Re: Ornithopoda + (Heterodontosauridae + Marginocephalia) phylogeny

["Tim Williams" <twilliams_alpha@hotmail.com>]

Mike Keesey wrote:

> Sereno 1998 defined it as Clade(_Heterodontosaurus_ + _Parasaurolophus_). 
> I'm
> not aware of any other formal definitions in the literature.

Thus, if heterodontosaurids+marginocephalians form a clade, then Cerapoda 
(another of Sereno's names) effectively becomes a junior synonym of 
Ornithopoda.

To further muddy the waters, the basal relationships of ornithischians are 
still being sorted out, and might produce a few surprises.  I'm looking 
forward to seeing what impact Chinese ornithopods(?) such as _Jeholosaurus_, 
_Yandusaurus_ and _Agilisaurus_ have on ornithischian phylogeny.  Not to 
mention the more recently discovered thyreophorans.

Most current phylogenies sort the majority of ornithischians into either a 
Ornithopoda+Marginocephalia clade (Cerapoda) or the Thyreophora (stegosaurs, 
ankylosaurs, and their basal kin like _Scelidosaurus_).  The only exceptions 
are those critters (e.g., _Pisanosaurus_, _Lesothosaurus_) that diverged 
before the great Cerapoda-Thyreophora split, and dwell at the very base of 
the Ornithischia.  But if in future analyses thyreophorans come out as 
closer to euornithopods (hypsilophodonts, iguanodonts, hadrosaurs), and 
heterodontosaurids are united with marginocephalians, then the Ornithopoda 
(under Sereno's definition) would include *all* ornithischians, except for 
the basalmost forms (_Pisanosaurus_, _Lesothosaurus_, etc).

> Personally, I feel these definitions should be revised. A stem-based 
> _Ornithopoda_ and a node-based _Euornithopoda_ would be very useful.

Agreed.




Tim

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