Pachycephalosauria (Greek for 'thick headed lizards') is a clade of ornithischian dinosaurs. Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from the Middle Jurassic Period of what is now Kyrgyzstan and Stenopelix valdensis from the Early Cretaceous Period of what is now Germany, although Sullivan (2006) doubts that either of these species are pachycephalosaurs. Most known pachycephalosaurs lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous animals with thick skulls; in some species the skull roof is domed and several inches thick. The dome may be surrounded by nodes (for instance, Pachycephalosaurus) or spikes (Stygimoloch).