Dinosaur (Greek, “terrible lizard”), extinct reptile that flourished between about 220 and 63 million years ago and then suddenly disappeared. Dinosaurs dominated the land life during most of this period and occurred in a wide variety of forms, some no bigger than a chicken and others weighing many tons. The dinosaurs arose in the Triassic Period (early Mesozoic Era) from a group of small reptiles called thecodonts. They evolved into 2 great groups, or orders, the Saurischia, or “lizard-hipped,” and the Ornithischia, or “bird-hipped.” Saurischian dinosaurs include 2-legged carnivores-the theropods and 4-legged herbivores-the sauropods. The sauropods were the giant dinosaurs, including such types as the apatosaurus (brontosaurus) and the diplodocus, with tiny heads, very long necks and tails, and pillar-like legs. The theropods ranged from the fast-running coelurosaurs, which fed on insects and perhaps on eggs, to the carnosaurs like the Tyrannosaurus and allosaurus, which had enormous skulls and dagger-like teeth and could grow to a height of 20 ft (6m). Ornithischians diversified into many types, some producing strange body armor. All were herbivorous. Four-legged forms include the stegosaurs, which had triangular bony plates along the back, the armadillo-like ankylosaurs, and the ceratopsians, such as the triceratops, which carried 3 horns and a bony frill on the skull. The 2-legged ornithischians included the duck-billed dinosaurs. At the end of the Cretaceous Period (about 65 million years ago) dinosaurs disappeared. The reason for their extinction is unknown. It has been suggested that disease killed them off or that mammals preyed excessively on their eggs. Climactic and other changes caused by the impact of a huge asteroid have also been proposed.
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