Before 1888, paleontologists classified the many different dinosaurs in various ways: sometimes according to tooth structure, sometimes according to foot structure. In 1888, British paleontologist Harry Seeley gave a lecture in which he proposed that dinosaurs should be grouped according to the structure of their pelvic bones and joints. The saurischian dinosaurs are “lizard-hipped,” while the ornithischian dinosaurs are “bird-hipped.”

The dinosaurs with hips structured similarly to lizards include the great sauropods (e.g., apatosaurs, brachiosaurs, and diplodocoids), and the carnivorous theropods (e.g., tyrannosaurs, and dromaeosaurs). The dinosaurs with hips that are more similar to birds in design include the stegosaurs, ceratops, hadrosaurs, and pachycephalosaurs.

Although one may be inclined to think (within an evolutionary worldview) that birds have evolved from bird-hipped dinosaurs, this is in fact not the case. Evolutionary scientists suggest that theropods are the main ancestors of today’s birds.

However, as will be seen on the following pages, the idea that dinosaurs and birds share a common ancestor is not scientifically viable.