Pachycephalosaurus (meaning “thick-headed lizard,”) is a genus of pachycephalosaurid dinosaurs. The type species, P. wyomingensis, is the only known species. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous creature which is primarily known from a single skull and a few extremely thick skull roofs, though more complete fossils have been found in recent years. Pachycephalosaurus was one of the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus.
Like other pachycephalosaurids, it was a bipedal herbivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest known pachycephalosaur.
The thick skull domes of Pachycephalosaurus and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. This hypothesis has been disputed in recent years.
Pachycephalosaurus is the last, largest, and most famous member of the Pachycephalosauria, or thick-headed dinosaurs. In the 1970s paleontologist Peter Galton proposed that male pachycephalosaurs used their dome heads as battering rams, like Bighorn sheep. The idea caught the public’s imagination, and two individuals are seen doing this in The Lost World: Jurassic Park (of course, these are genetically engineered dinosaurs and not necessarily exactly the same ones that lived 70 million years ago!). But by the 1990s, scientists began to question Galton’s head butting theory. It was pointed out that animals who do butt heads have a wide surface area where the heads come into contact to prevent “head slippage.” This happens when two animals butt heads at high speed and do not hit straight on. The risk breaking their necks when their heads suddenly snap to one side. Pachycephalosaurus has a domed, or rounded, head, which would minimize surface contact and therefore increase the risk of head slippage. This throws doubt on the idea of any high speed head-butting between pachycephalosaurs, but it does not exclude “head-pushing” of “head-ramming” against non-pachycephalosaurs.
The skull, which was almost 2 feet (60 cm) long, was nearly 8 inches (20 cm) thick at the central part of the dome. Pachycephalosaurus had triangular teeth with coarse serrations along the edges for shredding tough plant matter.
Nearly all fossils have been recovered from the Lance Formation and Hell Creek Formation of the western United States. Pachycephalosaurus possibly co-existed alongside additional pachycephalosaur species of the genera Sphaerotholus, Dracorex and Stygimoloch, though these may represent juveniles of Pachycephalosaurus itself, though Sphaerotholus is regarded as a valid species. Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus and a possible species of Parasaurolophus, ceratopsids like Triceratops, Torosaurus, Nedoceratops, Tatankaceratops and Leptoceratops, ankylosaurids Ankylosaurus, nodosaurids Denversaurus and Edmontonia, and the theropods Acheroraptor, Dakotaraptor, Ornithomimus, Struthiomimus, Anzu, Leptorhynchos, Troodon, Pectinodon, Paronychodon, Richardoestesia and Tyrannosaurus.