Ornithischia
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Pachycephalosaurus is the most famous pachycephalosaur, and obviously the group’s namesake. It’s also the only known pachycephalosaur to grow to a larger size, although 4.5 m / 15 feet long and under half a tonne isn’t exactly a giant among the Late Cretaceous fauna of North America.
As in other dome-headed dinosaurs, the large, solid bony cranial helmet was clearly important to their biology, and must have included some (or all) of defense against predators, display and combat between conspecifics. Evidence of injuries to the domes in some specimens has been interpreted as evidence of head-to-head collisions, although this isn’t universally accepted. While certainly head-butting for year after year would do damage to the skull and brains, nature isn’t particularly concerned, as long as it gets the job done. For example, a recent study showed that bighorn sheep males regularly get concussed while head-butting, and over time can develop CTE-like symptoms.
