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Re: Stormbergia dangershoeki, new Early Jurassic ornithischian from South Africa



On 10/9/05, Tim Williams <twilliams_alpha@hotmail.com> wrote:
>
> I'd only include marginocephalians in the Ornithopoda *if* they are found to
> form a clade with euornithopods to the exclusion of heterodontosaurids and
> thyreophorans.

Since _Euornithopoda_ is defined as clade(_Parasaurolophus_ not
_Heterodontosaurus_), Butler's findings would render marginocephalians
and possibly thyreophorans as euornithopods(!)

> Here's my problem with the current stem-based definition of Ornithopoda...
> What if current euornithopodan taxa ("hypsilophodontids", iguanodontians)
> form a paraphyletic array between heterodontosaurids and marginocephalians.
> If you look at Fig. 25 of Butler (2005), this would not require a major
> shift in topology.

Yes, but that's a ... septachotomy? The lack of resolution there
indicates he was not looking closely at relationships within that
clade.

Still, your point remains, I suppose.

> I agree that it is a bad idea to anchor any clade on Heterodontosauridae,
> including the clade Ornithopoda.  However, I think it is unwise to define
> Ornithopoda such that it explicitly excludes _Triceratops_, given the
> instability of Marginocephalia in current phylogenies.  Of course, good
> material from basal marginocephalians could resolve this situation.  But
> what if these basal marginocephalians show that pachies and ceratopsians
> arose from basal (eu)ornithopods.  I think any definition of Ornithopoda
> (either stem- or node-based) should allow for this possibility.

I guess the real question is which taxa you consider to be
"essentially" ornithopodan. A node-based definition of something like
clade(_Hypsilophodon_ + _Iguanodon_) for either "Ornithopoda" or
"Euornithopoda" might be more aesthetically pleasing, and allow for an
outside possibility of including marginocephalians.

But this is probably about as far as the discussion can go until
further studies are done. Looking forward to Butler's talk!
--
Mike Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com