Dinosaurs
'Dinosaur' is the common name given to any of certain extinct reptiles, often very large, that thrived worldwide for some 150 million years and that died out at the end of the Mesozoic Era, about 66.4 million years ago. The popular name comes from the Greek words deinos (³terrible²) and sauros (³lizard²).
The English anatomist Richard Owen proposed the formal term Dinosauria to designate certain giant extinct animals represented by large fossil bones that had been unearthed at several locations in southern England during the early part of the 19th century. Originally applied to just a handful of incomplete specimens, the category Dinosauria now encompasses more than 550 generic names and at least 1,000 species. Not all of these are valid taxa, however, because of either inadequate specimens, duplication of names, or misidentification of findings as dinosaurian. Nevertheless, certain characteristics of the dinosaurs, such as diversity, longevity, and ubiquitous distribution, are well documented by abundant fossil remains recovered from every continent on Earth.
The extensive list of genera and species is testimony of the many different kinds of animals, with widely divergent lifestyles and adaptations, that are known as dinosaurs. Their remains are found in sedimentary rock strata laid down over a period ranging from roughly 230 to 66.4 million years ago (from the Middle Triassic Epoch to the end of the Cretaceous). The abundance of their fossil bones is substantive proof that dinosaurs were the dominant form of terrestrial animal life during the Mesozoic Era. It is likely that the known remains represent a very small fraction, probably less than 0.0001 percent, of all the dinosaurs that once lived. New kinds are added to the roster every year through scientific explorations around the world.
Before Richard Owen introduced the word in 1841, there was no concept of anything like a dinosaur. Quite probably, large fossil bones had been observed long before that time, but there is little record, and no existing specimens, of such findings before 1818. Dragons of Asian and Western legends would seem to have been generated by very early fossil discoveries (which later might have proved to be dinosaur remains), but there is no historical evidence to that effect.
Early 19th-century discoveries
The earliest published record of fossil remains that still exist for verification as dinosaurian was a note in the 1820 American Journal of Science and Arts by Nathan Smith. The bones had been found in 1818 by Solomon Ellsworth, Jr., while he was digging a well at his homestead just east of the Connecticut River in Windsor, Conn., U.S. At the time, the bones were thought to be human, but much later they were identified as Anchisaurus . Even earlier (1800), large birdlike footprints had been noticed on sandstone slabs farther north, in Massachusetts. Pliny Moody, who discovered these tracks, attributed them to ³Noah's raven,² and Edward Hitchcock of Amherst College, who began collecting them in 1835, considered them to be those of some giant extinct bird. The tracks are now recognized as having been made by several different kinds of dinosaurs, and such tracks are still commonplace in the Connecticut River valley today.
Better known are the finds in southern England during the early 1820s by William Buckland, a clergyman, and Gideon Mantell, a physician, discoverers respectively of Megalosaurus and Iguanodon . In 1824 Buckland published a description of the original specimen of Megalosaurus, which consisted of a lower jawbone with a few teeth. The following year Mantell published his ³Notice on the Iguanodon, a Newly Discovered Fossil Reptile, from the Sandstone of Tilgate Forest, in Sussex,² based on several teeth and some leg bones. Both men collected fossils as an avocation and are credited with the earliest published announcements of what later would be recognized as dinosaurs. In both cases, their finds were too fragmentary to permit a clear image of either original animal. In 1834 a partial skeleton was found near Brighton, Eng., which corresponded with Mantell's fragments from Tilgate Forest. It became known as the Maidstone Iguanodon, named after the village where it was discovered. The Maidstone skeleton provided the first glimpse of what these creatures might have looked like.
