years. They were the “blood brothers” of the Jurassic. 5.18. Inner view of the teeth and lower jaw of Zofiabaatar

pulcher, a multituberculate mammal from the Morrison

Formation. Note the scale bar (0.04 = 1 mm), indicating