Dinosaur (Greek for "terrible lizard") is the name given to various kinds of large extinct reptiles of the Mesozoic Era (from 230 to 65 million years ago), the interval during which they were the dominant land animals on Earth. The term was proposed as a formal zoologic name in 1841 by the British anatomist Sir Richard Owen, in reference to large fossil bones unearthed in southern England, but it is used today only in an informal sense. The various kinds of dinosaurs are classified in two formal categories, the orders Saurischia and Ornithischia, within the reptilian subclass Archosauria. The first recorded dinosaur remains found consisted of a few teeth and bones. They were discovered (1822) in Sussex, England, by an English doctor, Gideon Mantell, who named them \TIguanodon\t ("iguana tooth"). About the same time, other fossil teeth and bones were found near Oxford, England, by Rev. William Buckland. These were named Megalosaurus ("great lizard"). Thousands of specimens have since been discovered nearly worldwide. Different types of dinosaurs varied greatly in form and size, and they were adapted for diverse habitats. Their means of survival can only be inferred from their fossil remains, and some inferences are in dispute. They ranged in weight from 2 to 3 kg (4 to 6 lb), in the case of \TCompsognathus\t, and up to 73 metric tons (160,000 lb), in the case of \TBrachiosaurus\t. Most dinosaurs were large, weighing more than 500 kg (1,100 lb), and few weighed less than 45 kg (100 lb). Most were herbivores, but some saurischians were carnivorous. The majority were four-footed but some ornithischians and all carnivores strode on their hind legs. Always classified as reptiles, dinosaurs have traditionally been assumed to have been reptilian in their physiology, cold-blooded and ectothermic (dependent on external heat sources). In recent years several different lines of evidence have been interpreted as indicating that dinosaurs may have had warm blood and high rates of metabolism, comparable to birds and mammals. Evidence supporting this view includes upright posture and carriage; mammallike microscopical structure of bones; skeletal features suggestive of high activity; and specialized food-processing dentitions and low ratios of dinosaurian predators to prey animals, both suggesting high food requirements. The evidence is not conclusive--all the facts can be alternatively explained--but some dinosaurs may have been endothermic (maintaining body temperature and heat production by internal metabolic processes). The reproductive means of most dinosaurs is as yet unknown. Fossil eggs, attributed to one of the horned dinosaurs (\TProtoceratops\t) and a sauropod (Hypselosaurus), have been discovered in Mongolia and France. Fragments that are presumed to be of dinosaur eggs have also been found in Brazil, Portugal, Tanzania, and in the United States, Colorado, Montana, and Utah. In Montana, Utah, and Alberta, Canada, fossils of unhatched dinosaur eggs have been discovered. This evidence indicates egg-laying (oviparous) reproduction in dinosaurs, like most modern reptiles. A few scientists believe that some dinosaurs may have given birth to living young, but no conclusive evidence has yet been found to substantiate this. The two orders of dinosaurs are distinguished by numerous features, the most diagnostic being the arrangement of the three bones of the \Tpelvis\t. In saurischians, these bones were arranged in a triradiate pattern similar to that of modern crocodilians and lizards; the term Saurischia means "lizard hip." The ornithischian pelvis was usually rectangular or tetraradiate; hence the name, which means "bird hip." SAURISCHIA The order Saurischia includes two different and perhaps distantly related kinds of dinosaurs: the carnivores (suborder Theropoda); and the huge herbivores, and their ancestors, suborder Sauropodomorpha. Aside from the saurischian pelvic arrangement, the two groups had few anatomical features in common, which suggests remote affinity and possibly separate ancestry. Theropods The theropods are subdivided into two categories, the small and lightly built coelurosaurs and the large and heavily built carnosaurs. Some evidence indicates that this subdivision may be arbitrary and may not accurately represent evolutionary relationships among the theropods. All theropods were obligatory bipeds--unable to assume a four-legged stance. The hind legs were strong and bore birdlike feet. In some, the hind limb proportions indicate high running speed. The forelimbs bore sharp, curved claws for seizing and holding prey. All theropods had long tails that functioned as dynamic stabilizers or counterbalances. The head was relatively large to huge, and the jaws of most contained numerous