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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:
> T. Michael Keesey
> >
> >Depends on which definition of _Ornithopoda_ you are following. Sereno
> >(1997) considered it a node-based clade, and anchored it on
> >_Heterodontosaurus_ and _Parasaurolophus_ (1998), but in the latest edition
> >of The Dinosauria, it is "all cerapodans closer to _Edmontosaurus_ than to
> >_Triceratops_", which explicitly excludes _Marginocephalia_.
>
> Using Butler's tree, these two definitions give an Ornithopoda that are very
> different in content.

Yes; incidentally, Butler seems to use _Ornithopoda_ in the stem-based sense.

> Yes, the traditional/classical Ornithopoda also included scelidosaurs,
> psittacosaurs and pachycephalosaurs.  So I can't see why Ornithopoda
> shouldn't be node-based and expanded to include pachies and ceratopsians,
> should the topology show that iguanodontians and marginocephalians are
> closer than previously thought.

You could say they are closer, or you could say that
heterodontosaurids are further. Six of one, half a dozen of the other.
(But I'd sooner say the latter, given the unstable position of
heterodontosaurids--read on....)

Why just marginocephalians? Why not also include thyreophorans, since
_Scelidosaurus_ adn especially _Scutellosaurus_ have been considered
ornithopodan?

Before you object, consider this quote from the abstract for Butler's
SVP talk this year: "The Heterodontosauridae form the sister group to
Genasauria and may represent the basalmost known ornithischians, a
position concordant with stratigraphy." His newer findings would
expand _Ornithopoda_ sensu Sereno to do just that! It would become a
node-based clade with nearly identical content to the stem-based
_Ornithischia_.

To me it seems like a bad idea for now to anchor any major clade on
heterodontosaurids. Better to use _Ornithopoda_ in the stem-based
sense than to have it be an unstable clade that may include
marginocephalians and even thyreophorans.
--
Mike Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com