Pachycephalosauria (pak-ee-sef-uh-lo-SAWR-ee-uh) is the infraorder of
“thick-headed” Marginocephalia dinosaurs. Members of this group had helmet-like
skulls, which were up to 9 inches (23 cm) thick, as in Pachycephalosaurus.
By comparison, a human skull is only ¼ inch (.64 cm) thick. Their
vertebrae also show strengthening that would be of use if pachycephalosaurs
used their heads as battering rams. It would make sense that the domes
of males would be thicker than those of females and that male pachycephalosaurs
may have butted heads, as rams do, to establish dominance in their social
hierarchy. The domes may have served a defensive purpose, but given the
equipment of predators, it is unclear how useful the domes would have been.
Pachycephalosaurs were bipedal and herbivorous, as were the ornithopods
from whom they were probably derived.