About Dinosaurs - 2

Dinosaurs - 2

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 rocks�the Santa Maria Formation of Brazil and the Ischigualasto Formation of Argentina, respectively�indicate 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 Tethys�the precursor of the Mediterranean Sea�which 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.

Herding behavior

That Deinonychus was a social animal should not come as a surprise. Many animals today are gregarious and form groups. Fossil evidence documents similar herding behaviour in a variety of dinosaurs. The mass grave in Bernissart, Belg., held a large assembly of Iguanodon. The dozens of skeletons of Coelophysis of all ages recovered in New Mexico indicate group association and activity. The many specimens of Allosaurus at the Cleveland-Lloyd Quarry in Utah may denote a herd of animals attracted to the site for the common purpose of scavenging.

These rare multiple occurrences of skeletal remains have repeatedly been reinforced by dinosaur footprints that register herding habits. First noted by Roland T. Bird in the early 1940s, a series of large, basin-size depressions along the Paluxy riverbed in central Texas proved to be a succession of giant sauropod footsteps preserved in the Early Cretaceous limestone of the region. Bird noticed that there were many trackways and that they were nearly parallel and progressed in the same direction. He concluded that �all were headed toward a common objective� and suggested that the sauropod track-makers �passed in a single herd.� Large trackway sites are known in the eastern and western United States, Canada, Australia, England, Argentina, South Africa, China, and other places. These sites, ranging in time from the Late Triassic to the latest part of the Cretaceous, document herding as common behaviour among a variety of dinosaur types.

Some dinosaur trackways register hundreds, perhaps even thousands, of animals, possibly recording mass migrations. They suggest the presence of great populations of sauropods, prosauropods, ornithopods, and probably most other kinds of dinosaurs. The majority must have been herbivores, and many of them were huge, weighing several tons or more. The impact of such large herds on the plant life of the time must have been devastating.

Growth and life span

Much attention has been devoted to dinosaurs as once-living animals�as moving, eating, growing, and reproducing biological machines. But how fast did they grow? How long did they live? How did they reproduce? The evidence concerning growth and life expectancy is sparse. Histological studies by Armand de Ricql�s in Paris and R.E.H. Reid in Ireland show that plexiform perichondral bone in dinosaur skeletons grew quite rapidly. The time required for full growth has not been quantified, but the life span of most dinosaurs would seem to have been short and probably did not exceed five or six decades. The largest varieties probably lived longer than the smaller ones, but no precise age has been determined for any kind.

Reproduction

As for reproduction, considerable evidence is now available. The idea that dinosaurs, like most living reptiles and birds, built nests and laid eggs had been widely debated before the 1920s, when a team of scientists from the American Museum of Natural History made an expedition to Mongolia. Their discovery of dinosaur eggs in the Gobi proved conclusively that at least one kind of dinosaur, Protoceratops, had been an egg layer and nest builder. These findings were substantiated in 1978 when John R. Horner discovered dinosaur nests in western Montana. A few other finds, mostly of eggshell fragments from a number of sites, established oviparity as the dinosaurian mode of reproduction.

The almost complete absence of juvenile dinosaur remains, however, was puzzling. Horner, first of Princeton University and later of Montana State University, demonstrated that most paleontologists simply had not been exploring the right territory. After a series of intensive searches for immature dinosaur material, he succeeded beyond all expectations. He unearthed the first such bones near Choteau, Mont., U.S., and during the 1980s he and his field crews discovered hundreds of nests, eggs, and newly hatched dinosaurs, mostly of the duck-billed variety. Horner observed that previous explorations had usually concentrated on geologically old lowland areas, where sediments were commonly deposited and most fossil remains were preserved.

He recognized that those regions were not likely to produce dinosaur nests and young because they would have been hazardous places for nesting and raising hatchlings. Upland regions would have been safer, but they were subject to erosion rather than deposition and therefore less likely to preserve nests and eggs. It was exactly in such ancient upland areas, though, close to the rising young Rocky Mountains, that Horner made his discoveries.

Egg Mountain, as the area was named, produced some of the most important clues to dinosaurian habits yet found. For example, the sites show that a number of different dinosaur species made annual treks to this same nesting ground (perhaps not all at the same time). Because of the stratigraphic succession of like nests and eggs one on top of the other, it is thought that particular species returned to the same site year after year to lay their clutches. As Horner concluded, �site fidelity� was an instinctive part of dinosaurian reproductive strategy. If site fidelity was a universal instinct among dinosaurs, that strategy could help to explain their success for some 150 million years. As mountain building increased toward the end of the Mesozoic era, geologic processes might have reduced appropriate nesting grounds and contributed to the decline and eventual extinction of dinosaur communities.

Body temperature There is no doubt about the dinosaurs' success. Their worldwide domination of the land during the Mesozoic and what brought it about is every bit as important as what caused their extermination, or more so. Understanding their success requires further consideration of them as living animals. Beyond eating, digestion, assimilation, reproduction, and nesting, there are many other processes and activities that go into making a successful biological machine. Breathing, fluid balance, temperature regulation, and other such capabilities are also required. Dinosaurian body temperature regulation, or lack thereof, has been a hotly debated topic among students of dinosaur life.

Body temperature

Ectothermy and endothermy

All land animals possess some degree of thermoregulation. Much of the terrestrial environment is highly variable and beyond the control of most organisms. The internal environment of the body is under the influence of both external and internal conditions. When the outside world is hotter than preferred, organisms usually respond by moving to a cooler spot. Some perspire or pant to increase cooling. When it is dangerously cold, organisms may move to warmer climates (migrate), generate heat (shiver), or conserve body heat and energy by lowering their metabolism (hibernate), and the human species, of course, can further adjust body temperature by artificial means. The so-called warm-blooded animals today are the mammals and birds; reptiles, amphibians, and most fish are labeled as cold-blooded. These two terms are imprecise and misleading. Some �cold-blooded� lizards have higher normal body temperatures than do some mammals, for instance. More precise terms for these conditions are �ectothermy� and �endothermy.�

Ectothermy is that state in which thermoregulation depends on the behaviorally and autonomically regulated uptake of heat from the external environment. Endothermy, on the other hand, depends on a high (tachymetabolic) and controlled rate of internal heat production. Mammals and birds have a high metabolism, which produces body heat internally. They possess temperature sensors that control heat production and switch on heat-loss mechanisms such as perspiration. Reptiles and amphibians are ectotherms that must gain heat energy from sunlight, a heated rock surface, or some other external source. The endothermic state is effective but expensive; the metabolic �furnaces� must produce heat continuously, and that requires correspondingly high quantities of �fuel� (i.e., food). On the other hand, endotherms can be active and can survive quite low external temperatures. Ectotherms do not require as much fuel, but most cannot deal as well with cold surroundings.

What, then, about the dinosaurs? From the time of the earliest discoveries in the 19th century, experts like Owen, Leidy, Marsh, and Cope classified all then-known dinosaur remains as reptilian because they exhibited a set of anatomic features that were typical of living reptiles like turtles, crocodiles, and lizards. Dinosaurs all had lower jaws constructed of several bones, featured a reptilian jaw joint, and possessed a number of other nonmammalian characteristics. Consequently, it was assumed that living dinosaurs were like living reptiles�scaly, cold-blooded ectotherms and not furry, warm-blooded creatures that gave live birth. A chauvinistic attitude seems to prevail that the warm-bloodedness of mammals is better than the cold-blooded reptilian state. Turtles, snakes, and other reptiles, however, do very well by regulating their body temperature in a different way.




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