'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.

Clues to dinosaurian metabolism

The physiology of dinosaurs is unknown for the simple reason that their temperatures cannot be measured, nor can their food consumption or carbon dioxide and solid waste output be determined (the usual methods of measuring an animal's metabolic rate). Indirect evidence is all that is available. The question whether any dinosaur species was a true endotherm cannot be answered, but some interesting anatomic facts suggest that possibility.

First of all, two dinosaurian clans, the hadrosaurs and the ceratopsians, featured highly specialized dentitions that obviously were effective food processors. Both groups were herbivorous, but unlike living reptiles they chewed their foliage thoroughly. Such highly efficient dental equipment implies that the hadrosaurs and ceratopsians were tachymetabolic. With the exception of the carnivores and possibly some ornithopod predecessors of the duckbills, like Heterodontosaurus and Iguanodon, other dinosaurs generally possessed very weakly developed dentitions.

On another tangent, certain of the predaceous dinosaurs had anatomic features that reflect a high capacity for activity. The 'ostrich dinosaurs' like Struthiomimus , Gallimimus, and Dromiceiomimus, for example, all were obligatory bipeds (two-footed animals) that, on the basis of their long hind legs, must have been very fleet. Further, the dromaeosaurs like Deinonychus , Velociraptor , and Dromaeosaurus , although they also were obligatory bipeds, killed their prey with the talons on their feet. It must have taken a high level of metabolism to generate the degree of activity and agility required by such a skill. The implication is compelling, but conclusive proof of endothermy is lacking.

Dinosaurian posture is also suggestive. Many (but not all) dinosaurs stood upright with the legs positioned directly beneath the hip sockets and, in some, the shoulder sockets. Such an erect posture is present in all nonaquatic endotherms (mammals and birds), but a sprawling or semierect posture is typical of all ectotherms (reptiles and amphibians). Bipedal stance and gait are not possible in any living ectotherm. Why is that? And what is the implication for all of the theropod dinosaurs?

Related to the upright posture of many dinosaurs is the inescapable fact that the head was usually positioned at a high level, often well above the level of the heart. In some extreme cases (Apatosaurus, Diplodocus, Brachiosaurus , and Barosaurus, for instance), the brain must have been several metres above the heart. The importance of this is that a four-chambered heart would need to have been present to pump freshly oxygenated blood to the brain. Brain death follows very quickly when nerve cells are deprived of oxygen, and to prevent it most dinosaurs must have required two ventricle pumps.

In a four-chambered heart, one ventricle pumps oxygen-poor venous blood at low pressure to the lungs to absorb fresh oxygen (low pressure so as not to rupture the pulmonary capillaries). A powerful second ventricle pump circulates the freshly oxygenated blood from the lungs to all other parts of the body at high pressure; the high systemic pressure is needed to overcome the weight of the column of blood that must be pumped from the heart to the elevated brain. In short, like birds and mammals, many dinosaurs apparently had the required double-pump heart that is necessary for an animal with a high metabolism.

The significance of thermoregulation can be seen by comparing modern reptiles with mammals. The rate of metabolism is usually measured in terms of oxygen consumed per unit of body weight per unit of time. The resting metabolic rate for most mammals is on the order of 10 times that of modern reptiles, and the range of metabolic rates of living mammals is about double that of reptiles. These differences mean that endothermic mammals have much more endurance than their cold-blooded counterparts. Some dinosaurs may have been so endowed. They seem to have possessed the cardiovascular system necessary for endothermy, but that capacity does not prove that they were endothermic. The probabilities are that dinosaurs were neither complete ectotherms nor complete endotherms but were somewhere in between.

Form and function

Differentiation of the dinosaurian orders

The two traditional orders of dinosaurs established by Seeley, Saurischia and Ornithischia, long believed to be closely related, are now widely believed to have evolved from a common ancestor‹an as-yet unrecognized (or undiscovered) primitive archosaurian reptile. The chief difference between the two orders was in the configuration of the pelvis. It was primarily on this distinction that Seeley established the orders and named them Saurischia ('Lizard Hips') and Ornithischia ('Bird Hips'), a differentiation still maintained today.

As in all four-legged animals, the dinosaurian pelvis was a paired structure consisting of three separate bones on each side that attached to the sacrum of the backbone. The ilium, above (attached to the spine), and the pubis and ischium, below, formed a robust bony plate at the centre of which was a deep cup‹the hip socket, or acetabulum. The hip socket faced laterally and was pierced or open at its centre for the articulation of the medially projecting proximal head of the thighbone. The combined saurischian pelvic bones presented a triangular outline as seen from the side, the pubis extending down and forward and the ischium projecting down and backward from the hip socket.

The massive ilium formed a deep vertical plate of bone to which the muscles of the pelvis, hind leg, and tail were attached. The pubis had a stout shaft, commonly terminating in a pronounced expansion or bootlike structure (presumably for muscle attachment), that joined its opposite mate in a solid symphysis. The ischium was slightly less robust than the pubis, but it too joined its mate in a midline symphysis. There were minor variations in this structure between different saurischian genera and families.

The ornithischian pelvis was constructed of the same three bones on each side of the sacral vertebrae, to which they attached by coossification. The lateral profile of the pelvis was quite different from that of the saurischians, with a long but low iliac blade above the hip socket and a modified ischium-pubis structure below. Here, the long, thin ischium extended backward and slightly downward from the hip socket. The pubis had a short to moderately long anterior blade, but posteriorly it stretched out into a long, thin postpubic process lying beneath and closely parallel to the ischium. The resulting configuration resembled that of birds, whose pubis is a thin process extending backward beneath the larger ischium.

These anatomic dissimilarities are believed to reflect important differences in muscle arrangements in the hips and hind legs of these two orders. Other marked dissimilarities between saurischians and ornithischians are found in their jaws and teeth, their limbs, and especially their skulls. Details on these differences are given in the following discussions of the major dinosaur groups. The table shows how the two orders are subdivided. It is important to note that the classification of dinosaurs involves a high degree of uncertainty, which may result in variations in the way dinosaurs are grouped depending on the authority.

Saurischia

The order Saurischia is known from specimens ranging from the Middle Triassic to the latest part of the Cretaceous in geologic time and recovered from every continent on the Earth. Two distinctly different suborders are traditionally included in the order‹the Sauropodomorpha (herbivorous sauropods and prosauropods) and the Theropoda (carnivorous dinosaurs). These groups are placed together only because both have the saurischian type of pelvis along with a few other primitive archosaurian features in common. No common ancestor has been widely recognized, and they could just as well be placed in separate orders. A little-known group, the Staurikosauria, is also classified in the order.

Included in this group as infraorders are the well-known sauropods, or ³brontosaur² types, and their probable ancestral group, the prosauropods. All were plant eaters.

Prosauropoda

Most primitive of the Sauropodomorpha were the early (Triassic) saurischians known as prosauropods or plateosaurs. Found in Late Triassic and Early Jurassic rocks (230 to 187 million years old), their remains are probably the most ubiquitous of all Triassic dinosaurs. They have been found in Europe (Germany), North America (New England, Arizona, New Mexico), South America (Argentina), Africa (South Africa, Lesotho, Zimbabwe), China (Yunnan), and Antarctica.

The best-known examples are Plateosaurus of Germany and Massospondylus of South Africa. Prosauropods were not large, as dinosaurs go, ranging from less than 2 metres (7 feet) in length up to about 7 metres (23 feet) and about 1 ton in maximum weight. Because their forelimbs were conspicuously shorter than their hind limbs, these animals (known from very complete skeletons) usually have been reconstructed poised on their two hind legs in a bipedal stance. Their anatomy, however, clearly indicates that some of them could assume a quadrupedal (four-footed) position. Footprints generally attributed to prosauropods appear to substantiate a quadrupedal form of locomotion.

