Pachycephalosauria

Pachycephalosauria ( /ˌpækɨˌsɛfəlɵˈsɔriə/; from Greek παχυκέφαλοσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some fossils, the skull roof is domed and several inches thick; in others it is flat or wedge-shaped. While traditionally regarded as distinct species or even families, the flat-headed pachycephalosaurs may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.