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On Tue, 23 Jan 1996, Stan Friesen wrote:
> > series. Also, how similar is the Pachyrhino nose bump to the new Two
> > Medicine ceratopsian ...
> Quite similar, actually. These two forms make up a transitional
> series with between the more typical Centrosaurus and Pachyrhinosaurus.
> >From these skulls it is clear that Pachy's horn is derived from
> a Cnetrosaurus horn by doubling over.
This is not clear at all. It is true that Einiosaurus and Achelosaurus
represent serially closer outgroups to Pachyrhinosaurus. This however
does not necessarily imply an anagenetic evolutionary lineage. Indeed in
the description of Einiosaurus, Scott Sampson suggested the strongly
procurved nasal horn of adults was an autapomorphy of the genus.There are
indications that a slightly older population (= Brachyceratops??) had an
erect nasal horn. This could be the ancestor of both Einiosaurus and the
Achelousaurus-Pachyrhinosaurus clade. As far as I know the bosses of
Achelousaurus and Pachyrhinosaurus could still be support enlarged horns
that would simply be too heavy if they retained the primitive bony core.