Two years before the Maidstone Iguanodon came to light, a different kind of skeleton was found in the Weald of southern England. It was described and named Hylaeosaurus by Mantell in 1832 and later proved to be one of the armoured dinosaurs. Other fossil bones began turning up in continental Europe: fragments described and named as Thecodontosaurus and Palaeosaurus by two English students, Henry Riley and Samuel Stutchbury, and the first of many skeletons named Plateosaurus by the naturalist Hermann von Meyer in 1837. Richard Owen named two other fragmentary specimens: a single large tooth that he called Cladeiodon and an incomplete skeleton composed of very large bones that he named Cetiosaurus . Having carefully studied most of these fossil specimens, Owen recognized that all of these bones represented a group of large reptiles that were unlike any living varieties. In a report to the British Association for the Advancement of Science in 1841, he termed these animals Dinosauria, and the word was first published in the association's Proceedings in 1842.
Reconstruction and classification During the decades that followed Owen's announcement, many other kinds of dinosaurs were discovered and named in England and Europe: Massospondylus in 1854, Scelidosaurus in 1859, Bothriospondylus in 1875, and Omosaurus in 1877. Popular fascination with the giant reptiles grew, reaching a peak in the 1850s with the first attempts to reconstruct two of them, Iguanodon and Hylaeosaurus, for the first world exposition, the Great Exhibition of 1851 in London's Crystal Palace. The sculptor Waterhouse Hawkins, under Owen's direction, created life-size models of these two genera, and in 1854 they were displayed together with models of other extinct and living reptiles, such as plesiosaurs, ichthyosaurs, and crocodiles.
Initially the category Dinosauria was adequate to include all of the large nonaquatic reptiles then known from Mesozoic strata of Europe. But by the 1880s it became evident that the Mesozoic fauna was more diverse and complex than had been realized. The first important attempt to establish a more instructive classification of the dinosaurs was made by the English biologist T.H. Huxley as early as 1868. Because he observed that these animals had a number of birdlike features, including their legs, he established a new order called Ornithoscelida. He divided the order into two suborders: first, the Dinosauria, including the iguanodonts, the large carnivores, or megalosaurids, and the armoured forms including Scelidosaurus; and second, the Compsognatha, for the very small, birdlike carnivorous form Compsognathus.
Huxley's classification was replaced by a radically new scheme proposed by his fellow Englishman H.G. Seeley in 1887. Seeley noticed that all dinosaurs possessed one of two distinctive pelvic designs, one like that of birds and the other like that of reptiles. Accordingly, he divided the dinosaurs into two orders, the Ornithischia (with a birdlike pelvis) and the Saurischia (with a reptilian pelvis). The Ornithischia included four suborders: Ornithopoda (Iguanodon and similar herbivores), Stegosauria (plated forms), Ankylosauria (Hylaeosaurus and other armoured forms), and Ceratopsia (horned dinosaurs, just then being discovered in North America). Seeley's second order, the Saurischia, included all the carnivorous dinosaurs, such as Megalosaurus and Compsognathus , as well as the giant herbivorous sauropods, including Cetiosaurus and several immense ³brontosaur² types that were turning up in North America.
In 1878 a spectacular discovery was made in the town of Bernissart, Belg., when several dozen complete, articulated skeletons of Iguanodon were accidentally uncovered in a coal mine during the course of mining operations. Under the direction of the Royal Institute of Natural Science of Belgium, in Brussels, thousands of bones were retrieved and carefully restored over a period of many years. The first skeleton was placed on exhibit in 1883, and today the public can view an impressive herd of Iguanodon. The discovery of these multiple remains gave the first hint that at least some dinosaurs may have traveled in groups. The supervisor of this extraordinary project was Louis Dollo, a zoologist who was to spend most of his life studying Iguanodon, working out its structure, and speculating on its living habits.
American hunting expeditions
England and Europe produced most of the early discoveries and students of dinosaurs, but North America soon began to contribute a large share of both. One leading student of fossils was Joseph Leidy of the Academy of Natural Sciences in Philadelphia, who named some of the earliest dinosaurs found in America, among them Palaeoscincus, Trachodon, Troodon, and Deinodon. Leidy is perhaps best known for his study and description of the first dinosaur skeleton to be recognized in North America, that of the duckbill found at Haddonfield, N.J., U.S., in 1858. He named the specimen Hadrosaurus foulkii. Leidy's theory that this animal probably was amphibious influenced views of dinosaur life for the next century.
