The ornithischian dinosaurs, except for some early primitive forms, are all members of a clade called the Genasauria: beaked, herbivorous dinosaurs that were the prey for carnivorous theropods. In one frequently used classification, the Genasauria are divided into the Neornithischia and Thyreophora. The Thyreophora are further divided into the Stegosauria and the Ankylosauria (although in some classifications, these two divisions are combined into one group; in another classification, Eurypoda is added with the Stegosauria and Ankylosauria); the Neornithischia are divided into several types of dinosaurs, the major one being the Ceropoda, which include the Ornithopoda and the Marginocephalia. All the Genasauria dinosaurs developed in the Middle to Late Triassic period, evolving and increasing in number by the Jurassic and Cretaceous periods.

All ornithischians have a few major features in common: a pubis pointing backward, running parallel with the ischium (the name Ornithischia means “bird-hipped,” and modern birds also have pelvises in which the pubis points backwards). Besides the hip structure of the animals, the other main characteristic shared by the ornithischian (Genasauria) dinosaurs was a deeply inset tooth row. This manifested itself in the skull as a deep concavity on each side, a structure some paleontologists use to support the idea of cheeks in some of these dinosaurs. (Since all ornithischian dinosaurs were herbivorous, cheeks would have been an important feature for keeping the food in the mouth during the chewing process.)