Dinosaurs arose at a time when the land was
                        dominated by crocodilelike phytosaurs (see
                        PHYTOSAUR), giant amphibians and archosaurian
                        carnivores, and large therapsids (see THERAPSID).
                        The first dinosaurs were small, lightly built, bipedal
                        carnivores or omnivores that were probably quicker and
                        more agile than their contemporaries, most of which
                        became extinct by the end of the Triassic period. During the Jurassic period and
                        Cretaceous period the dinosaurs evolved into myriad adaptive types, many of which
                        reached colossal size, including Argentinosaurus, a long-necked vegetarian
                        weighing about 100 tons, 20 tons more than Seismosaurus, another sauropod
                        previously thought to be the heaviest dinosaur; and Gigantosaurus carolinii, a land
                        carnivore of about 8 tons. Both of these dinosaurs roamed South America 30 million
                        years before Tyrannosaurus rex, their carnivorous North-American counterpart,
                        came into existence. 

                        Remains of dinosaurs were first discovered in England in the 1820s. By the 1840s
                        several genera were well enough known that the great comparative anatomist
                        Richard Owen gave them the name Dinosauria, a term coined from the Greek words
                        deinos, "terrifying" and sauros, "lizard." In doing so, he recognized the uniqueness
                        of their great size, their terrestrial habits, their upright posture, and the inclusion of
                        at least five vertebrae in their hip girdles. It was not, however, until the exploration of
                        the western U.S. in the 1880s and after the recovery of complete dinosaur skeletons
                        from the badlands there that dinosaurs were recognized as having been largely
                        bipedal?a most unusual stance for a reptile, and one that led to much speculation
                        about their locomotion, behavior, and physiology. 

                        In the 1880s, H. G. Seeley (1839?1909) saw that Dinosauria could be divided into
                        two groups based on the form of the hip girdle. The Ornithischia had pubic bones
                        that, like those of birds, faced posteriorly. The more conventional Saurischia had
                        pubic bones that faced anteriorly. Ironically, it was the "reptile-hipped" Saurischia
                        from which birds evolved, specifically from small carnivorous dinosaurs related to
                        Deinonychus and Compsognathus. The reversal of the pubis was an evolutionary
                        convergence in birds and ornithischians. 

                        Dinosaurs are distinguished by an erect posture in which the limbs are brought
                        more or less under the body in the fashion of birds and mammals, rather than
                        sprawling to the side as in crocodiles, lizards, and turtles. They share this
                        characteristic with pterosaurs (see PTEROSAUR), their closest relatives, as well as
                        with their descendants the birds. Their footprints show that the bipedal dinosaurs
                        walked as birds do, putting one foot in front of the other, toed slightly inward. Their
                        hands were prehensile, with thumbs somewhat opposable to the other digits. Their
                        brains were generally larger than average for reptiles, particularly in the carnivores
                        and duck-billed dinosaurs. 

                        Ornithischian Dinosaurs

                        The earliest ornithischians include the poorly known Triassic form Pisanosaurus
                        from South America, and Early Jurassic genera such as Heterodontosaurus and
                        Scutellosaurus. By the Early Jurassic the ornithischians had already split into
                        several major lineages. One lineage, known collectively as Thyreophora, includes
                        the plated stegosaurs and the armored ankylosaurs, and ranges from the Early
                        Jurassic through the Late Cretaceous. A second group, the Ornithopoda, includes
                        the duck-billed hadrosaurs and their relatives the iguanodonts, as well as the horned
                        ceratopsians and their relatives the pachycephalosaurs. The Ornithopoda also
                        ranged from the Early Jurassic through the Late Cretaceous. 

                        Ornithischian dinosaurs are distinguished by their "birdlike" hip girdles, and also by
                        the presence of a predentary bone on the tip of the lower jaw. All ornithischians
                        were herbivorous, and the predentary bone appears to have served, much like the
                        incisors of camels and horses, to crop vegetation. It also connected the two halves
                        of the lower jaws and enabled them to transmit and absorb force during chewing. In
                        duck-billed dinosaurs and ceratopsians, the jaws held dozens of teeth tightly
                        arranged to form a single beveled battery of chewing surface. As in all other
                        vertebrates except mammals, the teeth were continually replaced through life as the
                        roots were resorbed and new crowns developed. 