Prosauropods have long been seen as including the first direct ancestors of the giant sauropods, probably among the family of melanorosaurids. That view still prevails, largely because of their distinctly primitive sauropod-like appearance and also because of their Late Triassic­Early Jurassic occurrence. No better candidate has been discovered so far. In general body form they were rather stocky, with a long, moderately flexible neck (containing surprisingly long and flexible cervical ribs) and a head that was small in comparison with the body. The jaw was long and contained rows of thin, leaflike teeth suited for chopping up (but not grinding or crushing) plant tissues, although there is an indication of direct tooth-on-tooth occlusion.

Prosauropod forelimbs were stout and heavily built, with five complete digits. The hind limbs were about 50 percent longer than the forelimbs and even more heavily built. The foot was of primitive design, and its five-toed configuration could be interpreted as a forerunner of the sauropod foot. Walking apparently was semidigitigrade (partly on the toes), with the metatarsus held well off the ground. The vertebral column was unspecialized and bore little indication of the cavernous excavations that were to come in later sauropod vertebrae or of the processes and projections that were to buttress the sauropod vertebral column. The long tail probably served as a counterweight or stabilizer whenever the animal assumed a bipedal position.

A large number of skeletons of Plateosaurus were recovered in the 1920s from a site in Germany. The occurrence there of multiple remains, together with the nature of the enclosing sediments, led to the thought that this assemblage had been overcome by drought and sandstorm while migrating to more suitable environs. Whether migrating or not, the abundance of Plateosaurus individuals in one location lends support to the herding instinct that has been attributed to several kinds of dinosaurs.

The more widely known sauropods - the huge 'brontosaurs' - varied in length from 6 or 7 metres (about 20 feet) in the primitive ancestral sauropods Riojasaurus of South America and Vulcanodon of Africa up to 28 to 30 metres (90 to 100 feet) or more in advanced Jurassic North American forms like Apatosaurus (formerly known as Brontosaurus), Diplodocus, and Seismosaurus. Weights ranged from about 20 tons or less in the smaller kinds like Barapasaurus of India to 80 tons or more for the gigantic Brachiosaurus of Africa and North America. Sauropods were worldwide in distribution but have not as yet been found in Antarctica. In geologic time they ranged from the Late Triassic Riojasaurus to the Late Cretaceous Alamosaurus of North America and Laplatasaurus of South America. Their greatest diversity and abundance took place during the Late Jurassic (163 to 144 million years ago).

Sauropods are notable for their body form as well as their enormous size. Their large bodies were almost barrel-shaped, with long (sometimes very long) necks and tails. They had columnar legs, like those of elephants, with little freedom to bend at the knee and elbow. The legs were maintained in a nearly vertical position beneath the shoulder and hip sockets. Because of their great bulk, sauropods unquestionably were obligatory quadrupeds, and the largest forms could not have assumed a bipedal stance even momentarily.

The sauropod limb bones were heavy and solid. The feet were broad, close to plantigrade (adapted for walking on the soles), and graviportal (adapted for bearing great weight). The five toes were generally short, blunt, and broad, but some kinds featured a large straight claw on the first digit of the forefoot and the first and second toes of the hind foot. These claws probably improved traction. Movement for these animals must have been relatively slow, with short steps necessary because of the comparative inflexibility of the limbs. Running must have been stiff-legged and no better than an elephantine pace of 16 kilometres (10 miles) per hour, if that. Their tremendous bulk placed them out of the reach of predators and eliminated any need for speed.

The vertebrae of the backbone were highly modified, with numerous excavations and struts to reduce bone weight. Complex spines and projections for muscle and ligament attachment compensated for any loss of skeletal strength that resulted from reductions in bone density and mass. The long and sometimes massive tails, characteristic of so many sauropods, would appear to have been carried well off the ground. Tail drag marks associated with sauropod trackways are not known, and damaged (stepped-on) tails are also not known, even though these animals apparently traveled in herds (albeit of undetermined density).

The sauropod tail may have served as a modest whiplike weapon, but there is no evidence to that effect. Another possible use of the tail may have been thermal regulation‹improved heat loss through its large surface area. A more likely explanation of its function is that the massive muscular base of the tail was the critical anchor site of the large, powerful hind leg muscles that produced most of the walking force required to move the many tons of sauropod weight. The muscle arrangement of the tail was precisely that of modern alligators and lizards.

The most important part of any skeleton is the skull, because it provides the most information about an animal, its mode of life, and its general biology. Sauropod skulls were of several main types; the high, boxy Camarasaurus type (often incorrectly associated with Apatosaurus) and the low, narrow, streamlined Diplodocus type. The former had broad, spatulate teeth, while the latter had narrow, pencil-shaped teeth. Both kinds of teeth seem weak and totally ill-designed to crop or chew the volumes of plant food necessary to sustain such large animals. Correspondingly, the jaws were relatively weakly developed, and there is no special evidence indicating powerful jaw muscles to activate the feeding system.

Until recently, sauropods were visualized as swamp or lake dwellers because their legs were thought to be incapable of supporting their great weights or because such huge creatures would naturally prefer the buoyancy of watery surroundings. Not only is that thinking incompatible with their food requirements (food would seem to have been most plentiful on land), but research has refuted it. Experiments with fresh bone samples have shown that bone of the type which composed the sauropods' limb bones could easily have supported their estimated weights. Moreover, there is no feature in their skeletons that suggests an aquatic, or even amphibious, existence. In addition, numerous trackway sites clearly prove that sauropods could navigate on land, or at least where the water was too shallow to buoy up their weight. Accordingly, newer interpretations see these animals as forest inhabitants.

Still another blow has been dealt to the old swamp image by the physical laws of hydrostatic pressure, which prohibit the explanation that the long neck enabled a submerged animal to raise its head to the surface for a breath of fresh air. The depth at which the lungs were submerged would not allow them to be expanded by normal atmospheric pressure, the only force that fills the lungs. Consequently, the long necks of sauropods must be explained in terms of terrestrial functions such as elevating the feeding apparatus or the eyes. On all counts, sauropods are best seen as successful giraffelike browsers and only occasional waders.

Staurikosauria

Very little is known about the animals grouped in this suborder because so few specimens have been found, and all of those are fragmentary. The remains of Staurikosaurus, from the Middle Triassic of Brazil, the specimen for which the suborder was established, consist only of the vertebral column and pelvis, the hind legs lacking the feet, and the lower jaw. The skull and forelimbs, like the feet, are not known. Those parts that are known, however, are similar to those of the later theropods, and a flesh-eating habit is suggested by the piercing teeth of the lower jaw. The skeleton indicates a moderate-size animal of about two metres (seven feet) in length, possibly having a bipedal posture and gait. Appearing as it does in Middle Triassic rocks, it may be the oldest kind of dinosaur known‹if in fact it proves to be a dinosaur.

Several other fragmentary remains from South America may be of related types. Specimens of Herrerasaurus and Ischisaurus from Middle to Late Triassic rocks of Argentina are those of carnivores about the same size as Staurikosaurus or slightly heavier. But again, the material is so incomplete that relationships are still uncertain. What is preserved suggests a theropod identity.

Theropoda

This group includes all the other known carnivorous dinosaurs. No herbivores are recognized in the group. Theropods ranged in size from the smallest known adult dinosaur, Compsognathus , the size of an ordinary chicken and probably weighing 1 or 2 kilograms (2 to 4 pounds), up to the great Tyrannosaurus , which was 15 or more metres (50 feet) long and over 5 metres (16 to 18 feet) tall and which weighed 6 tons or more. In all theropods the hind leg bones were hollow to varying degrees‹extremely hollow and lightly built in small to medium-size animals like Compsognathus, Coelurus, and Ornitholestes and more solid in the larger forms like Allosaurus, Daspletosaurus, and Tarbosaurus. Theropods have been recovered from rocks of the Late Triassic through the latest part of the Cretaceous and from all continents except Antarctica.

