DINOSAUR NATIONAL MONUMENT

area in northwestern Colorado and northeastern Utah, U.S., set aside in 1915 to preserve rich fossil beds that include dinosaur remains. The monument was enlarged from 80 acres (32 hectares) to about 328 square miles (850 square km) in 1938 and to 330 square miles (855 square km) in 1978 to protect the scenic canyons of the Green and Yampa rivers. These two rivers cut through some 20 tilted and highly coloured geologic formations of the Uinta Plateau, representing 20,000 feet (6,000 m) of strata that are exposed in deeply incised meanders. The rivers join near Steamboat Rock and flow westward as the Green. Wildlife in the area includes deer, beaver, bighorn sheep, antelope, eagles, hawks, and owls. The canyons contain evidence of prehistoric Indian life. The fossil quarry, visitor centre, a museum, nature trails, and campsites are provided for tourists. Natural history Habitats Dinosaurian habitats must have been as diverse as the animals themselves. One can infer something about the habitats of particular dinosaurs from a variety of clues, such as the kind of sedimentary rock in which the remains are preserved, other animal or plant fossils associated with them, and certain anatomic features like claws or hoofs. The kind of rock, its mineral composition, and sedimentary structures such as scour marks are especially important clues. The presence of ripple marks, for example, indicates a shallow-water environment. Fossil plants indicate something about climate. Associated animal remains like turtle, crocodile, or fish scales point to a nearby aquatic environment. Whatever habitat is inferred from clues like these, however, one must keep in mind that it is only an inference and does not necessarily reflect the actual living conditions of the dinosaur in question. Rather, such clues reflect the animal's death environment or burial situation. The condition of the skeleton and its bones and their degree of disarticulation help to reveal the extent of preburial transport. Anatomic features indicate that all dinosaurs were basically terrestrial animals. All had well-developed legs and feet; none had fins or flippers; most had long tails, but only those of the duckbills and their near relatives were deep and flat-sided as might be expected in swimmers. In general, it can be concluded that none were primarily aquatic animals. Of course, that does not preclude aquatic activity; most animals can swim if necessary, but the ability cannot always be predicted from their anatomy. The earliest dinosaurs known are from South America, found in Argentina and Brazil in rocks of the Middle and Late Triassic epochs. The oldest are carnivorous varieties named Eoraptor, Staurikosaurus, and Herrerasaurus. Until 1989, the only known specimens were far from complete, but they suggested that all three kinds occupied distinctly terrestrial habitats with sufficiently large prey communities (not yet discovered) to support their predaceous habits. The encompassing sedimentary rocksthe Santa Maria Formation of Brazil and the Ischigualasto Formation of Argentina, respectivelyindicate lowland, coastal plain environments and lowland streams and lakes. It is not clear which of these predators came first (stratigraphic correlations between Argentina and Brazil are still under study). Associated with Herrerasaurus remains are fragments of another predator, Ischisaurus, and a smaller herbivore, Pisanosaurus. All four predators in question are considered to have been exceedingly primitive theropods (two-legged carnivorous dinosaurs). Eoraptor is the most primitive dinosaur yet discovered, closely resembling the original dinosaur. Presumably, they preyed on small herbivores like Pisanosaurus and on the rhynchosaurs and mammallike reptiles that were abundant at the time. These few specimens represent a meagre beginning (probably because of a highly incomplete early record) of the dinosaurian reign. Before that time, all the continents of the world had joined together to form one very large supercontinent called Pangaea. But movements of the Earth's great crustal plates were changing its geography. By Early Triassic time (245 to 240 million years ago), as dinosaurs were beginning to gain a foothold, Pangaea had started to split apart at a rate averaging a few centimetres a year. The initial separation was an east-west breach called Tethysthe precursor of the Mediterranean Seawhich divided Pangaea into a northern and a southern landmass. The northern landmass, known as Laurasia, consisted of the North American and Eurasian continental plates; the southern landmass, called Gondwanaland, was composed of the African, South American, Indian, Australian, and Antarctic plates. These landmasses continued to break up to form separate continents. In short, it appears