sharp, bladelike teeth with serrated edges--clear evidence of flesh-eating habits. The "ostrich" dinosaur (Struthiomimus), however, had toothless jaws that probably were covered with a birdlike beak--evidence that perhaps all theropods were not carnivorous. The smallest known theropod is Compsognathus (Late Jurassic, Europe) and the largest is Tyrannosaurus (Late Cretaceous, North America), which grew to a weight of 5 or 6 metric tons (about 10,000 lb), a height of 6 m (18 ft), and a length of 15 m (45 ft). First appearing in mid-Triassic, theropods thrived until the end of the Cretaceous. Sauropods The suborder Sauropodomorpha comprises two subcategories: the Sauropoda, which resembled the well-known member of the group, Brontosaurus; and the Prosauropoda, their less well-known ancestral stock. Prosauropods were common during the last part of the Triassic and have been found on all continents except Antarctica and Australia. Except for certain resemblances in the skull and teeth, they were quite different from the sauropods. The prosauropods were much smaller, ranging from a few hundred kilograms to perhaps 1,500 kg (3,300 lb). All were quadrupedal, but many were capable of bipedal posture and gait. They all appear to have been herbivores, although carnivorelike teeth have been found associated with skeletal remains of some kinds. Best known of the prosauropods is Plateosaurus (Late Triassic, Europe). Usually considered semiaquatic or amphibious, the sauropods include the largest of the dinosaurs. The smallest varieties were larger than elephants, and the largest, Brachiosaurus (Late Jurassic, East Africa) and a new, unnamed discovery, are estimated to have weighed 73 metric tons (about 160,000 lb) or more. Best known are Brontosaurus (more correctly, Apatosaurus) and \TDiplodocus\t from the Late Jurassic of North America. All sauropods had the same basic structure: large, barrellike body; sturdy, columnar legs; long, heavy tails; and a small head at the end of a long and stout neck. The hypothesis that the sauropods lived in semiaquatic swamp environments is based on the belief (probably incorrect) that their limb bones were not strong enough to support such massive weights, thus requiring the buoyant effect of water. According to this theory, the long neck enabled the animal to reach the surface for air. Footprints show that some sauropods did migrate across land; thus the long neck might have functioned like that of a giraffe. The sauropods were the dominant herbivores throughout the Jurassic but apparently were of only minor importance during the Cretaceous. ORNITHISCHIA The Ornithischia are divided into four or five suborders; the four always included are the bipedal ornithopods, the plated stegosaurs, the armored ankylosaurs, and the horned dinosaurs, or ceratopsians. Some authorities put the dome-headed, bipedal pachycephalosaurs in a fifth suborder, rather than in the Ornithopoda. In addition to the birdlike pelvis common to the ornithischians, all except a few primitive kinds had a toothless front part of the mouth, which was covered by a horny beak or bill. The ornithopods were the most successful and diverse of the ornithischians. The earliest are Middle and Late Triassic in age (Pisanosaurus, South America; Heterodontosaurus, Africa). At the close of the Cretaceous, 140 million years later, ornithopods were the most abundant of the herbivores. All were adept bipeds, but they were also able to assume a four-legged stance and gait. The most interesting feature of the ornithopods is the specialized dentition many had for crushing and grinding plant material. This dental apparatus was most highly developed in the Late Cretaceous duck-billed dinosaurs (Corythosaurus, Anatosaurus); their grinding dental batteries composed of hundreds of tightly compacted teeth are evidence of high food-volume intake, related perhaps to high metabolism. Aside from \TCamptosaurus\t (Late Jurassic, North America) and Iguanodon (Early Cretaceous, Europe), the duck-bills of Asia and North America are best known. The plated dinosaurs (Stegosauria) were never as important as the other dinosaurs. The few fossils found have been in Jurassic rocks, mostly from North America, East Africa, and Europe. Like all other ornithischians, they were herbivores, but with a small head and few small teeth that seem strangely designed to feed the animal's large body. The distinctive feature of \TStegosaurus\t is the two rows of alternating upright bony plates that developed along the back. Long thought to have been protection against predators, these plates have recently been shown to be highly vascularized and probably heavily perfused with blood during life. This has led to the suggestion that they may have been