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

[Tim Williams <twilliams_alpha@hotmail.com>]

T. Michael Keesey wrote:

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

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.

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.  Under this scenario, with the current stem-based 
definition, the Ornithopoda would only include iguanodontians.  This is 
because Ornithopoda is defined as "all cerapodans closer to _Edmontosaurus_ 
than to _Triceratops_", so Ornithopoda cannot include ceratopsians.  Future 
analyses may indeed recover the Marginocephalia as the sister taxon to 
Iguanodontia, which would suggest that marginocephalians evolved quite late 
in ornithischian evolution (this also would accord with the non-existent 
pre-Tithonian record of marginocephalians).

> 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_.

I think we can reach a compromise in which (a) heterodontosaurids are not 
used in any definition of Ornithopoda, and (b) Ornithopoda is defined such 
that allows for the possibility that marginocephalians (either once, or 
pachies and ceratopsians separately) evolved from basal ornithopods, or even 
basal euornithopods.

> 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.

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.

Cheers

Tim