The recent description of Acrotholus has provided me with a new insight into the early evolution of pachycephalosaurs in North America, as this is the oldest genus of domed pachie from the continent and an undescribed pachycephalosaur from the Wahweap Formation of southern Utah is also fully domed. Given that the oldest pachie, Amtocephale, is not flat-headed, does the discovery of Amtocephale and Acrotholus prove that the oldest pachies were domed and not flat-headed?