Pachycephalosaurus ( /ˌpækɨˌsɛfələˈsɔrəs/; meaning "thick headed lizard," from Greek pachys-/παχυς- "thick", kephale/κεφαλη "head" and sauros/σαυρος "lizard") is a genus of pachycephalosaurid dinosaur. 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 or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs. This dinosaur is monotypic, meaning the type species, P. wyomingensis, is the only known species. 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, Pachycephalosaurus was a bipedal omnivore 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 intraspecific combat. This hypothesis has been disputed in recent years.
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 25 cm (10 in) 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 vision 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. Pachycephalosaurus was probably bipedal and was the largest of the pachycephalosaurid (bone-headed) dinosaurs. It has been estimated that Pachycephalosaurus was around 4.5 metres (15 ft) long and weighed 450 kilograms (990 lb). 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.
Nearly all Pachycephalosaurus fossils have been recovered from the Lance Formation and Hell Creek Formation of the western United States. Pachycephalosaurus co-existed alongside juvenile pachycephalosaurs Dracorex and Stygimoloch. Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus, ceratopsid Triceratops, ankylosaurid Ankylosaurus, and the theropods Ornithomimus, Troodon, and Tyrannosaurus.
Scientists once suspected that Pachycephalosaurus and its relatives were the bipedal equivalents of bighorn sheep or musk oxen; that male individuals would ram each other headlong. It was also believed that they would make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, it is now believed that the pachycephalosaurs could not have used their domes in this way.
Foremost, the skull roof could not have adequately sustained impact associated with such ramming. Also, there is no evidence of scars or other damage on fossilized Pachycephalosaurus skulls. Furthermore, the cervical and anterior dorsal vertebrae show that the neck was carried in an "S"- or "U"-shaped curve, rather than a straight orientation, and thus unfit for direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.
It is more probable that the Pachycephalosaurus and other pachycephalosaurid genera engaged in flank-butting in intraspecific combat. In this scenario, an individual may have stood roughly parallel or faced a rival directly, using intimidation displays to cow its rival. If intimidation failed, the Pachycephalosaurus would bend its head downward and to the side, striking the rival pachycephalosaur on its flank. This hypothesis is supported by the relatively broad width of most pachycephalosaurs, a trait that would have protected vital organs from harm. The flank-butting theory was first proposed by Sues in 1978, and expanded upon by Ken Carpenter in 1997.
In 2012, a study showed that cranial pathologies in a P. wyomingensis specimen were likely due to agonistic behavior. It was also proposed that similar damage in other pachycephalosaur specimens previously explained as taphonomic artifacts and bone absorptions may instead have been due to such behavior.
Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, fruit and insects. The sharp, serrated teeth would have been very effective for shredding plants.