In stance and gait, theropods were obligatory bipeds. Their bodies conformed to a common shape in which the hind legs were dominant and designed for support and locomotion. The forelimbs, on the other hand, had been modified from the primitive design and entirely divested of the functions of locomotion and body support. Hind limbs were either very robust and of graviportal (weight-bearing) proportions, as in Allosaurus, Megalosaurus, and the tyrannosaurids, or very slender, elongated and of cursorial (adapted for running) proportions, as in Coelurus, Coelophysis, Ornitholestes, and the ornithomimids.

Theropod feet, despite the group's name, which means ³beast (i.e., mammal) foot,² usually were designed like those of birds. Three main toes were directed forward and splayed in a V-arrangement; an additional inside toe was directed medially or backward; and the whole foot was functionally digitigrade, with the ³heel² elevated well above the ground. Toes usually bore sharp, somewhat curved claws.

The forelimbs varied widely from the slender, elongated ones of Struthiomimus, for example, to shorter, more massively constructed grasping appendages like those of Allosaurus, to the greatly abbreviated arms and hands of Tyrannosaurus. The hands typically featured long, flexible fingers with pronounced, often strongly curved claws, which must have borne sharp piercing talons. Primitive theropods like Coelophysis had four fingers, but the majority were three-fingered. Tyrannosaurids (Albertosaurus, Daspletosaurus, Tarbosaurus, and Tyrannosaurus) and apparently the diminutive Compsognathus were notable for their two-fingered hands on unusually short arms. This separation of function between fore and hind limbs set theropods apart from all other dinosaurs.

As obligatory bipeds, the theropod body plan was reorganized so that the animal, large or small, balanced at the pelvic pedestal by using its heavy tail behind and thrusting the thighs and knees forward to positions directly below the centre of gravity. To shift the centre of gravity back toward the supporting hind legs, the head and neck were arched upward, especially so in those kinds with very large heads. In addition, the bipedal stance required reinforcing the backbone by enlargement of the interspinous ligaments (between the back vertebrae) and the ligaments of the neck. The tail probably served not only as a counterweight to the large body and head but also as a dynamic inertial stabilizer, moving up and down and from side to side to counteract changes in the animal's movement and direction such as lunging and turning during an attack.

Bipedality has sometimes been explained as an adaptation for fast running or for energy conservation. The latter seems unlikely in view of several experiments showing that it requires no less energy to run or walk on two legs than it does on four. Speed does not seem to have been the primary factor either, although some investigators have claimed that a seven-ton Tyrannosaurus could achieve an unlikely velocity of 70 kilometres (45 miles) per hour - faster than a greyhound or a racehorse. Rather, because of their great weight, tyrannosaurids probably could have barely kept ahead of a charging elephant (20 kilometres per hour), whereas the more cursorial-limbed ornithomimids might have been able to keep up with a modern ostrich (70 kilometres per hour). Rather than as a specialization for running, bipedality may have come into being and perfection more as an enhancement of viewing range. Theropods all had large eyes and a wide field of vision.

Theropods that featured large heads, like Allosaurus and Tyrannosaurus, had long, strong lower jaws that undoubtedly were powered by massive jaw muscles. The skull was a highly fenestrated strut work, both for lightness and for strength, providing ample attachment areas for muscles. The jaws are noted for their complement of sharp, bladelike teeth. In nearly all theropods these laterally compressed blades had steak-knife-like serrations along the rear edge and often along the front edge as well. Among the predatory adaptations displayed by most kinds of theropods, the characteristic teeth were the most conspicuous. The diversity of the suborder Theropoda, with its various modes of predation and carnivory, is suggested by the following summary of the group's two infraorders.

Ceratosauria

These are the basal or primitive theropods of medium size like Ceratosaurus, Dilophosaurus, and perhaps Coelophysis from the Late Triassic and Late Jurassic. They may include ancestral stock of most later theropods.

Theropoda

Tetanurae

These comprise all the nonceratosaurian theropods. The tetanuran theropods are subdivided into five distinct categories: the coelurosaurs, the ornithomimosaurs, the maniraptors, the segnosaurs, and the carnosaurs.

The coelurosaurs were small to medium-size carnivores, the smallest known being Compsognathus. Coelurosaurs had very long legs of cursorial proportions and had forelimbs and hands ranging from short (Compsognathus) to long and grasping (Ornitholestes).

Ornithomimosaurs were medium-size to large theropods. They were toothless and apparently beaked, with very long legs and arms. A well-known example is Struthiomimus. Most were ostrich-size and were designed for fast running. The largest kind was Deinocheirus from Asia, known only from one specimen consisting of complete arms and hands almost three metres (nine feet) long‹nearly four times longer than those of Struthiomimus. These animals' cursorial design, toothlessness, and hands unsuited for seizing prey leave their lifestyle and feeding habits unclear.

The maniraptors are also known as deinonychosaurs and include the oviraptors and troodontids. These medium-size predators had long, grasping arms and hands, moderately long legs, and a specialized tail that could be held high for active balance control. Their feet bore the primary killing device, large slicing talons on the inside toes. The best-known examples are Deinonychus of North America and Velociraptor of Asia.

Segnosaurs were medium-size Asian theropods known only from a few examples. The mouth had bladelike teeth at the back but apparently no teeth at the front. The pelvis differed markedly from the normal saurischian design. They are very inadequately understood but seem to have been unlike all other theropods.

The carnosaurs were large to very large (up to 6- or 7-ton) carnivores with blade-toothed jaws, twice or more as long as the arms and hands, powerful hind legs, and taloned feet. It is not certain whether they were predators or carrion feeders. Tyrannosaurus is the most commonly cited example.

Ornithischia

The order Ornithischia, unlike the Saurischia, appears to be a natural group of closely related animals. All were plant eaters, and all are thought to have descended from a common ancestor. The rationale for postulating such an ancestor is based on the common existence of a uniquely ornithischian feature‹a median predentary bone that joined the two lower jaws at the symphysis. Further, a distinctive tooth form, crenulated along the upper edges, occurred in some members of all suborders. Collectively, these features point to a close common ancestry. The order has traditionally been divided into four suborders. However, recent studies have regrouped the members of this order into two major categories, the suborders Cerapoda and Thyreophora.

Ornithischia

Cerapoda

The suborder Cerapoda is divided into three infraorders: Ornithopoda, Pachycephalosauria, and Ceratopsia.

Ornithopoda

The ornithopods ranged in size from the small fabrosaurids, 1 to 2 metres long, to the huge duck-billed types that reached lengths of 10 to 12 metres or more. Ornithopods appear to have flourished the longest of the dinosaur varieties, thriving from the Late Triassic to the latest part of the Cretaceous. They inhabited all land areas except Antarctica.

Ornithopod families included the early and somewhat primitive fabrosaurids, mostly from Eurasia and southern Africa, and heterodontosaurs, also largely from southern Africa. In the latter group was the oldest of the ornithischians, Pisanosaurus, a single very fragmentary specimen from the Late Triassic of Argentina. Better known are the slightly larger hypsilophodonts and much larger iguanodonts, mostly from North America and Europe. Representative of these groups are Hypsilophodon, about three metres in length, and the famed Iguanodon, about nine metres long. Another group, the hadrosaurs, sometimes called trachodonts, were the large duck-billed ornithopods of the Late Cretaceous, especially of North America and Eurasia. An abundant and diverse family, they included more than two dozen known genera. (The Late Triassic or Early Jurassic Scutellosaurus had some similarities to the ornithopods, but its affinities are still uncertain.)

