The marginocephalians (meaning “fringed headed”) were actually latecomers in dinosaur evolution, first appearing in the Early Cretaceous, and chiefly in North America and Central Asia. They are divided into the pachycephalosaurs (Pachycephalosauria, or “thick-headed reptiles”) and the ceratopsians (Ceratopsia or “horned dinosaurs”).

The marginocephalians are considered a clade of ornithischian dinosaurs. They were closely related to the ornithopods; some scientists believe the marginocephalians may have originated from the ornithopods, especially the heterodontosaurs. The marginocephalians were herbivorous dinosaurs that had a slight shelf or frill at the back of their skull. The frill, or shelf, differed in the two main subgroups of the marginocephalians: the “bone-headed” pachycephalosaurs and the frilled ceratopsians.