DINOSAUR

Dinosaur family tree. 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 (terrible) and sauros (lizard). 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. any member of a great series of reptiles that were the dominant land animals during most of the Mesozoic Era (from 245 to 66.4 million years ago) but became extinct at its close. The term, derived from the Greek, meaning terrible lizard, refers to the gigantic proportions of some of these beasts. Dinosaurs belonged to two distinct but related ordersthe Saurischia and the Ornithischiathat were distinguished from each other chiefly by differences in the structure of their pelvic girdles. The dinosaurs belonged to the Archosauromorpha infraclass of reptiles. Among the other archosaurs (Greek: ruling reptiles) were the early crocodilians and extinct flying reptiles. A considerable number of dinosaurs were flesh eaters, but many others abandoned this primitive reptilian mode of life for a plant diet. The early dinosaurs may have descended from archosaurs that were bipeds; many dinosaurs remained bipedal throughout the group's history. In both dinosaur orders, however, many of the herbivore types developed a four-footed mode of locomotion. It is believed that the saurischian dinosaurs evolved from small, bipedal archosaurs called thecodonts. Of the three known suborders of saurischians that evolved, the two main types were the theropods and the sauropods. The coelurosaurs, carnosaurs, and other theropods were all flesh eaters that walked upright on two legs. The slender-limbed, lightly built, fast-running coelurosaurs probably preyed upon smaller reptiles. They ranged from 2 to 8 feet (0.7 to 2.4 m) in length and had birdlike feet. The carnosaurs tended to reach a larger size. Among the best-known types was Tyrannosaurus, which had a huge skull with many sharp teeth that were highly effective biting and tearing instruments. It reached a height of about 16 to 18 feet (5 m) and a length of about 50 feet (15 m). Such an animal was powerful enough to attack any of its dinosaur contemporaries. The prosauropods evolved into the largest of all dinosaurs, the sauropods. Among these huge, four-footed plant eaters were Diplodocus, which reached a length of 87 feet (26 m), and the heavier Brachiosaurus, which weighed as much as 80 tons. Sauropods had massive bodies, powerful limbs (to support their great weight), a long tail, a long neck, and a small head. The ornithischian dinosaurs, like the saurischians, evolved from thecodonts. The ornithischians did not grow as large as some of the saurischians but were notable for their armour and other strange adaptations. They comprised two main groups. One group, the cerapods, included the ornithopods, pachycephalosaurs, and ceratopsians. The ornithopods included the hadrosaurs and the iguanodonts. These bipeds had ducklike mouths and several hundred teeth that they used to grind hard vegetable matter. The pachycephalosaurs were bipeds with dome-shaped skulls, while the four-footed ceratopsians, such as Triceratops, had a broad frill of bone protecting the head and neck and some type of long horn or horns projecting from the skull. The other main ornithischian group, the thyreophorans, included the stegosaurs and the ankylosaurs. The stegosaurs had high-arched bodies and a row of large vertical bony plates running along their backs. The ankylosaurs were heavily armoured dinosaurs with extra bone plating over most of their bodies for defensive purposes. Most dinosaurs had long tails, but they held these tails straight out and off the ground for help in maintaining their balance, rather than dragging them along the ground as had been previously thought. Dinosaurs' means of regulating their body temperature is a continuing point of controversy. They were long thought to be ectothermic, or cold-bloodedi.e., dependent on the uptake of heat from the external environment to maintain their body temperature. But recent consideration of their postures, rates of predation, and certain anatomical details has led many paleontologists to conclude that at least some dinosaurs were endothermic, or warm-bloodedi.e., capable of regulating their body temperature by internal means. Contrary to the traditional image of dinosaurs as sluggish, slow-moving beasts, many of them were swift-moving creatures with relatively high metabolic rates. Most dinosaurs probably relied on a combination of endothermic and ectothermic mechanisms for thermoregulation. Most types of dinosaurs continued to flourish until the very latest phases of the Cretaceous Period. Then, within the next million years, they disappeared completely from the geologic record, and succeeding rock strata show not the slightest trace of a dinosaur. The cause of this sudden demise is not at all clear. One widely accepted explanation for their mass extinction has been that a major geologic cycle of mountain building at the end of the Cretaceous Period reduced the lowland areas in which dinosaurs flourished and also changed the world's climate, thus stimulating evolutionary changes in the plant life upon which dinosaurs fed. A more recent theory postulates an astronomical catastrophe as the cause: a collision between an asteroid and the Earth generated a huge dust cloud that caused a period of darkness lasting as long as three years. This blockage of sunlight made photosynthesis virtually impossible, and the resulting collapse of the food chain led to the worldwide extinction of the dinosaurs and many other life forms. Although this hypothesis has been partially substantiated by geologic evidence, the apparent survival of some types of dinosaurs for as long as 1 million years after the presumed asteroid impact raises doubts that this catastrophe was the primary cause of the dinosaurs' eventual