The postcranial anatomy of the ornithopods was fairly uniform. All members had hind legs that were much longer and sturdier than their forelegs. The thighbone (femur) was always shorter than the shinbones (tibia and fibula) and usually bore a prominent process, called the fourth trochanter, just above mid-length for the attachment of the retractor, or walking, muscles. The pelvis was expanded, usually with an elongated and broad prepubic process for the attachment of the protractor, or recovery, leg muscles. The tail was long and sometimes quite deep and flat-sided (commonly equated with tails that afford a sculling action in swimming animals). The vertebral spines of the tail and thoracic region were reinforced by a rhomboidal latticework of ossified tendons. These features, taken together, have led to the conclusion that all ornithopods were at least facultatively bipedal but that most kinds could assume a quadrupedal stance and gait. Some may have been obligatory bipeds‹for example, Fabrosaurus, whose forelimbs were only one-third the length of the hind limbs and whose hands seem better constructed for grasping than for walking.

Ornithopod feet were modified from the primitive five-toed pattern. The three middle toes served as the functional foot; the inside toe was shortened and often held off the ground; and the outside toe was greatly reduced or absent altogether. The toes terminated in broad, almost hooflike bones, especially in the duckbills. The hand reflected the primitive five-part design, although the first, or inside, finger may have been missing in some. The fingers usually ended in broad, blunt bones rather than in claws. In the duckbills the fingers apparently were encased in a mittenlike device sometimes thought to have been a paddle to aid in swimming; more probably it simply broadened the hand to better support the animal's weight on soft ground.

Members of the infraorder differ from one another in the structure of their skulls. In particular, their jaws and teeth show considerable variation among the different families. In the fabrosaurids the teeth were simple, leaf-shaped, laterally compressed elements arranged in a single front-to-back row in each jaw. They were not set in from the outer cheek surface as in most ornithopods. Small incisor-like teeth existed on the premaxillary bones above, but no teeth were present on the predentary below. The lower jaw had no coronoid process for large muscle attachment, and the upper temporal fenestra (the jaw muscle site) was relatively small. Upper and lower teeth alternated in position when the jaw was closed; they did not occlude directly.

In heterodontosaurs the cheek teeth were crowded together into long rows and set in slightly from the outer cheek surface. They occluded directly to form distinct chisellike cutting edges with a self-sharpening mechanism maintained by hard enamel on the outer side of the upper teeth and the inner side of the lower. There were prominent upper and lower tusklike teeth at the front of the mouth, the upper tusks in the premaxillaries, the lower tusks on the dentary bones of the jaw and not on the predentary. The upper temporal fenestra, larger than that of the fabrosaurids, and a prominent coronoid process beneath it indicate the existence of much larger jaw muscles than in the fabrosaurids.

The hypsilophodonts had cheek teeth arranged in tightly packed rows set well in from the outer cheek surfaces. The teeth occluded directly, and the opposing rows formed a long shearing edge similar to that of the heterodontosaurs. There was, however, no canine tusk either above or below. The premaxillaries had small, simple, incisor-like teeth above the beak-covered, toothless predentary. Strong coronoid processes extended up from the lower jaws toward the moderate-size upper temporal fenestrae.

In the iguanodonts the coronoid processes and temporal openings provided for still larger jaw muscles, but the cheek teeth were less regular and compacted than in the primitive ornithopods and consequently did not occlude as uniformly. Both the premaxillaries and the predentary were toothless but probably were sheathed in horny beaks.

Specialization of the teeth and jaws reached a pinnacle in the hadrosaurs, or duck-billed ornithopods. Here a very prominent, robust coronoid process jutted out and up at the back of the stout lower jaw. A large adductor muscle chamber was present above this process and beneath the lateral and upper temporal fenestrae‹clear evidence of powerful jaw muscles. The dentition consisted of numerous tightly compacted teeth crowded into large grinding batteries. The battery in each jaw was composed of as many as 200 functional and replacement teeth with distinct, well-defined wear or grinding surfaces that resulted from very exact occlusion. Both the predentary and premaxillaries were toothless but were enclosed in broad horny beaks, or bills. These bills apparently had edges sharp enough to close, clamlike, for shredding and stripping leaves or needles from low shrubs and branches. Pine needles have been identified in duck-billed dinosaur remains and presumably represent stomach contents.

Some varieties of hadrosaurs are also noted for the peculiar crests and processes on the top of the head. These structures were expansions of the skull composed almost entirely of the nasal bones. In genera like Corythosaurus , Lambeosaurus , Parasaurolophus, and a few others, the crests were hollow, containing a series of median and lateral chambers that formed a convoluted passage from the nostrils to the trachea.

Aside from passing air along to the lungs, the function of these narial crests is not widely agreed upon. Sound production (honking), an improved sense of smell, or a visually conspicuous ornament for species recognition are some of the variously accepted suggestions. Since these animals are no longer considered to have been amphibious, ideas like snorkeling and extra air storage space have generally been discarded.

Pachycephalosauria

In important respects the pachycephalosaurs conformed to the basic ornithopod body plan (some experts include them with the Ornithopoda rather than as a separate infraorder). All appear to have been bipedal, possessed the typical ornithopod ossified tendons along the back, and had simple leaf-shaped teeth, although the teeth were enameled on both sides. The ornithischian type of pelvis was present, but the obturator process of the ischium was not. The pachycephalosaurs are known as domeheads, because of their most distinctive feature‹a marked thickening of the frontoparietal (forehead) bones of the skull (see illustration). The thickness of bone was much greater than might be expected in animals of their size. The suggestion has been made that this forehead swelling served as protection against the impact of head-butting activities such as those seen today in animals like bighorn sheep.

Stegoceras and Pachycephalosaurus of the North American Cretaceous were the smallest and largest members of the group, the former attaining a length of about 2.5 metres and the latter twice that. Pachycephalosaurs existed almost entirely in the Late Cretaceous (although Yaverlandia is from the Early Cretaceous) and have been found mostly in North America and Asia. The origin of the group is not known, but most likely it derived from some undiscovered Late Triassic or Early Jurassic hypsilophodont.

Ceratopsia

The first ceratopsian dinosaur remains were found in the 1870s by E.D. Cope, who named the animal Agathaumus, but the material was so fragmentary that its unusual design was not at once recognized. The first inkling that there had been horned dinosaurs did not emerge until the late 1880s with the discovery of a large horn core, first mistaken for that of a bison. Shortly afterward, dozens of large skulls with horns were found‹the first of many specimens of Triceratops .

Ceratopsians were a relatively short-lived dinosaur group, flourishing from the Middle Cretaceous to the 'great extinction' at the end of the Cretaceous Period. Triceratops, together with Tyrannosaurus, was one of the very last of all dinosaurs. Ceratopsians had a peculiar geographic distribution: the earliest and most primitive kinds, Psittacosaurus and Protoceratops, are known only from Asia - Mongolia and China specifically; all the advanced ceratopsians, with the exception of a few fragmentary and doubtful specimens, have been found only in North America.

Sizes ranged from relatively small animals the size of a collie to the nine-metre-long, four- to five-ton Triceratops. Although commonly compared to the modern rhinoceros, Triceratops grew to a weight and bulk several times that of the largest living rhinoceros, and its behaviour probably was correspondingly different. The most distinctive feature of nearly all members of the group was the horns on the head, giving them the name ceratops (horn face). Correlated with the various arrays of head horns in the different taxa was the unusually large size of ceratopsian heads. Great bony growths extended from the back of the skull, reaching well over the neck and shoulders. This neck shield, or frill, resulted in the biggest head that ever adorned any land animal; the length of the Torosaurus skull was almost three metres, longer than a whole adult Protoceratops.

Several theories have been proposed to explain this frill structure: a protective shield to cover the neck region, an attachment site of greatly enlarged jaw muscles, an attachment site of powerful neck muscles for wielding the head horns, or a sort of ornament to present a huge, frightening head-on profile to potential attackers. Most probably, all these functions were involved at some stage in the development of the frill. The most unusual thought is that the structure was none of these, but rather acted as a giant heat-control apparatus, with its entire upper surface covered in a vast network of blood vessels pulsing with overheated blood or absorbing solar heat. The idea is interesting but fails to account for several important associated features -the massive coronoid process of the lower jaw, for example - that are better explained as mechanical and muscular adaptations.

