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| Dinosaur, any member of a group of about 1300 reptiles that first appeared in the late Middle or early Late Triassic period, about 200 million years ago. | ||||||||
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Most dinosaurs are classified into two types, the "bird-hipped" Ornithischia, for example Iguanodon and Triceratops, and the "lizard-hipped" Saurischia, for example Apatosaurus and Tyrannosaurus. The earliest known dinosaurs, such as Staurikosaurus and Herrerasaurus from South America, are too primitive to be classified within either order. Dinosaurs died out about 65 million years ago, but one lineage, the birds, survives to the present. |
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| The first dinosaurs were
small, lightly built, bipedal (standing on two legs) 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, about 208 million
years ago. During the following Jurassic period and Cretaceous period
the dinosaurs evolved into myriad adaptive types, many of which reached
colossal size. 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 (Greek, "terrible lizard"). In doing so, he recognized the uniqueness of their great size, their land-living habits, their upright posture, and the inclusion of at least five vertebrae in their hip girdles. It was not until the explorations in the 1880s, however, and the recovery of complete fossil dinosaur skeletons, 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, behaviour, and physiology. In the 1880s, H. G. Seeley saw that Dinosauria could be divided into two groups based on the form of the hip girdle. The Ornithischia had pubic bones that, superficially resembled those of birds. The more conventional Saurischia had pubic bones that resembled those of extant reptiles. Ironically, it was the "lizard-hipped" Saurischia from which birds evolved, specifically from small carnivorous dinosaurs related to Deinonychus and Compsognathus. |
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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, 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 inwards. Their brain size compared to body size was variable and was lowest in the Sauropods (see below) and highest in small carnivores. "Dinosaur," Microsoft(R) Encarta(R) 97 Encyclopedia. (c) 1993-1996 Microsoft Corporation. All rights reserved. |
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