attributable to Pachycephalosaurus were discovered in Scollard Formation, Alberta, Canada, implying dinosaurs of this era were cosmopolitan and didn't have discrete faunal provinces. Description The anatomy of Pachycephalosaurus is poorly known, as only skull remains have been described. Pachycephalosaurus is famous for having a large, bony dome atop its skull, up to thick, which safely cushioned its tiny brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. The spikes were probably blunt, not sharp. The skull was short, and possessed large, rounded eye sockets that faced forward, suggesting that the animal had good eyesight and was capable of binocular vision. Pachycephalosaurus had a small muzzle which ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck. Younger individuals of Pachycephalosaurus maybe have had flatter skulls, with larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out, as the dome grew. Pachycephalosaurus was probably bipedal and was the largest of the pachycephalosaurid ("bone-headed") dinosaurs. It has been estimated that Pachycephalosaurus was about long and weighed about . Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short fore limbs, a bulky body, long hind legs and a heavy tail, which was likely held rigid by ossified tendons. Classification Pachycephalosaurus gives its name to the Pachycephalosauria, a clade of herbivorous ornithischian ("bird hipped") dinosaurs which lived during the Late Cretaceous Period in North America and Asia. Despite their bipedal stance, they were likely more closely related to the ceratopsians than the ornithopods. Pachycephalosaurus is the most famous member of the Pachycephalosauria (though not the best-preserved member). The clade also includes Stenopelix, Wannanosaurus, Goyocephale, Stegoceras, Homalocephale, Tylocephale, Sphaerotholus and Prenocephale. Within the tribe Pachycephalosaurini, Pachycephalosaurus is most closely related to Alaskacephale. Dracorex and Stygimoloch have been synonymized with Pachycephalosaurus. In 2010, Gregory S. Paul proposed that while Stygimoloch and Dracorex possibly represent a juvenile stage of Pachycephalosaurus, Stygimoloch might represent a different species, P. spinifer. This idea has been regarded as a way of interpretation by Mark Witton and Thomas Holtz. A phylogenetic analysis from 2021 by Evans and his colleagues accepted the validity of the genus Stygimoloch on the basis of it being found in later rock layers than Pachycephalosaurus, but agreed with the consensus that Dracorex represents