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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 more basal;

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).