The ancestors of dinosaurs had a hipbone, the pubis, which pointed forward. The pubis evolved to point backward in two groups of dinosaurs: ornithischians and birds. Even though they share the same hip shape, many other features show that birds are actually saurischian dinosaurs. The evolution of a bird-like hip in ornithischians and birds is a case of convergent evolution, in which the same structure or shape evolves more than once.

Archosaurs are a group of specialized reptiles which ruled the Earth during the Age of Dinosaurs. The only archosaurs that survive today are crocodiles and birds. Birds are the only group of dinosaurs that survived the extinction at the end of the Cretaceous period, 65 million years ago. Your holiday turkey is a saurischian dinosaur, like Apatosaurus , Tyrannosaurus , and Velociraptor .