A saurischian was a 'lizard-hipped' dinosaur, in other words, any of a group of dinosaurs having pelvic girdles typical of lizards, with three prongs to each side. They include the two-legged carnivorous theropods, such as Tyrannosaurus and Allosaurus, with enormous skulls and large teeth; and the four-legged herbivorous sauropods, such as Apatosaurus and Diplodocus, with very small heads and long necks and tails.
Living birds had common ancestors on the theropod lineage. Oddly enough, birds are derived from the "lizard-hipped" dinosaurs and not from the 'bird-hipped' ornithischian dinosaurs. The 'bird-hipped' condition of a pubis pointing toward the back of the animal occurred twice independently, once in the ornithischians and once in the lineage leading to birds – an example of convergent evolution.