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Re: Patagonian Mesozoic Reptiles

On 7/11/07, Tim Williams wrote:
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.

Indeed. The authors don't give any support though for why they favor
an ornithopod position for heterodontosaurids (and consequently
_Pisanosaurus_ as well due to their inclusion of it  within the

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.

It's a shame that this remains untested in published works thus far.

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.

One gets the impression from the relevant portions of the text, that "Bayosaurus" was intended to have been described in Coria's chapter in this volume, but was apparently omitted.

Nick Gardner