disappearance. It is possible that both climatic change and an asteroid impact played a part in the extinction of dinosaurs. Additional reading General works Advanced textbooks on vertebrate evolution and paleontology include Edwin H. Colbert and Michael Morales, Evolution of the Vertebrates: A History of the Backboned Animals Through Time, 4th ed. (1991); Michael J. Benton, Vertebrate Palaeontology (1990); Robert L. Carroll, Vertebrate Paleontology and Evolution (1988); and Alfred S. Romer, Vertebrate Paleontology, 3rd ed. (1966). David B. Weishampel, Peter Dodson, and Halszka Osmlska (eds.), The Dinosauria (1990), gives extensive and comprehensive reviews of the major kinds of dinosaurs, with descriptions, reconstructions, and definitions. Edwin H. Colbert, Dinosaurs: An Illustrated History (1983), is written for a general audience. David Norman, The Illustrated Encyclopedia of Dinosaurs (1985), provides an excellent general treatment, well written and illustrated, for the specialist and nonspecialist alike. Spencer G. Lucas, Dinosaurs: The Textbook (1994), is a general college-level introductory text. David Lambert, The Ultimate Dinosaur Book (1993), presents a comprehensive survey of the most familiar varieties in detail with good illustrations. The search for dinosaurs Louie Psihoyos and John Knoebber, Hunting Dinosaurs (1994), is an impressive photographic assemblage of the discoveries and the people responsible for them. Edwin H. Colbert, Dinosaurs: Their Discovery and Their World (1961), an authoritative treatment of the subject by a world authority, includes extensive photographic and line-drawing coverage, and his Men and Dinosaurs: The Search in Field and Laboratory (1968) provides a thorough illustrated history of the discovery, collection, and study of dinosaurs. John R. Horner and James Gorman, Digging Dinosaurs (1988), is a fascinating account of the search for and discovery and collecting of dinosaur eggs and nests, told by the discoverers. John H. Ostrom and John S. McIntosh, Marsh's Dinosaurs: The Collections from Como Bluff (1966), illustrated for technical professionals, contains a historical study of one of the most famous dinosaur localities. John H. Ostrom, The Terrible Claw, Discovery (New Haven), 5(1):19 (1969), gives a popular account of the finding and interpretation of an unusual dinosaur variety. Natural history John H. Ostrom, Were Some Dinosaurs Gregarious? Paleogeography, Paleoclimatology, Paleoecology, 11(4):287301 (1972), a somewhat technical report, suggests that dinosaurs may have been herding animals; his essay Social and Unsocial Behavior in Dinosaurs, in Matthew H. Nitecki and Jennifer A. Kitchell (eds.), Evolution of Animal Behavior: Paleontological and Field Approaches (1986), pp. 4161, comments on what can and cannot be said about dinosaur behaviour. Alan Charig, A New Look at the Dinosaurs (1979), offers an illustrated historical and technical treatment of the biology of dinosaurs by one of the world's authorities. John R. Horner, The Nesting Behavior of Dinosaurs, Scientific American, 250(4):130137 (April 1984), gives a short account of the discovery of dinosaur eggs and their importance. Kenneth Carpenter, Karl F. Hirsch, and John R. Horner (eds.), Dinosaur Eggs and Babies (1994), a technical, multiauthored work, reviews nearly all recently discovered aspects about dinosaur reproductive biology. Three works by Robert T. Bakker are also useful: The Superiority of Dinosaurs, Discovery (New Haven), 3(2):1122 (1968), one of the first general comments on the new biology of the dinosaurs, Dinosaur Renaissance, Scientific American, 232(4):5872, 7778 (April 1975), a general account of the author's view of hyperheated and hyperactive dinosaurs, and The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction (1986), a highly authoritative and imaginative extension of the author's earlier papers, written for the general public and fully illustrated but deficient in documentation. Extinction The first thoroughly documented account of the asteroid theory of dinosaur extinction, by the original proponents, can be found in Luis W. Alvarez et al., Extraterrestrial Cause for the Cretaceous-Tertiary Extinction: Experimental Results and Theoretical Interpretation, Science, 208(4448):10951108 (June 6, 1980), a highly technical paper. Popular reviews of the general issue include Dale A. Russell, The Mass Extinctions of the Late Mesozoic, Scientific American, 246(1):5865 (January 1982); Steven M. Stanley, Mass Extinctions in the Ocean, Scientific American, 250(6):6472 (June 1984); and Rick Gore, Extinctions, National Geographic, 175(6):662699 (June 1989). John H. Ostrom Classification Annotated classification Dinosaur family tree. 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. Critical appraisal 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 subcategoriesCoelurosauria, Ornithomimosauria, Maniraptora, Segnosauria, and Carnosauriathat 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. Extinction 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 timeapparently 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 successionsearly 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 approximatelyusually 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 moments 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. 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 ancestoran 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. Types of dinosaur pelvis. 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 cupthe 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 (see illustration). 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 (see below Critical appraisal). 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 orderthe 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.