Re: Silesaurus and Trialestes

["Tim Williams" <twilliams_alpha@hotmail.com>]

_Trialestes romeri_ (Reig, 1963 )
(= _Triassolestes romeri_ Reig, 1963; _Triassolestes_ preoccupied, Tillyard, 
1918 [Odonata])

PVL 2561 (holotype): Partial skull lacking the skull roof and braincase, 
portions of two forelimbs including a scapula, a humerus, a radius, ulnae, a 
radiale and an ulnare, and several cervical and 16 caudal vertebrae.  The 
forelimb elements of the holotype were associated with the skull; but Reig 
(1963) referred the forelimb to _Proterochampsa_ (a crocodylian, according 
to Reig), because he believed the forelimb to display crocodylian characters 
(elongate carpals), and the partial skull did not.  However, the forelimb 
was referred back to _Trialestes_ when it was realized that _Proterochampsa_ 
is not a crocodylomorph (Bonaparte, 1972).
PVL 2559 (referred; Reig, 1963): Partial articulated pes.  Clark et al. 
(2000) believe this represents the same animal as the holotype.
PVL 3889: (referred; Bonaparte, 1978): Partial postcranium, including 
forelimb elements (not the carpus) most of the pelvis and hindlimb, and 
several vertebrae.
All specimens come from the Ischigualasto Formation (Late Triassic; Carnian) 
of northwestern Argentina.

PVL 3889 exhibits features considered diagnostic for dinosaurs: perforated 
acetabulum with a well-developed supraacetabular crest; inturned femoral 
head that is more distinct than that in sphenosuchians; mesotarsal ankle 
joint; functionally tridactyl pes (Clark et al., 2000).

Says Clark et al. (2000): "Although the forelimbs of both specimens 
[holotype and PVL 3889] are very similar, nearly all of their similarities 
can be interpreted as plesiomorphic character-states or features that are 
found in both crocodylomorphs and basal dinosaurs."  The authors note the 
"striking" length of the forearm (radius+ulna) relative to the humerus 
(~1.15 times).  The same ratio in _Silesaurus_ is ~1.1, but no measurements 
are given for the putative carpals (Dzik, 2003).

Clark et al. (2000) propose two taxonomic dispositions for the _Trialestes_ 
material:  (1) the material represent a single taxon that combines 
dinosaurian and crocodylomorph characteristics; or (2) the specimens 
represent two different taxa, one a basal crocodylomorph (PVL 2561) and the 
other a basal dinosaur (PVL 3889), but both sharing the same forelimb 
proportions.  Jaime alluded to a third possibility: (3) the holotype is a 
chimera, composed of crocodylomorph and dinosaurian elements.  (This reminds 
me of what has been claimed by some authors for _Procompsognathus_ - part 
crocodylomorph, part dinosaur.  I don't know if this idea is still current 
for _Procompsognathus_.)



Tim

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