Prenocephale =/= Homalocephale?

[ZoPteryx]

Pachycephalosaurs were hit hard by hard by the theory of ontogenetic lumping, their diversity nearly slashed in half.  While some cases for this look pretty solid, there's is one that might not be.  Enter the lumping of flat-skulled Homalocephale into the dome-headed Prenocephale, both from the Late Cretaceous Nemegt in Mongolia.  The notion was that the smaller Homalocephale just hadn't grown into its dome yet, and thus was just a juvenile of Prenocephale.  That seemed reasonable at the time, but now a new paper has cast some doubt on this assertion.  New specimens referable to Prenocephale include an individual that was similar in size to Homalocephale, but it and all the other specimens, retained the same basic dimensions of their skull bones and arrangement of ornamentation.  Thus the authors suggests Homalocephale was a distinct taxon after all!

http://www.sciencedirect.com/science/article/pii/S0031018217306685

It's important to remember that this probably won't be the end all for this discussion, sexual differences or chronospecies could still be at play here.


Prenocephale (A), Stegoceras (B), Homalocephale (C)

[Loon]

And just when it looked like the lumpers were having their day, the pachys told them what's what!

[Halichoeres]

I seem to recall reading somewhere that a large fraction of pachycephalosaur remains consist of isolated skulls that washed into wetlands and riverbeds from upstream, and thus were pretty weathered when they fossilized. I can imagine that that process would erase a lot of phylogenetically and ontogenetically important characters.