A dinosaur is any of a large number of terrestrial reptiles that lived during the Mesozoic era, between about 225 and 65 million years ago. Dinosaurs were mostly egg-laying animals, ranging in size from 90 centimeters (31 inches) to 27 meters (90 feet). There were two orders: Saurischia (lizard-hipped), which included the bipedal carnivores and the giant carnivores, and Ornithischia (bird-hipped), which were smaller herbivores. Their posture, with limbs vertically beneath the body, distinguishes them from other reptiles. The name "dinosaur", which comes from the Greek for "terrible lizard", was coined by the English paleontologists Richard Owen in 1842.


Dinosaurs diverged from their archosaur ancestors about 230 million years ago during the middle to late Triassic period, some 20 million years after the great Permian-Triassic extinction event, which wiped out an estimated 95% of all life on Earth. The last of the dinosaurs died out at the Cretaceous-Tertiary boundary, about 65 million years ago. There is good evidence that birds are the living descendants of ornithischians. It has also been speculated that had the dinosaurs not become extinct, some ornithischians could have evolved to become fairly intelligent (see intelligent dinosaurs).