Acrotholus: Oldest Pachie From North America

[DD1991]

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?

[Auspex]

What are the chances, given the paucity of anything other than fragmentary remains, that non-domed ornithischian specimens would be called a Pachycephalosaur? Currently, the defining characteristic is the domed skull, really.

[Ridgehiker]

Interesting question but As Aupex states...no complete record so all is speculation. I've found two etremely distinct pachy domes if a two formations separated by a million years at most...this aspect of their anatomy could probably evolve quickly and would be quite variable.

Head shape could evolve flat, round, etc. a dozen times back and forth without any fossil record of it. Also, we have no idea of knowing who is 'first' as all we have is a fossil that is older than another. My father is older than me but that doesn't make him older than previous ancestors. I'm taller than my father so does that make me taller than all of his ancestors? No way of knowing.

Anyways, features don't evolve in just one direction but atropphy depending on natural selection of a particular moment. Dinosaurs can evo;ve to be larger but then smaller...teeth sharper but then more blunt...long legs to run fast but then shorter. A flatter pachy head could have been rounder.