The infraorder is usually divided into three families: the primitive psittacosaurids, including the putative ceratopsian ancestor, Psittacosaurus; the protoceratopsids, including Protoceratops of Asia and Leptoceratops of North America; and the ceratopsids, encompassing all the advanced and better-known kinds such as Triceratops, Torosaurus, and Monoclonius, all from North America.

Psittacosaurus has sometimes been classified as an ornithopod, largely because of its relatively long hind limbs and short front limbs, probably resulting in bipedal stance and locomotion. Resembling ornithopods in body form, it had a shorter neck and tail and was much smaller (only two metres long) than the most advanced ornithopods like the iguanodonts and hadrosaurs. Psittacosaurus, however, possessed a ceratopsian-like beak, the beginnings of a characteristic neck frill at the back of the skull, and teeth that prefigured those of the more advanced ceratopsians. Also, its remains have been found in rocks that predate those containing the ceratopsids.

The best-known of the protoceratopsids is the genus Protoceratops. Dozens of skeletal specimens have been found and studied, ranging from near hatchlings to full-size adults. This rare treasure, the first to include very young individuals unmistakably associated with mature individuals, was the result of the series of American Museum of Natural History expeditions in the 1920s to the Gobi. Their collection provided the first good growth series of any dinosaur. Even more momentous was their discovery of several nests of eggs loosely associated with Protoceratops skeletons‹the first certifiable dinosaur eggs. Additional specimens of Protoceratops and its nests of eggs were recovered on subsequent expeditions by Soviets, Poles, and Chinese.

The skeletal anatomy of the protoceratopsids foreshadowed that of the more advanced ceratopsids. The ceratopsian skull was disproportionately large for the rest of the animal, comprising about one-fifth of the total body length in Protoceratops and at least one-third in Torosaurus . The head frill of Protoceratops was a modest backward extension of the upper temporal fenestral arches but became the enormous fan-shaped ornament of later forms. Protoceratops also displayed a short but stout horn on the snout, a development of the nasal bones; this too was a precursor of the prominent nasal horns of ceratopsids like Monoclonius , Centrosaurus, Chasmosaurus, Styracosaurus, Torosaurus, and Triceratops. The last two genera developed two additional, larger horns above the eyes. These horns undoubtedly were covered by horny sheaths or soft tissue, as is evidenced by impressions on them of superficial vascular channels for the nourishing blood vessels.

Ceratopsian jaws were highly specialized. The lower jaw was massive and solid to support a large battery of teeth similar to those of the duckbills. The front of the lower jawbones met in a long, strong symphysis capped by a stout beak formed of the toothless predentary. This structure itself must have been covered by a sharp, horny, turtlelike beak. Continuous dental surfaces extended over the rear two-thirds of the jaw. The tooth batteries, however, differed from those of the hadrosaurs in forming long, vertical slicing surfaces as upper and lower batteries met, operating much like self-sharpening shears.

From the lower jaw a massive coronoid process jutted up toward a relatively small upper temporal fenestra, providing a strong attachment point for jaw muscles and powerful leverage for their action. Because the upper fenestra was so small, and the teeth so specialized for cutting, it has generally been accepted that the powerful jaw muscles were attached to the upper frill surface and connected to the coronoid process beneath by means of a stout tendon passing through the upper fenestra. At least one factor in the expansion of the skull frill would seem to have been the size of the required jaw muscles. (That, however, would not have precluded other functions for the frill as well‹shielding the neck, enlarging the head-on profile, or even controlling temperature.)

As in the hadrosaurs, each dental battery consisted of about two dozen or more tooth positions compressed together into a single large block of teeth. At each tooth position there was one functional, or occluding, tooth (the duckbills had two or three) along with several more unerupted replacement teeth beneath. (All toothed reptiles, living and extinct, have a lifelong supply of replacement teeth.) An intriguing question arises: what did the ceratopsians feed on that required such an unusual and powerful feeding mechanism? The answer remains hidden, but the suggestion is that they fed on something exceedingly tough and fibrous, like the fronds of palms or cycads, both of which were plentiful during late Mesozoic times.

Except for Psittacosaurus, all ceratopsians were obligate quadrupeds with a heavy, ponderous build. The leg bones were stout and the legs themselves muscular, the feet were semiplantigrade for graviportal stance and progression, and all the toes ended in ³hooves² rather than claws. As in most other four-legged animals, the rear legs were significantly longer than the front legs (again suggesting to some experts that such animals had a bipedal ancestor). The hind legs were positioned directly beneath the hip sockets and held almost straight and vertical. The front legs, on the other hand, projected out to each side from the shoulder sockets in a ³push-up² position. Consequently, the head was carried low and close to the ground. This mixed posture was probably related to the large horned head and its role in combat, the bent forelegs providing a wide stance and stable base for directing the horns at an opponent and resisting attack.

The first four neck vertebrae of ceratopsians were coossified, presumably for strength and impact resistance during hostile engagements. The first joint of the neck was unusual in that the condyle of the skull formed a nearly perfect sphere that fitted into a cuplike socket of the fused neck vertebrae. Such an arrangement would seem to have provided solid connections along with maximum freedom of the head to pivot in any direction without having to turn the body. There can be no doubt that ceratopsians used their head horns in an aggressive manner, but whether they used them as defense against possible predators or in rutting combat with other male ceratopsians, or both, is not so clear. Evidence of puncture wounds in some specimens suggests rutting encounters, but the fact that both sexes apparently had horns seems to indicate defense as their primary use.

Ornithischia

Thyreophora

The suborder Thyreophora is divided into two infraorders: Stegosauria, the plated dinosaurs, and Ankylosauria, the armoured dinosaurs.

Stegosauria With their unique bony back plates, the stegosaurs were one of the most distinctive dinosaur varieties. Relatively few specimens have been found, chiefly from North America, Africa, Europe, and Asia. Stegosaurian remains have appeared in Early Jurassic to Early Cretaceous strata. Their immediate ancestry is uncertain, and no descendants are known. The most familiar genus is Stegosaurus , found in the Morrison Formation (Late Jurassic) of western North America. Stegosaurus was 3.7 metres in height and 9 metres in length, probably weighed two tons, and had a broad, deep body. Not all varieties of the infraorder were this large; for example, Kentrosaurus , from eastern Africa, was less than 2 metres high and 3.5 metres long.

All stegosaurs were graviportal and undoubted quadrupeds, although the massive legs were of greatly disparate lengths - the hind legs more than twice as long as the forelegs. Some authorities have interpreted the short front legs as evidence that stegosaurs were secondarily quadrupedal and had a bipedal ancestry, probably some ornithopod. This theory of ancestry may be correct, but such a precursor need not have been bipedal. In fact, among virtually all quadrupedal animals, it is the rule rather than the exception that the front legs are shorter than the hind - often much shorter. (Notable exceptions are the giraffe and, among dinosaurs, the giant sauropod Brachiosaurus, whose front legs were much longer than its rear legs.)

Whatever walking and running skills were possessed by the stegosaurs, their limb proportions must have made them move extremely slowly. The humerus of the upper arm was longer than the bones of the forearm, the femur was much longer than the shinbones, and the metapodial bones of the feet were all very short, which means that the stride must have been short. The feet were graviportal in design, with no cursorial adaptations. The stegosaurian skull was notably small, long, low, and narrow with weakly developed dentition of small, laterally compressed, leaf-shaped teeth in short, straight rows. There was no significant coronoid process on the lower jaw, nor was there much space for sizable jaw muscles. This combination of features seems entirely incompatible with the large, bulky body, especially in view of the apparent absence of Jurassic vegetation of suitable nature. The weak dentition would indicate that the food eaten must have required little preparation by the teeth and yet have provided adequate nourishment.

