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RE: Patagonian Mesozoic Reptiles



Nick Gardner wrote:

- Coria and Cambiaso include _Pisanosaurus_ with heterodontosaurids
(which they consider basal ornithopods) while covering the Laguna
Colorada heterodontosaurid (_Heterodontosaurus_ sp.).

If _Pisanosaurus_ is a bona fide ornithopod (as opposed to a basal ornithischian, which is what most analyses find), then the Ornithopoda goes all the way back to the Late Triassic. So too would the Marginocephalia (as sister group to Ornithopoda) and Thyreophora (as sister group to Cerapoda). This requires a very deep-branching Ornithischia - even if marginocephalians evolved later (such as within the Ornithopoda or Heterodontosauriformes).


At the other extreme, wre have the ornithischian phylogeny of Butler et al. (the _Eocursor_ paper), which recovers a much younger Ornithopoda which may not have appeared until the Middle Jurassic. Under this phylogeny, taxa such as _Pisanosaurus_, _Eocursor_, Lesothosaurus_, _Stormbergia_, and heterodontosaurids are either basal ornithischians or basal genasaurs. Further, some traditional "hypsilophodontid-grade ornithopods" (e.g., _Agilisaurus_, _Hexinlusaurus_, _Othnielosaurus_) are found to lie outside the Ornithopoda, as non-cerapodan neornithischians.

The nice thing about the Butler &c hypothesis is that it gels with the very poor pre-mid-Jurassic record of ornithischians, especially given that many alleged Late Triassic ornithischians have been 'outed' as non-dinosaurian archosauriforms. (Though maybe the 'silesaurs' will one day turn out to be true ornithischians.) It also accords with the complete absence of verifiable marginocephalians before the Late Jurassic.

They predict that SAm
ornithopods (_Notohypsilophodon_, _Gasparinisaura_, _Anabisetia_,
_Talenkauen_) will fall out close together, but don't provide any
support for this.

This reminds of a similar idea regarding some Aussie ornithopods. It's been suggested that _Atlascopcosaurus_, _Qantassaurus_ and _Muttaburrasaurus_ may constitute an endemic clade of Australian ornithopods.


Gasparini et al's chapter on faunal succession mentions _Bayosaurus
pubica_ and references Coria's chapter in this volume, which mentions nothing of it. Apparently it is a new abelisauroid from the late
Cenomanian-early Turonian of Patagonia. No material was figured or
assigned in the book.

The name "Bayosaurus" was mentioned in...

Currie, Philip J., Carabajal, Ariana P., and Coria, Rodolfo A. (2006) A new abelisauroid theropod from northwestern Patagonia. Canadian Journal of Earth Sciences 43: 1283-1289.

However, the name only appeared in a figure, not in the text. So, if it's not described in the "Patagonian Mesozoic Reptiles" volume, it's still a nomen nudum.

Cheers

Tim

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