Re: Patagonian Mesozoic Reptiles

[Nick <nick.gardner@gmail.com>]

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
heterodontosaurs).
> 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