The best explanation is that the digestive tract may have contained a flora of fermenting bacteria capable of breaking down the cellulose-rich Jurassic plant tissues. Perhaps that process was assisted by a crop or gizzard full of pulverizing stomach stones (gastroliths), although none have so far been discovered in stegosaurian specimens. Even so, such features would still not explain how these animals, with such small mouths and dentition, could feed themselves adequately to sustain their great bulk. (The same problem has been encountered in speculations about the feeding habits of sauropods.)

The most distinctive stegosaurian feature was the row, or rows, of large, diamond-shaped bony plates on the back. A controversy has raged ever since the first specimen was collected, in 1877 in Colorado, U.S., as to their purpose and how they were arranged. The evidence and a general consensus argue in favour of the traditional idea that the plates projected upward and were set in two parallel rows in a staggered, or alternating, arrangement rather than side by side. Others, however, suggest that they did not project above the back at all, but lay flat to form flank armour. A few others maintain that the plates were set in a single midline row along the length of the back, beginning with small ones just behind the head, increasing in size to the hips, and then diminishing along the tail. The end of the tail bore at least two pairs of long bony spikes, indicating some sort of defensive role for the tail but not necessarily for the back plates.

The discovery in 1976 that the bony plates of Stegosaurus were highly vascularized, fed by large, hose-size blood vessels, led to the notion that these fins were large ³radiators,² or cooling vanes, to dissipate excess body heat in much the same manner as does the large surface area of an elephant's ears. The staggered arrangement in parallel rows might have maximized the area of cooling surface by minimizing any leeward breeze ³shadow² that would have resulted from a paired configuration. Asymmetry is a bizarre anatomic condition, and‹right or wrong‹this certainly is an imaginative explanation of its presence in this animal. No other stegosaur, however, had such a peculiar feature. Rather, all other taxa had a variety of paired body spikes that seem best explained as passive defense or display adaptations rather than cooling mechanisms.

Another variety of dinosaur may be mentioned here, although its correct systematic assignment is still under review, and it is known from very few specimens. The animal has been named Scutellosaurus after one of its most distinctive features, an armour of small bony scutes. The specimens were recovered from very late Triassic or the earliest Jurassic strata of the American Southwest. In general body form, limb proportions, and tooth form, Scutellosaurus resembled the early ornithopods and was at least preferentially bipedal. Its body armour, however, was unlike anything known in other ornithopods. An array of small to moderate-size, separated bony scutes, set in multiple rows, covered the back and flanks. These attachments apparently corresponded in location with the ribs and with the vertebrae along the entire length of the backbone.

Ornithischia

Thyreophora

Ankylosauria The ankylosaurs lived during the Cretaceous Period and, like the stegosaurs, were a relatively brief dinosaurian experiment. They are called armoured dinosaurs after their most distinctive feature, an extensive mosaic of small and large interlocking bony plates that completely encased the back and flanks. Most varieties, like Euoplocephalus , Nodosaurus , and Palaeoscincus were relatively low and broad in body form and built close to the ground, with short, stocky legs and a quadrupedal stance. Again, the hind legs were longer than the front legs but not so extremely disproportionate as those of Stegosaurus. Like the stegosaurs, however, their limbs were stout and columnar, the thighbone and upper arm were longer than the shin and forearm, and the metapodials were stubby. These features point to a slow, graviportal mode of locomotion. The feet were semiplantigrade and possibly supported from beneath by pads of cartilage. The terminal phalanges of both fore and hind feet were broad and hooflike elements rather than claws.

The ankylosaur skull was a low, broad, boxlike affair with dermal scutes, or osteoderms, that often fused with the underlying skull bones. In one form, Euoplocephalus, even the eyelid seems to have developed a protective bony covering. The jaws were weak, with a very small predentary and no significant coronoid process for jaw muscle attachment. The small jaw muscle chamber was largely covered over by dermal bones rather than fenestrated. The teeth were small, loosely spaced leaf-shaped structures reminiscent of the earliest primitive ornithischian teeth. All taxa had very few teeth in either jaw, a marked contrast to the highly specialized, numerous teeth of other ornithischians. These features of the jaws and teeth lead to the impression that the animals must have fed on some sort of soft, pulpy plant food.

Not a particularly diverse or abundant group, the ankylosaurs are known only from North America, Europe, and Asia. They are divided into two families, the primitive Nodosauridae and the advanced Ankylosauridae. The most conspicuous difference between the two families is the presence of a massive bony club at the end of the tail in the advanced ankylosaurs; no such tail structure is present in the nodosaurs.

Dinosaurs have broad public, as well as scientific, interest partly because they are extinct. It is widely believed that all dinosaurs died out at the same time‹apparently quite suddenly at the end of the Cretaceous Period. This belief is not entirely correct, however. It is also usually supposed that dinosaurs left no direct descendants, a view that has been challenged and is now a matter of intense reexamination by paleontologists and evolutionary biologists.

Faunal changes

During the 150 million years or so in which dinosaurs existed, there were repeated changes in the dinosaur communities. The stratigraphic record is too incomplete to establish whether these faunal turnovers were uniform, taking place at a steady rate, or episodic, but it seems to indicate the latter. The evidence shows a moderately rich Late Triassic fauna of plateosaurs and other prosauropods, primitive ornithopods, and theropods. Most of these kinds of dinosaurs are not represented in Early Jurassic strata, and by Late Jurassic time the fauna was very different, with sauropods, more advanced ornithopods, stegosaurs, and a variety of theropods predominating.

Early Cretaceous strata contain few sauropods (all new), a few stegosaurian holdovers, new kinds of theropods and ornithopods, and the first ankylosaurs. By Late Cretaceous time, sauropods apparently were rare, and advanced ornithopods (duckbills) had become the dominant browsers. A variety of new theropods of all sizes were widespread, stegosaurs no longer existed, and the ankylosaurs were represented by a collection of new kinds that were prominent in the North American and Asian faunas. A totally new group of dinosaurs, the horned ceratopsians, had appeared in Asia and had successfully colonized North America. The overall picture is quite clear: throughout Mesozoic time there was an ongoing turnover, or dying out and renewal, of dinosaurian life.

It is important to note that extinction is a normal, universal occurrence. On balance, it is as commonplace as is the appearance of new species. Old life-forms decline and diminish in numbers beyond the critical threshold below which the reproduction rate can no longer sustain the population. Ecological space and opportunities are created as a result of the void left by an extinguished species.

Sometimes new forms that originate by phylogenetic diversification are suitably adapted to make use of the vacated niche. That does not always happen, however, and the niche may remain empty or be parceled out among many occupants.

In a sense, the history of animal and plant life is replete with successions -early primitive kinds replaced by new and often more advanced kinds. In most instances, the stratigraphic record gives too little information to show whether the old forms were actually displaced by the new successors or the new kinds simply expanded into the declining population's ecological niches. Nor is the stratigraphic sequence adequate to document actual evolutionary lineages except in the most general way. For example, among dinosaurs, the sauropod group is generally thought to have originated from melanorosaurid prosauropods, but the sequence of ancestral to descendant species is not known specifically. Likewise, the hadrosaurs are widely believed to have derived from an Early Cretaceous iguanodont-like ornithopod (perhaps Probactrosaurus of Asia), but again the exact lineage is unknown.

Because of such stratigraphic gaps, it is not possible to say precisely how long dinosaur species or genera actually existed. Moreover, because of the somewhat inconsistent, and thus inexact, anatomic definitions of the various dinosaur taxa, the duration of any particular kind can be gauged only approximately‹usually by stratigraphic boundaries and presumed first and last occurrences.

