Wikipedia
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. 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 centimeters thick; in others it is flat or wedge-shaped. While the flat-headed pachycephalosaurs are traditionally regarded as distinct species or even families, they may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.
Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from Middle Jurassic Period strata of Kyrgyzstan and Stenopelix valdensis from Early Cretaceous strata of Germany, although R.M. Sullivan has doubted that either of these species are actually pachycephalosaurs.