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

On 10/8/05, Tim Williams <twilliams_alpha@hotmail.com> wrote:
> Michael Mortimer wrote:
> >      `--+--Lesothosaurus
> >         |--SAM-PK-K8025
> >         `--+--Stormbergia
> >            `--+--Agilisaurus
> >               `--+--"Yandusaurus" multidens
> >                  `--+--+--Heterodontosaurus
> >                     |  `--Abrictosaurus
> >                     `--+--Jeholosaurus
> >                        |--Hypsilophodon
> >                        |--Gasparinisaura
> >                        |--Thescelosaurus
> >                        |--Parksosaurus
> >                        |--Iguanodontia
> >                        `--+--Pachycephalosauria
> >                           `--Ceratopsia
> >
> >Genasaurian (and neornithischian) Lesothosaurus, non-cerapodan
> >heterodontosaurids...
> ... and ceratopsians and pachycephalosaurs as derived ornithopods.  At
> least, that's how I read Butler's tree.

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_ (or at very least one genus therein).

Note that in phylogenies where _Heterodontosauridae_ is the sister
group to _Marginocephalia_, Sereno's definition also includes
_Marginocephalia_ in _Ornithopoda_. However, the topology found by
Sereno excluded it, so I don't think he meant for this.

"Classic" Ornithopoda is a massively paraphyletic assemblage roughly
equivalent to "unarmored _Ornithischia_" or "bipedal _Ornithischia_".
In adapting it to cladistic taxonomy, there seems to be a consistent
desire to restrict it to a particular lineage (a la _Amphibia_) rather
than expand it to include descendants (a la _Dinosauria_), probably
because the very popular term "Ornithischia" is already available for
the expanded group (not to mention "Predentata").

My feeling is that, given that all three possible arrangements of
heterodontosaurids, marginocephalians, and iguanodonts +
hypsilophodonts have been found by different studies, the stem-based
definition is more congruent with the modern concept of _Ornithopoda_
than the node-based definition, even though the node-based definition
was published earlier.
Mike Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com