RE: II CLPV talk summaries: Day 2

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

Tim Williams (twilliams_alpha@hotmail.com) wrote:

<Doesn't all this make you want to have another look at _Pisanosaurus_?  ;-)>

  While the idea of silesaurids on many continents is very intruiging, if true,
or based on a grade of taxa preceding the origin of Dinosauria itself but
postdating the divergence of "lagosuchids" and *Pseudolagosuchus* (which might
be *Lewisuchus*, which might not even be [and probably isn't] dinosaurian
itself), I find that the term should be applied with regards to shared derived
characters, which to date given only *Silesaurus* may be unknown. That said,
features of a basal dinosaurian with buccal emargination, waisted and vaguely
leaf-like teeth, a procumbent pubis that curves ventrally at the puboischiadic
plate, and other features such as a lateral ridge on the maxilla, are known
among MANY dinosauromorphans, including, *Eoraptor*, "*Thecodontosaurus*", the
Isalo II maxilla from Madagascar which Flynn et al. (2005) recently regard only
as an "archosaur", and probably several other taxa I'm not willing to mention
just yet. *Silesaurus* lacks this character.

  That said, evidence for *Pisanosaurus* as a dinosaur, much less an
ornithischians, appears to be based on a few features:

  1. "Twisted" tibia with cranial distal notch for receiving the ascending
     process of the astragalus (Weishampel & Witmer, 1990, _the Dinosauria_);

  As an ornithischian:

  2. Five sacral vertebrae (Bonaparte, 1976);
  3. Maxilla and dentary with buccal emargination and coincident lateral
laminae
     demarcating thses emarginations (Weishampel & Witmer, 1990);
  4. Maxillary and dentary teeth symmetrical (Weishampel & Witmer, 1990);
  5. Maxillary and dentary teeth with a constriction between root and crown (or
     waisting) (Weishampel & Witmer, 1990);
  6. Largest teeth in middle of tooth row (Weishampel & Witmer, 1990);
  7. Posterior margin of the dentary turned upward into a coronoid eminence
     (Weishampel & Witmer, 1990);

  As a non-genasaur:

  8. Medial metatarsals overlap lateral metatarsals (Weishampel & Witmer,
1990);
  9. Low coronoid eminence (Weishampel & Witmer, 1990);
  10. Distal tibial notch positioned laterally near calcaneal facet (Weishampel
     & Witmer, 1990);

  As a heterodontosaur(?):

  11. Extensive wear of the maxillary and dentary teeth [as in
     heterodontosaurids] (Weishampel & Witmer, 1990; Sereno, 1991).

  Of these features, *Pisanosaurus* shares with *Silesaurus* 1, 3-6 & 8. Six
out of eleven ain't bad, but it's too close to half for my comfort. The
anterior dentary is apparently intact as in said to bear a predentary structure
(which is not present (Casamiquela, 1967), but may be broken and could very
well exhibit a "silesaurid" rostral hook just as easily; indeed the same
evidence in *Silesaurus* can claim that, if a fracture on the rostrum of the
dentary (Dzik, 2003, fig. 5E-F) is complete mediolaterally, the dentary would
look almost identical in structure to *Pisanosaurus*. The maxillary fragment
tells us nothing about the antorbital region. Margins of the mandibular bones
are indistinct and largely obliterated. As noted by Sereno (1991), the jaws
seem to be MORE derived than *Lesothosaurus* is bearing emarginations of the
maxilla and dentary with elongated laminae demarcating them and extensive wear
of the teeth (which is found in marginocephalians and ornithopods). A possibly
very large articular may account for the large ridge on the lateral margin of
the angular/surangular/articular contact region. Unlike *Silesaurus*, Bonaparte
identified three cervicals with elongated prezygapophyses, very short
postzygapophyses, and lateral fossae (pleurofossae, sensu Tidwell and
Carpenter, in press) on he centra. The apex of the ascending process is
laterally convergent with the lateral tibial margin rather than more medial, as
in *Technosaurus*, but unlike *Silesaurus* in which the ascending process is
more medially located. So it's at least possible it's more primitive than the
earliest otherwise known ornithischian *Lesothosaurus* and thus may not be
ornithischian, or even dinosaurian. Then again, these features may diagnose
Dinosauria if *Silesaurus* is a basal member allied to ornithischians. 

<I know Jalil and Knoll (2002) put _Azendohsaurus_ outside the Dinosauria, but
do they (or Parrish) say what it might be.>

  I know that teeth extend nearly to the incomplete rostral tip of the
dentaries and that there wouldn't be much room for an articulating predentary
as a consequence, thus may not be ornithischian. The teeth exhibit a single
strong lateral ridge and relatively coarse denticulation with a constriction
and are very reminiscent of prosauropod jaws. There are no interdental plates,
and the meckelian groove is shallow. Despite figures and labels, there is no
evidence for a coronoid eminence. The teeth are otherwise very similar to
prosauropods. Presence of a cingulate-like structure on the crown above the
crown/root constriction is particular to ornithischians and the crowns are
apparently very much mediolaterally compressed.

  Anyways, I only have Jalil, 1999, on the subject of Algerian tetrapods. Is it
possible there is a copy of Jalil & Knoll (2002) that can be sent my way? Yes,
trolling for treasure.

<_Alwalkeria_ and _Protoavis_ may contain dinosaurian (theropod?) elements; but
as the hypodigms for both do appear to be chimeric, I'm not sure which
particular element gets to carry the name of the genus.  I fits the cranial 
material, then neither genera would appear to be dinosaurs.>

  In the case of Chatterjee, the data and material he uses to "set" his cases
should carry the names should they prove chimaeric. The paratype, rather than
the holotype, gives much of the more robust dinosaurian features of the
braincase, whereas the holotype skull belongs it seems to a drepanosaurid. This
means we may get *Protoavis* attached to a non-dinosaur avecephalan, which may
suit the "Rubenites" very well. In the case of a single specimen of collected
bones (*Alwalkeria*) I have a hard time seeing ANY of it as a dinosaur, and the
whole animal may end up crurotarsan. This includes the vaguely-coelophysoid
astragalus, which possesses an intricate but apparently croc-normal type ankle.
Elongation of the dentary symphysis and lack of dental serrae on the conical
teeth also appear to argue the skull is non-dinosaurian but crocodyliform. The
vertebrae, with elongated but apparently typical cervicals, are seen in
"poposaurid"/"rauisuchid"/"teratosaurid" rauisuchians (incl. *Postosuchus*).

<_Barapasaurus_ was named and described by Jain, Kutty, Roy-Chowdhury and
Chatterjee (1975).  It appears to be safe within the Dinosauria.  :-)>

  Why can't it be the rest of the plesiosaurian skeleton of *Dravidosaurus*?
Come on, Tim, use your imagination ;).

  Cheers,

Jaime A. Headden

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


                
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