During the Late Triassic, the ancestor of dinosaurs gave rise to two different groups of dinosaurs. One of these is the Saurischian (lizard-hipped dinosaurs), which has a hipbone with the pubis pointing forward. This is very much like the lizards of today. The second group is the Ornithischians (bird-hipped dinosaurs), which has a hipbone with a pubis pointing back in the same ways bird hipbones do. Despite the name, birds evolved from lizard-hipped dinosaurs, not from bird-hipped dinosaurs.

Lizard-hipped dinosaurs followed two different evolutionary paths, leading to Theropod (beast-feet) and Sauropod (lizard-feet) dinosaurs. Beast-feet dinosaurs were bipedal meat-eaters and are best represented by T-rex and Velociraptors. Lizard-feet dinosaurs were longneck gigantic plant-eaters. The giant Brachiosaurs are the best examples of this group. Bird-hipped dinosaurs were all plant-eaters. These include the Stegosaurus of the Jurassic period and the Triceratops of the Cretaceous.