Pachycephalosaurus - Paleobiology

Paleobiology

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.