The latter often coincide with geologic age boundaries; in fact, the absence of particular life-forms usually defines geologic boundaries. The Mesozoic ³moments² of apparently high extinction levels among dinosaurs were around the end of the Triassic (208 million years ago), the end of the Jurassic (144 million years ago), and of course the end of the Cretaceous (66.4 million years ago). Undoubtedly, there were lesser extinction peaks at other times in between, but these are poorly documented by fossil records.

The K­T boundary event

It was not only the dinosaurs that disappeared at the end of the Mesozoic. Many other organisms became extinct or were greatly reduced in abundance and diversity. Among these were the flying reptiles (pterosaurs), sea reptiles (plesiosaurs and mosasaurs), and ichthyosaurs, the last disappearing slightly before the Cretaceous­Tertiary boundary‹known as the K­T boundary. Strangely, turtles, crocodilians, lizards, and snakes were not affected or were affected only slightly. Effects on amphibians and mammals were mild. But other organisms, such as the molluscan ammonites, the belemnites and certain bivalves, the bryozoans, the crinoids, and a number of planktonic life-forms‹like foraminifera, radiolarians, coccolithophores, and diatoms‹were decimated.

Whatever factor or factors caused it, there was a major, worldwide biotic change at about the end of the Cretaceous. But the extermination of the dinosaurs is the best-known change by far and has been a puzzle to paleontologists, geologists, and biologists for two centuries. Many theories have been offered over the years to explain dinosaur extinction, but few have received serious consideration. Proposed causes have included everything from disease, heat waves and resulting sterility, freezing cold spells, and the rise of egg-eating mammals, to X rays from a supernova exploding nearby. Since the early 1980s, attention has focused on the so-called asteroid theory put forward by the American geologist Walter Alvarez, his father, the physicist Luis Alvarez, and their coworkers.

The ongoing debate pits catastrophists against gradualists. Were the extinctions simultaneous and instantaneous, or were they nonsynchronous and spread over a long time period? The precision with which geologic time can be measured, by either radiometric means, paleomagnetic reversal stratigraphy, or the more traditional approach of measuring the fossil content of stratigraphic layers, leaves much to be desired. Only rarely does an ³instantaneous² event leave a worldwide‹or even regional‹signature in the geologic record in the way that a volcanic eruption does locally. Attempts to pinpoint the K­T boundary event, even using the best radiometric dating techniques, result in a margin of error on the order of a half million years. Consequently, the actual time involved in the mass extinctions of that period, or in any of the preceding or subsequent extinctions, has remained undetermined.

The asteroid theory

The discovery of an abnormally high concentration of the rare metal iridium at, or very close to, the K­T boundary, however, provides what has been recognized as one of those rare instantaneous geologic time markers that seem to be worldwide. This iridium anomaly, or spike, was first found by Walter Alvarez in the Cretaceous­Tertiary stratigraphic sequence at Gubbio, Italy, in the 1970s. The spike has subsequently been detected at localities in Denmark and elsewhere, both in rock outcrops on land and in core samples drilled from ocean floors. Iridium normally is a rare substance in rocks of the Earth's crust (about 0.3 parts per billion). At Gubbio, the iridium concentration is more than 20 times greater (6.3 parts per billion), and it is even greater at other sites.

Because the levels of iridium are higher in meteorites than on the Earth, the Gubbio anomaly is thought to have an extraterrestrial explanation. The level of iridium in meteorites has been accepted as representing the average level throughout the solar system, and by extension, the universe. Accordingly, the iridium concentration at the K­T boundary is widely attributed to a collision between the Earth and a huge meteor or asteroid. The size of the object is estimated at about 10 kilometres (6.5 miles) in diameter and one quadrillion metric tons in weight; the velocity at the time of impact is reckoned to have been several hundreds of thousands of miles per hour. The crater resulting from such a collision would be some 100 kilometres or more in diameter. No crater of this sort has been recognized, but, in view of the fact that the Earth's surface is two-thirds ocean, it is likely that such an impact site (called an astrobleme) would be hidden on the ocean floor.

The asteroid theory is widely accepted as the most probable explanation of the K­T iridium anomaly, but it does not appear to account for all the paleontological data. An impact explosion of this kind would have ejected an enormous volume of terrestrial and asteroid material into the atmosphere, producing a cloud of dust and solid particles that would have encircled the Earth and blocked out sunlight for many months, possibly years. The loss of sunlight could have eliminated photosynthesis and resulted in the death of plants and the subsequent extinction of herbivores and their predators and scavengers.

The K­T mass extinctions, however, do not seem to be fully explained by this hypothesis. The stratigraphic record is most complete for extinctions of marine life‹foraminifera, ammonites, coccolithophores, and the like. These life-forms apparently died out suddenly and simultaneously, and their extinction accords best with the asteroid theory. The fossil evidence of land dwellers, however, suggests a gradual decline in dinosaurian diversity, and possibly abundance, rather than a sudden change at the K­T boundary. Alterations in terrestrial life seem to be best accounted for by environmental factors such as the consequences of seafloor spreading and continental drift. With the rearrangement of the continental masses, disrupting and deflecting oceanic current patterns and causing repeated changes in sea level, there undoubtedly occurred many climatic changes, which in turn would have affected terrestrial organisms and their distribution.

Finally, in the controversy between the gradualist and catastrophist explanations of the dinosaurs' extinction, it should be noted that one phenomenon does not preclude the other. It is entirely possible that a culmination of ordinary biological changes and some catastrophic event both took place around the end of Cretaceous time.

Dinosaur descendants

Contrary to the commonly held belief that the dinosaurs left no descendants, the rare (seven) specimens of Archaeopteryx (see photograph ) provide compelling evidence that birds are closely related to, and probably are direct descendants of, small theropod dinosaurs. In fact, today Archaeopteryx actually is classified as a dinosaur and a bird by many experts, and some even categorize all living birds as dinosaurs.

Many ornithologists do not agree with this taxonomy, although many (if not most) accept a dinosaurian origin of birds.

The specimens of Archaeopteryx contain particular anatomic features that also are exclusively present in certain theropods, such as Ornitholestes, Coelurus, Velociraptor, Deinonychus, Saurornithoides, Troodon, and Sinornithoides.

All these dinosaurs existed too late in time to have given rise to Archaeopteryx and thereby to later birds. However, it is believed that they arose from a common theropod ancestor that possessed the beginnings of these shared anatomic features found in Archaeopteryx, the oldest-known unquestionable bird.

Dinosaurs traditionally have been placed in the reptilian subclass Diapsida, reptiles with two pairs of temporal openings in the skull. As diapsids, dinosaurs are grouped with the crocodilians, thecodonts, and pterosaurs, all of which have socketed teeth and a number of other features in common. These are the so-called archosaurian reptiles. In recent years it has been suggested that dinosaurs be ranked as a class of their own, comparable to the classes Mammalia and Aves. That idea has not been universally accepted as yet. At the present time, the term Dinosauria is not used as a taxonomic category to include all dinosaurs. Instead, they are classified in their two orders, as either Saurischia or Ornithischia.

Order Saurischia

The reptile-hipped dinosaurs.

Suborder Sauropodomorpha

All the reptile-hipped herbivorous dinosaurs. Late Triassic to Late Cretaceous.

Infraorder Prosauropoda

Facultative bipeds; primitive forerunners of sauropods. Late Triassic to Early Jurassic.

Family Anchisauridae

Primitive prosauropods including Anchisaurus and Plateosaurus . Late Triassic to Early Jurassic.

Family Melanorosauridae

Advanced prosauropods such as Melanorosaurus and Riojasaurus; probably includes the sauropod ancestry. Late Triassic to Early Jurassic

Infraorder Sauropoda

Large to gigantic obligatory quadrupeds; all herbivorous. Early Jurassic to Late Cretaceous.

