The first true dinosaurs first appeared about 230 million years ago. The first dinosaur fossils are distinguished from those of their reptile ancestors by their more upright stance, with legs directly beneath the body instead of splayed to the side, as with other reptiles. The earliest dinosaur fossils were found in South America, although a December 2012 fossil discovery in southern Africa casts some doubt on a South American origin.
The first dinosaurs were a group of species known as theropods. They walked on two legs and varied in size from the 20-pound Eoraptor to the 400-pound Herrerasaurus. The group spread relatively quickly from South America to North America, and then further to Africa and Eurasia. Many famous types of dinosaur, including tyrannosaurs and allosaurs, were later species of theropod.
Dinosaurs with four legs evolved from theropods, but they are not considered theropods themselves. These species, which were mostly herbivores, were split into two groups by their hip structures. Lizard-hipped species included the giant sauropods. Bird-hipped groups included the armored anklyosaurs and the horned ceratopsians. Scientists think that all bird-hipped dinosaurs descend from a species called Eocursor, a species from South Africa, which, in turn, was probably descended from Eoraptor.