Re: Rabdomorpha question

[Tim Williams <twilliams_alpha@hotmail.com>]

Jaime A. Headden wrote:

>   It also doesn't help that "Hypsilophodontia" appears to be a 
> paraphyletic grade of three or more groups of ornithopods, and some 
> "hypsilophodontians" appear to be non-ornithopods, perhaps thyreophorans or 
> more basal;

It has been long suspected that some "hypsilophodontians" are not 
ornithopods, and that some "hypsilophodontians" might really be basal 
marginocephalians or even basal thyreophorans.  But I wasn't aware that 
there was any concrete evidence for this.  To complicate matters even 
further, I know some researchers hold grave doubts concerning the monophyly 
of Marginocephalia (i.e., a Ceratopsia+Pachycephalosauria clade).

> *Heterodontosaurus* appears to be allied to Marginocephalia, but not as a
> member of it (the current -- and only? -- definition excludes it).

One possibility would be to define Heterodontosauria to include 
heterodontosaurids, ceratopsians and pachycephalosaurians (e.g., "all 
descendents of the most recent common ancestor of _Heterodontosaurus_, 
_Triceratops_, and _Pachycephalosaurus_".)  If heterodontosaurids remain in 
the Ornithopoda, then this Heterodontosauria disappears, as it becomes a 
junior synonym of Cerapoda (if Marginocephalia is monophyletic).

Cheers

Tim