dinosaur, Any of the extinct reptiles that were the dominant land animals during most of the Mesozoic Era (251–65.5 million years ago). The various species appeared at different times—with the first form likely appearing some 245 million years ago—and not all overlapped. The shape of the teeth reveal whether a given dinosaur was a carnivore or an herbivore. Dinosaurs are classified as either ornithischians or saurischians, based on pelvic girdle structure. Most had a long tail, which they held straight out, apparently to maintain balance. Most, if not all, were egg layers. Some were probably warm-blooded. Dinosaur fossils have been found on every continent, including Antarctica. Most types of dinosaurs flourished until late in the Cretaceous Period (65.5 million years ago), then disappeared within the next million years. Two theories for the cause of this mass extinction following some 180 million years of existence are that volcanism and mountain-building cycles altered the habitat and changed the climate or that one or more asteroids hit Earth, resulting in immense dust clouds that blocked sunlight for several years. Birds and crocodiles are thought to be living descendants of the dinosaurs. See also carnosaur; sauropod.
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