Figure 9. Mechanically relevant and species recognition characters mapped onto pachycephalosaur phylogeny from Figure 8. The common ancestor of Stegoceras and Stygimoloch is interpreted as having a large dome and extra ornamentation, with additional display ornament evolving in subsequent lineages. Extra ornamentation in large-domed taxa, such as Pachycephalosaurus and Tylocephale, suggests that the dome was not solely for display and had a strong agonistic function such as head-butting. Enlarged ornamentation and a small dome (states not mapped) in Stygimoloch suggest greater emphasis on display and less on dome-to-dome combat.