Re: Where are the basal ornithischians...

["Jaime A. Headden" <qilongia@yahoo.com>]

David Marjanovic (david.marjanovic@gmx.at) wrote:

<I wonder if *Silesaurus* could be a _really_ basal ornithischian without both
a predentary and a retroverted pubis. I still haven't compared its femora to
those of other basal dinosauriforms, though (...I guess someone else is doing
that anyway...).>

  Dzik, actually, has already compared the postcrania of *Silesaurus* and basal
dinosaurs and already indicated a dearth of ornithischian features, but more
rather features of the teeth seem to either imply affinities to non-dinosaurian
dinosauromorphs, or to non-ornithischian, non-theropod dinosaurs such as
*Thecodontosaurus*. The case for *Silesaurus* having possible affinities to
Ornithischia lies soley with the jaws, which lack a predentary but possess a
homologue in the form of a modified rostral dentary "beak". Morphology of the
teeth is said to resemble *Stagonolepis*, and whether mentioned or not, the
teeth of *Revueltosaurus,* neither of which it appears pertain to dinosaurs,
but to crurotarsans (the later is apparently a pseudosuchian per Parker et al.
[2005, _Proceedings of the Royal Society, B_ 272:963-969]). Otherwise, the
jaws, dentition, and postcrania are not at all consistent with what we
currently know of ornithischians and as such hypothetically calling it a "most
basal ornithischian" is beyond the knowledge of its anatomy as studied to date.
Dzik mentioned this at the end of his analysis. While it is likely someone will
include, is including, or has included *Silesaurus* in a rigorous phylogenetic
analysis (which hasn't already been mentioned onlist -- both Mickey and myself
did this, if I recall correctly, but neither of us explicitly testing for
affinities of the taxon), current data does not support any unambiguous support
for intra-dinosaurian relationships of *Silesaurus*.

Full references for the above:

  Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from
    the early Late Triassic of Poland. _Journal of Vertebrate Paleontology_
    23(3):556-574.

  Parker, W. G.; R. B. Irmis, S. J. Nesbitt, J. W. Martz & L. S. Browne. 2005.
    The Late Triassic pseudosuchian *Revueltosaurus callenderi* and its
    implications for the diversity of early ornithischian dinosaurs.
    _Proceedings of the Royal Society, B_ 272:963-969.

  Cheers,

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
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learn to walk soft, walk small, see the world around us rather than zoom by it.

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