n.
Any of the extinct reptile s that were the dominant land animals during most of the Mesozoic Era (24865 million years ago).
The various species appeared at different times, and not all overlapped. The shape of the teeth reveal whether a given dinosaur was a carnivore or herbivore. Dinosaurs are classified as either ornithischian s or saurischian s, 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 million years ago), then disappeared within the next million years. Two theories for the cause of this mass extinction, following some 140 million years of existence, are that mountain-building cycles altered habitat and changed climate or that an asteroid hit the Earth, resulting in immense dust clouds that blocked sunlight for several years. Birds are thought to be living descendants of the dinosaurs. See also carnosaur ; sauropod .