Two Americans whose work in the second half of the 19th century had worldwide impact on the science of paleontology in general, and the growing knowledge of dinosaurs in particular, were O.C. Marsh of Yale College and E.D. Cope of the University of Pennsylvania and the Academy of Natural Sciences in Philadelphia. All previous dinosaur remains had been discovered by accident in well-populated regions with temperate, moist climates, but Cope and Marsh astutely focused their attention on the arid North American West, which had wide expanses of bare, exposed rock. In their intense quest to find and name new dinosaurs, these scientific pioneers became fierce and unfriendly rivals.
Marsh's field parties explored widely, exploiting dozens of now famous areas, among them Yale's sites at Morrison and Canon City in Colorado and, most important, Como Bluff in southeastern Wyoming. The discovery of Como Bluff in 1877 was a momentous event in the history of paleontology, generating a burst of exploration and study and a widespread public enthusiasm for dinosaurs. Como Bluff brought to light one of the greatest assemblages of dinosaurs, both small and gigantic, ever found. For decades the site went on producing the first known specimens of renowned dinosaurs like Stegosaurus, Camptosaurus, Camarasaurus, Laosaurus, Coelurus, and others. From the Morrison site came the original specimens of Allosaurus, Diplodocus, Atlantosaurus, and Apatosaurus (sometimes called Brontosaurus). Canon City provided bones of a host of dinosaurs, including Stegosaurus, Brachiosaurus, Allosaurus, and Camptosaurus.
Another major historic site was the Lance Creek area of northeastern Wyoming. There J.B. Hatcher discovered and collected dozens of horned dinosaur remains for Marsh and Yale College, among them the first specimens of Triceratops and Torosaurus. Marsh was aided in his work at these and other localities by the skills and efforts of many other collaborators like Hatcher‹William Reed, Benjamin Mudge, Arthur Lakes, William Phelps, and Samuel Wendell Williston, to name a few. Marsh's specimens now form the core of the Mesozoic collections at the National Museum of Natural History of the Smithsonian Institution and the Peabody Museum of Natural History at Yale University.
Cope's dinosaur explorations ranged as far as, or farther than, Marsh's, and his interests encompassed a wider variety of fossils. Due to a number of circumstances, however, Cope's dinosaur discoveries were fewer and his collections far less complete than those of Marsh. Perhaps his most notable achievement was finding and proposing the names for Coelophysis and Monoclonius . Cope's dinosaur explorations began in the eastern badlands of Montana, where he discovered Monoclonius in the Judith River Formation of the Cretaceous period. Accompanying him there was a talented young assistant, Charles H. Sternberg. Later Sternberg, with his three sons, went on to recover countless dinosaur skeletons from the Late Cretaceous Oldman and Edmonton formations along the Red Deer River of Alberta, Canada.
Dinosaur ancestors During the early decades of dinosaur discoveries, little thought was given to their evolutionary ancestry. Not only were few specimens known, and those specimens so unlike any living animal, but the concept of evolution itself was still a radical idea. With the growing acceptance of Charles Darwin's theory on the mutability of species during the last half of the 19th century, the question of dinosaurian origins acquired respectability and serious thought.
Early on, it was recognized that, as a group, dinosaurs appear to be most closely allied with crocodilians. Two anatomic features‹socketed teeth and a doubly fenestrated (diapsid) skull‹are present in both. The earliest crocodilians occurred nearly simultaneously with the first known dinosaurs, so neither could have given rise to the other. The most likely ancestry of dinosaurs lies within a poorly understood group of Triassic reptiles termed pseudosuchian ("false crocodile") thecodonts ("socket-toothed reptiles").
