Harry Govier Seeley split dinosaurs into two groups, the Order Saurischia ("lizard-hipped" dinosaurs) and the Order Ornithischia ("bird-hipped" dinosaurs). Both orders probably had a common ancestor that lived sometime during the Middle Triassic. Birds belong to the saurischian dinosaur clade.
As in all land animals, there were three bones in each side of the pelvis. The left and right ilia (singular: ilium) firmly gripped the spine in the sacrum. The left and right pubes (singular: pubis) extended down beneath the ilia. The left and right ischia (singular: ischium) extended down and back beneath the ilia and behind the pubes. In some dinosaurs, the pubes extended down and forward, as they do in lizards. This is why Seeley called them saurischian, or "lizard-hipped" dinosaurs. In other dinosaurs, the pubes extended down and back, running beneath and parallel to the ischia, as they do in birds. Seeley called these dinosaurs ornithischian, or "bird-hipped" dinosaurs.
Ornithischian dinosaur pelves (the plural of pelvis) developed from evolutionary changes of the primitive saurischian pelvis. Ornithischians also had other traits that grouped them together: a bone, called the predentary, was at the front of the lower jaw. Also, an "eyelid" bone rimmed the upper part of the eye socket.
Some dinosaurs were neither saurischians nor ornithischians. The earliest, most primitive dinosaurs, such as Staurikosaurus and Herrerasaurus, fit into neither order. They were too specialized to be the direct ancestors of the dinosaurs.