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Re: Revueltosaurus not a dinosaur

Ben Creisler (bh480@scn.org) reported:

<William G. Parker , Randall B. Irmis, Sterling J. Nesbitt , Jeffrey W. Martz ,
Lori S. Browne. 
  The Late Triassic pseudosuchian Revueltosaurus callenderi and its
implications for the diversity of early ornithischian dinosaurs
  A new discovery of skeletons of Revueltosaurus callenderi from the Upper
Triassic Chinle Formation of Petrified Forest National Park, Arizona clearly
shows that Revueltosaurus is not an ornithischian dinosaur as previously
supposed. Features such as the presence of a postfrontal, crocodile-normal
ankle and paramedian osteoderms with anterior bars place R. callenderi within
the Pseudosuchia, closer to crocodylomorphs than to dinosaurs. Therefore,
dental characters previously used to place Revueltosaurus within the
Ornithischia evolved convergently among other archosaur taxa, and cannot be
used to diagnose ornithischian dinosaur teeth. As a result, all other putative
North American Late Triassic ornithischians, which are all based exclusively on
teeth, are cast into doubt. The only reasonably well-confirmed Late Triassic
ornithischians worldwide are Pisanosaurus mertii and an unnamed
heterodontosaurid from Argentina. This considerably changes the understanding
of early dinosaur diversity, distribution and evolution in the Late Triassic.>

  Well, though I should hold off on commenting until I see the published paper
itself, I am glad to see this abstract from SVP published, given the
interesting skull restored in their poster.

  My big major comment here is not to just hold off on confirming ornithischian
status, but to call into question the rather broad means by which the original
authors attributed the original type teeth as ornithischian as they in fact had
very, very few universal basal ornithischian characters of the dentition, save
for "swollen crown" or "coarse dentition" which implied ornithischian identity
in the original paper (they even used a question mark) and was later "affirmed"
as wholly ornithischian. This shows a need to narrow criteria for assigning
taxa. Well ... wait and see. The skull is a good one, and I can't wait to see
the paper.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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