Nose Horns

[Adam Yates <zooamy@zoo.latrobe.edu.au>]

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.

Adam Yates