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Re: Yandusaurus multidens in SVP2003 abstracts



Tim Williams (twilliams_alpha@hotmail.com) wrote:

<Thank you for a very interesting post.  I have seen _Y. multidens_ called
_Agilisaurus multidens_.  I would addd that further complicating the taxonomy of
this group is the status of the genus _Xiaosaurus_.>

  *Xiaosaurus* may or may not be chimaeric: the type cranial material possesses
large dentition for its vertebral size, a marginocephalian feature, and
extremely short humeri, a pachycephalosaurian feature; however, the humeri are
extraordinarily tiny, and may not correspond to the same individual; the
lindlegs as complete are pretty generic in that most "hypsi" grade
ornithopods/cerapods have similar limbs; the fibula is extremely slender,
though, another pachycephalosaurian feature, and these differ from those of
*Yandusaurus,* *Agilisaurus,* and *"Y." multidens.* The cranial material is
brief and virtually incomparable in that the material is "general" to the grade
of ornithischians sampled. It doesn't help that the supposed synonyms Dong has
created appear to clade in different places in the Genasaurian tree.

<A monophyletic Hypsilophodontidae is looking a little shaky these days, and not
just on account of the position of
_Othnielia_/_Yandusaurus_/_Agilisaurus_.  It has been suggested that some
Australian "hypsilophodontids" (_Atlascopcosaurus_, _Qantassaurus_) may be
closer to their larger compatriot _Muttaburrasaurus_, and thus represent an
endemic clade of Aussie ornithopods.>

  This is based a lot on the absence of a single, strong vertical crest on the
teeth, and large secondary ridges, forming a continuous, crenellated tooth
margin appearing as scalloped clam valves. This feature occurs in
*Muttaburasaurus,* *Tenontosaurus tillettorum* (but not *T. dossi*), *Rhabdodon*
and *Zalmoxes,* *Qantassaurus* and *Atlascopcosaurus.* *Q.* and *A.* differ only
in relative tooth count with similar size, otherwise they'd be specific
synonyms.

<I also wonder if some of the taxa thought to be fabrosaurids or basal
ornithischians might prove to be basal marginocephalians - before they "bloomed"
into the pachycephalosaurs and ceratopsians.>

  Well, they would have to lack possible thyreophoran features as well.

  Cheers,

  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)