Re: Silesaurus and Trialestes

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

Mickey Mortimer (mickey_mortimer111@msn.com) wrote:

<Nope, I can't take credit for that.  I just reported Clark et al.'s 
findings, which had nothing to do with Silesaurus, as the taxon wasn't 
described yet.  Tim's idea is new.>

  I was refering to *Trialestes* as a dinosaur. Clark et al. don't
actually suggest *T. romeri* is a dinosaur, only suggest that referred
material from Bonaparte could be dinosaurian, as well as that the
braincase doesn't show any unambiguous crocodylomorphan synapomorphies.
While I don't have Bonaparte's work on the subject, I do have Reig (1963)
and Clark et al. (2000) and unfortunately all that is illustrated is the
foot and the lateral skull bones, minus the braincase, so this appears to
be incomparable based on just the descriptions (Reig never mentions of the
skull morphology aside from orbit shape and maxilla extent, and the narrow
compression of the jaws).

  Theropodan features of the pes include a transverse compression of the
metatarsals to one another, narrower than occurs in *Herrerasaurus* and
similar to *Eoraptor,* but not as cylindrical as in *Silesaurus.*
Metatarsals appear to be appressed to one another along their lengths.
Pedal phalanges reduce in length towards the distal ends of the digits.
There is no indication of how large the fifth metatarsal is, nor how long
the first was, as it is broken into proximal and distal ends; this is
based on PVL 2559, the refered specimen associated with the arms that bear
the elongated radiale and ulnare (Clark et al., 2000).

  The skull, on the other hand, bears an expanded symphysis dorsally with
radially-arranged teeth in a pattern seen in *Coelophysis* (incl. species
referred to *Megapnosaurus*) and *Dilophosaurus,* which underlies a
maxillary dorsal curvature similar to the "notch" of dilophosaurs; unlike
in crocodylomorphans, this curvature flattens out prior to the contact
with the premaxilla, and is further back on the jaw than at the anterior
margin, to fit a more rear-ward placement of the "caniniform" teeth. The
largest dentary tooth is the third, as in some "coelophysoids." The tooth
row does not extent as further back as under the orbit, unlike basal
crocodylomorphans or basal non neotheropodan theropods. Teeth are
apparently laterally compressed. The jugal is deep under the orbit, and a
fossa extends onto the lateral surface between caudal and dorsal processes
(quadratojugal contact and postorbital contact, respectively). The
lachrymal is preserved as a massive bar, but details are largely
unapparent in Reig's line drawing, so other conditional identifications
are weak. Sigmoidal curvature of the dentary dorsal margin, rostral
maxilla curved into a notch, and short maxillary tooth row imply
dinosaurian affinities, especially to "coelophysoid" theropods, rather
than affinities with *Silesaurus.* The pes may likely be crocodylomorphan,
as some sphenosuchians have long, appressed metatarsals with a similar
straight transverse margin of the proximal metatarsals. Elongate forelimb
material also occurs in basal crocodylomorphans, moreso in hesperosuchine
sphenosuchians, but coupled with elongated metapodials appears to resemble
the condition of *Hesperosuchus* very well ... the condition in
*Silesaurus* is unknown.

=====
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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