Dinosaur

Dinosaurs are a group of reptiles that first appeared in the Triassic Period, more than 230 million years ago, and dominated life on land through the Jurassic and Cretaceous. They belong to the archosaurs, the larger branch that also includes crocodiles and the flying pterosaurs. What sets dinosaurs apart lies mostly in the skeleton, especially an upright stance with the legs held directly beneath the body and a hip socket that is open in the middle, rather than limbs sprawling out to the sides as in lizards.

Paleontologists traditionally divide dinosaurs into two groups by the shape of the hip bones: the lizard-hipped saurischians, which include the long-necked sauropods and the mostly meat-eating theropods such as Tyrannosaurus, and the bird-hipped ornithischians, such as Triceratops and Stegosaurus. Confusingly, birds evolved from the lizard-hipped theropods, not from the bird-hipped group. About 66 million years ago an asteroid impact helped trigger a mass extinction that killed every dinosaur lineage except the birds.

Because birds are living dinosaurs, scientists call the extinct forms non-avian dinosaurs. Pterosaurs, plesiosaurs, ichthyosaurs and mammoths are often mistaken for dinosaurs, but none of them belongs to the group. The name was coined by Richard Owen in 1842.