Pachycephalosauroidea

Pachycephalosauria is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. Pachycephalosaurs lived exclusively in the Northern Hemisphere during the Cretaceous period, with all members being found in North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. Skulls can be domed, flat, or wedge-shaped depending on the species, and are all heavily ossified. The domes were often surrounded by nodes and/or spikes. Partial skeletons have been found of several pachycephalosaur species, but to date no complete skeletons have been discovered. Often isolated skull fragments are the only bones that are found.