that, just as the dinosaur line arose and experienced its initial diversification during the last half of the Triassic Period, the land areas of the world were in motion, splintering and drifting apart. Their respective inhabitants, dinosaurs and others, were consequently isolated from each other. Throughout the Mesozoic Era the ocean barriers grew wider and the separate faunas became increasingly different. As the continents drifted apart, successive assemblages arose on each landmass, diversified, waned, and disappeared, to be replaced by a new fauna. By Late Cretaceous time each continent occupied its own unique geographic position and climatic zone, and its fauna reflected that separation. Food and feeding During the passage of time from the Triassic through the Jurassic and into the Cretaceous, the Earth's vegetation changed slowly from forests rich in gymnosperms (cycadeoids, cycads, and conifers) to angiosperm-dominated forests of palms and hardwoods. Although conifers continued to flourish at high latitudes, palms were increasingly confined to subtropical and tropical regions. These forms of plant life, the vast majority of them high in hard-to-digest cellulose and low in calories and proteins, were the foodstuffs of the changing dinosaur communities. Accordingly, certain groups of dinosaurs, such as the ornithopods, included a succession of types that were increasingly adapted for efficient food processing. At the peak of the ornithopod lineage, the hadrosaurs (duck-billed dinosaurs of the Late Cretaceous) featured large dental batteries, in both upper and lower jaws, consisting of many tightly compressed teeth that formed a long crushing or grinding surface. The preferred food of the duckbills cannot be certified, but at least one specimen found in Wyoming offers an intriguing clue: fossil plant remains in the stomach region have been identified as pine needles. Other Late Cretaceous contemporaries, the ceratopsians (horned dinosaurs), had similarly compacted teeth, forming solid dental batteries that consisted of dozens of teeth. But here the upper and lower batteries occluded in serrated shearing blades rather than crushing or grinding surfaces. Ordinarily, slicing teeth are found only in flesh-eating animals, but the bulky body and the unclawed, hooflike feet of dinosaurs like Triceratops clearly are those of plant eaters. The sharp beaks and specialized shearing dentition of the ceratopsians suggest that they probably fed on tough, fibrous plant tissues, perhaps palm or cycad fronds. The giant sauropods like Diplodocus and Apatosaurus must have required large quantities of plant food, but there is no direct evidence as to the particular plants they preferred. Since angiosperms rich in calories and proteins did not exist during most of the Mesozoic, it must be assumed that these sauropods fed on the abundant conifers and palm trees. Such a cellulose-heavy diet would have required an unusual bacterial flora in the intestines to break down the fibrous tissues. A digestive tract with one or more crop chambers containing millstone batteries might have aided in the food-pulverizing process, but such gastroliths, or stomach stones, have only rarely been found in association with any dinosaur skeleton (the Seismosaurus specimen and its several hundred such stones is an important exception). The food preference of herbivorous dinosaurs can be inferred to some extent from their general body plan as well as the form of their teeth. It is probable, for example, that low-built animals like the ankylosaurs, stegosaurs, and ceratopsians fed on low shrubbery (but not grasses, which had not yet appeared). The tall ornithopods, especially the duckbills, and the long-necked sauropods probably browsed on high branches and treetops. The flesh-eating dinosaurs must have eaten anything they could catch, since predation is a highly opportunistic lifestyle. In several instances the prey victim of a particular carnivore has been established beyond much doubt. Remains of the small predator Compsognathus were found containing a tiny skeleton of the lizard Bavarisaurus in its stomach region. In Mongolia two different dinosaur skeletons were found together, a nearly adult-size Protoceratops in the clutches of its predator Velociraptor. Two of the many skeletons of Coelophysis discovered at Ghost Ranch in New Mexico contained bones of several half-grown Coelophysis, apparently an early Mesozoic example of cannibalism. The skeletons of Deinonychus unearthed in Montana were mixed with fragmentary bones of a much larger victim, the herbivore Tenontosaurus. This last example is significant because the multiple remains of the predator Deinonychus associated with the bones of a single large prey animal, Tenontosaurus, strongly suggests that Deinonychus hunted in packs.