An early candidate for ancestor of the dinosaur was an advanced thecodont of South Africa, Euparkeria , of the Early Triassic epoch. Euparkeria was a diapsid with socketed teeth, a preorbital fenestra (opening), and semierect hind limbs‹conditions all equivalent to, or approaching, those of dinosaurs. New discoveries suggest an even more dinosaur-like creature in the Middle Triassic small South American form Lagosuchus.
The earliest appearance of "true dinosaurs" is almost impossible to pinpoint. First, it can never be known with certainty that the very first (or last) specimen of any kind of organism has been found. The stratigraphic succession is discontinuous and contains many gaps in the geologic record. Similarly, the fossil record of dinosaurs and other creatures contained in the rock strata is far from complete.
Second, evolution from ancestral to descendant form is usually a gradational process; consequently, in the transformation from a theoretical thecodont ancestor to a recognizable dinosaur, it is extremely difficult to determine at exactly what point every diagnostic feature of the dinosaurian condition first appeared. A true dinosaur possessed all of the following anatomic features: a diapsid skull, a preorbital fenestra, a mandibular fossa, a perforated hip socket, an offset femoral head, a fourth trochanter of the femur, a mesotarsal ankle joint, digitigrade feet, and at least four sacral vertebrae. The first such animal is still being sought.
Pre-Triassic and Early Triassic reptiles that had acquired some of these features, the archosaurians ("ruling lizards"), diversified along a variety of evolutionary pathways. Only a few, however‹possibly one‹passed on to the dinosaurs an improved stance and posture with a resulting improved gait, increased efficiency of food gathering and processing, apparently higher metabolic rates and cardiovascular nourishment, and, for most, an overall increase in size. All these trends, individually or in concert, probably contributed to the collective success of dinosaurs, which resulted in their dominance among the terrestrial animals of the Mesozoic.
Modern studies During the first century or more of dinosaur awareness, workers in the field more or less concentrated on the search for new specimens and new kinds of animals. Their discoveries then required detailed description and analysis followed by comparisons with other known kinds in order to classify the new finds and develop theories about dinosaur evolutionary relationships. All these pursuits continue, but newer methods of exploration and analysis have been adopted. Emphasis has shifted from purely descriptive procedures to quantitative analytical and multivariate statistical analysis and the application of such analysis to functional anatomic systems.
Functional anatomic studies make extensive use of living analogues that, together with both mechanical and theoretical models, make it possible to visualize certain aspects of the once living animal. For example, reconstruction of the limb musculature, combined with examination of the biomechanics of the leg and joint skeletomuscular system and analysis of trackways, can provide information about an animal's locomotion‹walking and running‹and estimates of normal walking and maximum running speeds. The same method has been applied to jaw mechanisms and tooth wear patterns for a better understanding of feeding habits and capabilities.
Original colours and patterns cannot be known, but it is possible to speculate on them with an understanding of the ecological functions of pattern and colour in modern analogues. Are large animals mostly brightly coloured or drab? How important is colour vision, and what kinds of organisms see colours? Dinosaur skin texture has rarely been preserved, but there are a few examples. Most show a knobby or pebbly surface and not a scaly texture such as might be expected in reptiles. What might that indicate about dinosaur environments or about dinosaur relatives? In short, modern inquiry focuses more on the biology of dinosaurs and their various modes of life than on their immense size and strange design.
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 ³original² 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 ³stomach stones,² 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 ³all were headed toward a common objective² and suggested that the sauropod track-makers ³passed in a single herd.² 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 Ricqlès 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, ³site fidelity² 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 ³cold-blooded² lizards have higher normal body temperatures than do some mammals, for instance. More precise terms for these conditions are ³ectothermy² and ³endothermy.²
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 ³furnaces² must produce heat continuously, and that requires correspondingly high quantities of ³fuel² (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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