Dracorex

Classification and species

In 2010, Gregory S. Paul proposed that, while Stygimoloch and Dracorex possibly represent different growth stages of Pachycephalosaurus, Stygimoloch might represent a different species, P. spinifer. In his supplementary material of a 2017 paper, Fowler noted that Stygimoloch is only known from younger rock layers than Pachycephalosaurus, so he tentatively classified both to be separate, though Dracorex was included as a synonym of either taxa. A 2021 phylogenetic analysis by Evans and colleagues accepted the validity of the genus Stygimoloch based on the same reason as Fowler (2017), but agreed with the consensus that Dracorex represents an ontogimorph of either Stygimoloch or Pachycephalosaurus instead of a distinct taxon. In 2025, Wroblewski treated Stygimoloch as a separate genus to which he attributed three specimens from the Ferris Formation, providing further evidence that this taxon was restricted to the uppermost Maastrichtian, but agreed with the consensus that Dracorex is morphologically identical to Stygimoloch (or Pachycephalosaurus).