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Re: Rabdomorpha question
Jaime A. Headden wrote:
It also doesn't help that "Hypsilophodontia" appears to be a
paraphyletic grade of three or more groups of ornithopods, and some
"hypsilophodontians" appear to be non-ornithopods, perhaps thyreophorans or
It has been long suspected that some "hypsilophodontians" are not
ornithopods, and that some "hypsilophodontians" might really be basal
marginocephalians or even basal thyreophorans. But I wasn't aware that
there was any concrete evidence for this. To complicate matters even
further, I know some researchers hold grave doubts concerning the monophyly
of Marginocephalia (i.e., a Ceratopsia+Pachycephalosauria clade).
*Heterodontosaurus* appears to be allied to Marginocephalia, but not as a
member of it (the current -- and only? -- definition excludes it).
One possibility would be to define Heterodontosauria to include
heterodontosaurids, ceratopsians and pachycephalosaurians (e.g., "all
descendents of the most recent common ancestor of _Heterodontosaurus_,
_Triceratops_, and _Pachycephalosaurus_".) If heterodontosaurids remain in
the Ornithopoda, then this Heterodontosauria disappears, as it becomes a
junior synonym of Cerapoda (if Marginocephalia is monophyletic).