RE: Patagonian Mesozoic Reptiles

[Tim Williams <twilliams_alpha@hotmail.com>]

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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