                        Saurischian Dinosaurs

                        Saurischian dinosaurs include two major groups: (1) the herbivorous
                        Sauropodomorpha, which comprise the giant, long-necked Sauropoda such as
                        Diplodocus and Apatosaurus, plus the less-well-known Prosauropoda, which include
                        Plateosaurus; and (2) the carnivorous Theropoda, which comprise all the large and
                        small meat-eaters from Coelophysis, Compsognathus, and Deinonychus up through
                        the giant meat-eaters such as Allosaurus and Tyrannosaurus, as well as their
                        descendants the birds. Larger, bulkier sauropods, called Supersaurus and
                        Ultrasaurus, which are related to Brachiosaurus or Diplodocus, have been found in
                        Colorado. 

                        Saurischians have long necks and large claws on the first digits of their hands and
                        feet; the fourth and fifth fingers of the hand are reduced or absent (as in birds, which
                        retain only the first three digits). The sauropods became very large and quadrupedal,
                        and evolved some interesting adaptations for feeding on high vegetation. In
                        Diplodocus, for example, the neck and tail are both very long; the lower (haemal)
                        arches of the tail vertebrae, at the point where the tail would normally reach them,
                        are canoe-shaped instead of tongue-shaped, and appear to have assisted the
                        animal in using the tail as a prop for the hindlimbs in a tripod stance as it fed on
                        high vegetation, with its forelimbs raised off the ground. In Brachiosaurus, by
                        contrast, the neck is very long but the tail is short, so the tripod stance was
                        impossible; however, Brachiosaurus had forelimbs that were longer than its
                        hindlimbs, and this length plus the long neck enabled it to reach high vegetation by
                        different means than Diplodocus used. 

                        Large carnivorous theropods such as Tyrannosaurus had very large heads with great
                        jaws filled with recurved, doubly serrated teeth. Their forelimbs were reduced but
                        their hips and hindlimbs were massive, and clearly the predator had little to do
                        beyond grasping its prey in its jaws, planting its feet, and tearing and ripping the
                        flesh until the prey suffered massive shock and blood loss. Smaller theropods such
                        as Deinonychus must have been much more agile hunters that chased down prey
                        and attacked in packs, ripping with teeth and claws. However, some lineages of
                        small theropods, such as Struthiomimus and its relatives, were toothless, as were
                        all but the first birds (see; HESPERORNIS) and may have been more omnivorous in
                        their habits. 

                        Warm-Bloodedness

                        Were the dinosaurs warm-blooded? The evidence is mixed. Like birds and
                        mammals, dinosaurs had rapid rates of growth, and their bones show evidence of
                        secondary reworking (Haversian canal systems). They could not sprawl like most
                        living reptiles, and their obligate erect posture implies continual expense of
                        metabolic energy. Their footprints and long limbs show that they were capable of
                        high speeds. In addition, the birds, which are technically living dinosaurs, are
                        warm-blooded. Probably no single thermal strategy can describe all dinosaurs: In
                        these respects such mammals as bats, cats, elephants, and whales differ
                        considerably, and the dinosaurs were probably equally varied. New evidence that
                        dinosaurs were warm-blooded and that their brooding behavior was similar to that of
                        modern birds came with the 1995 discovery of the remains of a nesting dinosaur in
                        the Gobi Desert. The 8-foot-long ostrichlike dinosaur of the genus Oviraptor was
                        found with its legs folded beneath its body and its forelimbs surrounding a clutch of
                        at least 15 eggs. 

                        Extinction

                        Dozens of explanations for dinosaurian extinction have been offered, most of them
                        fanciful or outside the resolution of evidence. Until recently dinosaurs were
                        considered to have died off gradually through the Late Cretaceous. The recent
                        discovery at the Cretaceous-Tertiary boundary of the rare element iridium and of
                        shocked quartz granules, both signatures of the impact of a large asteroid or comet,
                        fueled speculation that such an impact could have triggered climatic catastrophes
                        that ended the dinosaur?s reign. Although such events, which are known to be
                        common in geologic history, might have had some environmental consequences,
                        the vast majority of dinosaurs were long extinct by that time. Moreover, other
                        organisms such as crocodiles, turtles, fishes, birds, and amphibians, which might
                        have been expected to suffer equally from such a cataclysm, survived with only
                        minor losses?a pattern that has yet to be explained by any catastrophe
                        hypothesis. It is known that through the Late Cretaceous the climate was becoming
                        more unstable and seasonal, and waves of extinctions had long been affecting both
                        marine and terrestrial life. Although the effects of extraterrestrial impact cannot be
                        ruled out, they do little to explain the observed evidence of extinction and survival at
                        the end of the Cretaceous.