Keep back! That domed head might not look threatening, but Pachycephalosaurus is certainly not afraid to use it!

The dome is a thick growth of solid bone - in fact, that's what the Greek root "pachy" translates to: "thick"! Hence its use in both Pachycephalosaurus - the thick-headed lizard... ... and the ceratopsian Pachyrhinosaurus - the thick-nosed lizard.

These little jerks are constantly injuring their skulls, because they won't stop hitting things with them! These injuries are so common that fossilized head wounds were convincing evidence for hypothetical head-butting behavior - until we brought them into the park, of course. Little good that does us... The vet bills just keep going up and up and up!

It’s remarkable how their skulls grow these domes... When they first pop out of the egg in the nursery, their skulls are actually flat! The transition through its awkward teenage growing years proved quite vexing for paleontologists, who originally thought that the flat skulls and the big domed ones belonged to different species.