saurischian

According to Sereno, the primitive nature of the Eoraptor skull supports the theory that dinosaurs arose from a common ancestor and only later developed specializations that would split them into ornithischians and saurischians. The new discovery also confirms earlier suspicions about what the ancestral dinosaur looked like.

Because the various groups had such dissimilar anatomies, early dinosaur investigators presumed that the bird-hipped ornithischians had evolved from a different stock of reptiles than had the sauropods and theropods, which together form a group called saurischian dinosaurs, characterized by their lizard-like pelvises.