By the end of the last century, the assortment of dinosaur skeletons that had already accumulated were classified into two groups of approximately equal size based on the divergent plan of their hip bones: Saurischia, the “lizard-hipped” dinosaurs, and Ornithischia, the “bird-hipped” dinosaurs (Seeley, 1888). Today we continue to follow this early dinosaur classification, albeit for somewhat different reasons, and consider that it captures a fundamental, ancient bifurcation in dinosaur evolutionary history.