cooling fins and functioned as temperature regulators. The Ankylosauria succeeded the stegosaurs in the Cretaceous, apparently filling that vacated niche. To varying degrees, all ankylosaurs were encased in a mosaic-work shield of bony plates and spikes that covered the back and flanks. All were relatively low, broad animals with short, pillarlike legs. The large body indicates a herbivorous diet, but the small, weakly developed teeth seem inconsistent with the necessary food intake. \TPalaeoscincus\t and Ankylosaurus (Late Cretaceous, North America) are the best-known examples. The horned dinosaurs (Ceratopsia) were the last of the major dinosaurian kinds to appear. The earliest-known kind, Protoceratops, a low-slung, calf-sized animal, is known only from the Upper Cretaceous rocks of the Gobi Desert, Mongolia. It probably descended from an earlier Mongolian ornithopod dinosaur, resembling the bipedal Psittacosaurus. Although all other horned dinosaurs arose from that Asiatic beginning, they have been found only in North America. Horned dinosaurs with large head and neck shields and horns evolved in North America, culminating in \TTriceratops\t and Torosaurus--two of the last surviving dinosaurs. Ceratopsians also bore specialized dental batteries for slicing, rather than grinding, plant food--further evidence of high food-volume requirements among some dinosaurs. EVOLUTION AND EXTINCTION Dinosaur origins are uncertain, although most authorities agree that both orders arose from an extinct group of primitive reptiles known as the thecodonts. The pronounced anatomical differences between ornithischians and saurischians may be evidence of separate thecodont ancestry. The pseudosuchian thecodonts Ornithosuchus and Euparkeria are often cited as likely anatomical types that could have given rise to saurischians, and possibly ornithischians, but the evidence is too fragmentary to be conclusive. During the 140-million-year reign of the dinosaurs, many new varieties evolved and older kinds died out. Not all kinds became extinct at once; but the last of the dinosaurs disappeared at the end of the Cretaceous. Many other animal kinds died out at about the same time, including \LIchthyosaur\ls, \Lmosasaur\ls, \Lplesiosaur\ls, flying reptiles, and a variety of lower organisms. What brought about such widespread extinction among so many different kinds of organisms is not known; it must, however, have involved major changes in environment. Their \Textinction\t has been attributed to many causes, including cosmic radiation, exploding supernova, world-wide fluctuations in sea level, acid rain caused by volcanic activity, climatic change, and \Tcontinental drift\t. Independent evidence indicates that sea levels did fall and temperatures dropped at the end of the Mesozoic Era, a time when continents were drifting apart and new mountain ranges were rising. Although none of these conditions is likely to have been solely responsible for the extinction of the dinosaurs, collectively they may have been important. A controversial extinction theory was proposed in the 1980s by physicists Luis and Walter \TAlvarez\t. They found a larger-than-normal amount of Iridium in samples of sedimentary layers between rocks of the Cretaceous and Tertiary period and concluded that the iridium came from outer space, most likely from an asteroid that struck the Earth at that time. They theorized that the impact caused an enormous cloud of dust to circle the Earth, striking at the food chain (including the dinosaurs) by killing many plants. Other scientists later suggested that such a collision would instead have caused the extinctions by setting off massive fires. Many geologists, while accepting the iridium evidence as meaningful, have suggested that the iridium resulted from climate-changing volcanic eruptions over a longer time period. Whatever the cause, the dinosaurs are now gone. In a way, however, they may remain. That is, many paleontologists consider birds almost certainly to have evolved from some small bipedal dinosaur during the Jurassic. If so, the children of the dinosaurs still exist today. John H. Ostrom Bibliography: Bakker, R. T., The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction (1986); Colbert, E. H., Dinosaurs (1986); Czerkas, S. J., and Olson, E. C., eds., Dinosaurs Past and Present, vols. 1 and 2 (1987-88); Dixon, Dougal, et al., The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals (1988); Horner, J.R., and Gorman, J., Digging Dinosaurs (1988); Paul, G.S., Predatory Dinosaurs of the World (1988); Raup, D.M., The Nemesis Affair: A Story of the Death of Dinosaurs and the Ways of Science (1986); Wilford, N.N., The Riddle of the Dinosaur (1987). See also: \Tevolution\t; \Tfossil record\t.