Family Cetiosauridae

Primitive and poorly studied sauropods such as Cetiosaurus ; specimens mostly from the Old World; vertebrae well-excavated to lighten bone weight. Early to Late Jurassic.

Family Diplodocidae

More advanced and better-known large sauropods; highly excavated vertebrae. Diplodocus and Apatosaurus (Brontosaurus) are the best known. Late Jurassic to Late Cretaceous.

Family Brachiosauridae

Largest of all the sauropods; greatly elongated necks and highly excavated vertebrae; front legs longer than back legs. Brachiosaurus is the most famous. Middle Jurassic to Late Cretaceous.

Family Titanosauridae

Advanced sauropods, primarily from the southern continents. Examples are Titanosaurus and Alamosaurus.

Early to Late Cretaceous.

Family Camarasauridae

Moderate-size sauropods; relatively short necks and tails for this infraorder; teeth spatulate or spoon-shaped. Camarasaurus and Morosaurus are typical. Late Jurassic to Late Cretaceous.

Suborder Staurikosauria?

Primitive carnivorous dinosaurs resembling theropods; known only from South America. Classification is uncertain(?). Name bearer is Staurikosaurus. Middle to Late Triassic.

Suborder Theropoda

All the carnivorous dinosaurs except the staurikosaurs; obligatory bipeds. Late Triassic to Late Cretaceous.

Infraorder Ceratosauria

Small to medium-size, hollow-boned carnivores. Late Triassic to Late Jurassic. Includes the primitive theropods such as Ceratosaurus , Coelophysis , and Syntarsus.

Infraorder Tetanurae

All other theropods are grouped here into the five subcategories Coelurosauria, Ornithomimosauria, Maniraptora (previously Deinonychosauria), Segnosauria, and Carnosauria. Late Triassic to Late Cretaceous.

Family Compsognathidae (Coelurosauria)

Advanced; smallest of the theropods; all known specimens 2-fingered. Name bearer is Compsognathus .

Family Oviraptoridae (Maniraptora)

Small, toothless theropods with an odd skull form; perhaps related to the ornithomimids. Oviraptor is the best-known example. Late Cretaceous.

Family Dromaeosauridae (Maniraptora)

Includes Deinonychus and Velociraptor . Early to Late Cretaceous.

Family Troodontidae (Maniraptora)

Less specialized in foot structure, but close to the dromaeosaurids. Name bearer is Troodon. Late Cretaceous.

Family Megalosauridae (Carnosauria)

Primitive large theropods; often 4-fingered. Megalosaurus is the best known. Early Jurassic to Late Cretaceous

Family Allosauridae (Carnosauria)

More advanced large theropods; all 3-fingered except for Ceratosaurus. Best known is Allosaurus . Late Jurassic to Late Cretaceous

Family Tyrannosauridae (Carnosauria)

Largest of the theropods and the most advanced; all with just 2 fingers. Examples are Tyrannosaurus, Tarbosaurus, and Albertosaurus . Late Cretaceous.

Order Ornithischia

The bird-hipped, herbivorous dinosaurs; characterized by the diagnostic predentary bone of the jaw.

Suborder Cerapoda

Facultatively bipedal ornithischian dinosaurs plus the horned ceratopsian forms. Late Triassic to Late Cretaceous.

Infraorder Ornithopoda

Family Fabrosauridae Earliest and most primitive of the ornithopods; small and often hollow-boned. Best known is Fabrosaurus. Late Triassic to Early Cretaceous

Family Heterodontosauridae

More advanced small ornithopods, with the beginnings of specialized dentition. Best examples are Heterodontosaurus and the primitive Pisanosaurus . Late Triassic to Late Jurassic.

Family Hypsilophodontidae

More advanced small to medium-size ornithopods, with only a suggestion of specialized dentition. Hypsilophodon and Thescelosaurus are examples. Late Jurassic to Late Cretaceous.

Family Iguanodontidae

Medium to large ornithopods, with the first stages of specialized grinding dentition. Iguanodon and Camptosaurus are the best known. Late Jurassic to Late Cretaceous.

Family Hadrosauridae

The duck-billed ornithopods, with highly specialized grinding dentition; medium to large size. Edmontosaurus, Corythosaurus , and Lambeosaurus are well known. Late Cretaceous.

Infraorder Pachycephalosauria

The dome-headed ornithischians; closely related to the ornithopods; usually with a massively thick bony skull roof; bipedal. Stegoceras and Pachycephalosaurus are the best examples. Late Cretaceous.

Infraorder Ceratopsia The horned dinosaurs. Early to Late Cretaceous.

Family Psittacosauridae Ancestral and most primitive of the ceratopsians; represented by the hornless and bipedal Psittacosaurus. Early Cretaceous

Family Protoceratopsidae

Primitive quadrupedal ceratopsians, with short frills and very modest horns. Protoceratops and Leptoceratops are the best examples. Late Cretaceous.

Family Ceratopsidae

Advanced quadrupedal ceratopsians, with prominent horns and frills. Examples are Monoclonius, Torosaurus, and Triceratops. Late Cretaceous.

Suborder Thyreophora

The plated and armoured dinosaurs. Late Triassic or Early Jurassic to Late Cretaceous.

Infraorder Stegosauria

Family Scelidosauridae

Primitive stegosaurs, with less well-developed back plates. Scelidosaurus is the most primitive form; Scutellosaurus perhaps the most advanced. Late Triassic or Early Jurassic.

Family Stegosauridae

Advanced stegosaurs, usually with well-developed back plates and spines. Stegosaurus and Kentrosaurus are the best known. Middle Jurassic to Early Cretaceous.

Infraorder Ankylosauria

Family Nodosauridae

Primitive ankylosaurs, usually with less completely developed armour. Nodosaurus , Hylaeosaurus , and Sauropelta are well-known kinds. Early to Late Cretaceous.

Family Ankylosauridae

Advanced ankylosaurs such as Euoplocephalus and Ankylosaurus

Late Cretaceous

No universally accepted classification of dinosaurs exists. Fossil remains are often difficult to interpret, especially when only a few fragmentary specimens of a type have been found. Moreover, classifications may be constructed to serve different purposes that require different categories or organization.

Occasionally, for example, the Sauropodomorpha have been divided into more or fewer lower-rank categories (e.g., families, subfamilies); but the twofold division into the infraorders Sauropoda and Prosauropoda has stood the test of time and has been followed here.

Likewise, previous classifications divided the suborder Theropoda into two infraorders, the Carnosauria and the Coelurosauria. The former included all the larger animals and the latter all the smaller kinds. That arrangement did recognize certain distinctive anatomic features such as large heads and short necks in the Carnosauria and small heads and long necks in the Coelurosauria.

But great numbers of theropod discoveries around the world in the past several decades have blurred those anatomic distinctions and reduced the importance of size as a diagnostic criterion. Accordingly, infraordinal categories are not always used in current classifications of the Theropoda; sometimes only family groupings are listed. In the classification adopted here, the theropods are divided into two infraorders, the Ceratosauria and the Tetanurae.

The tetanuran theropods are further divided into certain subcategories - Coelurosauria, Ornithomimosauria, Maniraptora, Segnosauria, and Carnosauria - that are at a higher level than the families of this infraorder. It must be noted, however, that evolutionary affinities among all the theropod types are still being analyzed, and experts have not reached full agreement on a formal classification.

Within the order Ornithischia, two distinct subdivisions are generally given equal rank, currently as the suborders Cerapoda and Thyreophora.

A final example is the recently discovered Scutellosaurus, which has been assigned by some to the Fabrosauridae (Ornithopoda) and by others to the Stegosauria. Scutellosaurus might well represent an evolutionary link between the ornithopods and the later stegosaurs or ankylosaurs. Since its affinities are still unclear, it has here been tentatively placed with the Stegosauria.

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