[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
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
I'd only include marginocephalians in the Ornithopoda *if* they are found to
form a clade with euornithopods to the exclusion of heterodontosaurids and
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
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
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.