Pachycephalosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Pachycephalosauria > Pachycephalosauridae > Pachycephalosaurinae

Overview: There’s a lot of mystery surrounding the origins of the dome-headed pachycephalosaurs, though they do appear to have been well established by the end of the Early Cretaceous. Towards the end of the Cretaceous, however, they would see their peak of success and diversity. Among the many genera from that time, there was Pachycephalosaurus itself, the namesake of both the clade Pachycephalosauria and the family Pachycephalosauridae. Pachycephalosaurs are classified close to the horned ceratopsians, though unlike derived ceratopsians, such as Triceratops, none of these dome-headed creatures evolved into quadrupeds. Pachycephalosaurus is notable for being, by far, the largest known member of its lineage, at four to five meters in length. Most others averaged at two meters long. As with other pachycephalosaurs, this genus possessed a thickened, rounded mass of bone atop its skull, fringed by small, pointed hornlets at the rear. Similar hornlets can also be seen along its snout. Pachycephalosaurus had a beaked snout, paired with rows of shearing teeth farther back in its jaws. It was either a herbivore or possibly omnivorous.

Debate surrounds why Pachycephalosaurus and its kin evolved domed heads. Restorations of this animal often depict it engaging in direct, one-on-one head butting between competing individuals, primarily males. Their neck anatomy may’ve been ill suited to this, so side-to-side strikes are seen as more likely by some researchers. Pachycephalosaurus was established as a genus in 1943, but for a time, its fossils were associated with the teeth of the dubious genus Troodon (a maniraptoran theropod). The family Troodontidae was, for a time, incorrectly used for the pachycephalosaurids. Pachycephalosaurus lived towards the end of the Cretaceous, coming from the Hell Creek, Lance and other similar geological formations. Two genera, Dracorex and Stygimoloch, are also found in these formations, but they may merely be younger growth stages of Pachycephalosaurus; debate is still ongoing on this subject. Pachycephalosaurus coexisted with other famous herbivorous dinosaur genera like Triceratops, Edmontosaurus, Thescelosaurus and Ankylosaurus. Both it and these plant-eating genera would’ve been prey for theropods like Tyrannosaurus or Nanotyrannus.