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



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