Trialestes revisited (was RE: something's wrong here: Qianosuchus phylogeny)

[Tim Williams <twilliams_alpha@hotmail.com>]

Mickey Mortimer wrote:

> And yes, Triassolestes, Turfanosuchus and Pseudohesperosuchus were 
> included.  Crurotarsan phylogeny was quite odd though, and more characters 
> were needed for so many taxa.

_Trialestes_/_Triassolestes_ is a potentially tricky taxon, on account of 
the hypodigm.  The excerpt below is from Clark et al. (2000).  It's long, 
but it does lay out an argument for possible dinosaurian affinities for at 
least part of the _Trialestes_ hypodigm.  I wonder what your phylogenetic 
analysis found, Mickey.

Clark et al. (2000):

   "This taxon [_Trialestes romeri_] was founded by Reig (1963, under the 
preoccupied generic nomen _Triassolestes_, for which Bonaparte [1982] 
proposed the replacement name _Trialestes_) on the basis of two specimens, 
the holotype (PVL 2561) and a referred specimen (PVL 2559), from the Upper 
Triassic (Carnian) Ischigualasto Formation of northwestern Argentina. PVL 
2561 consists of a 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. PVL 
2559 now comprises a partial articulated pes. Reig (1963) provided 
measurements for a pubis, astragalus, and cervical and sacral vertebrae of 
this specimen, but JMC could not locate these elements in the collections of 
the Instituto Miguel Lillo in 1985. The forelimb elements of the holotype 
were associated with the skull. However, because the carpals are elongate, 
Reig considered it unlikely that a skull devoid of crocodylian characters 
would be associated with a forelimb exhibiting such a characteristic 
crocodylian feature. Instead, because he interpreted _Proterochampsa_ from 
the same horizon as a crocodylian (Reig, 1959; see also Sill, 1967), he 
referred the limbs to that taxon (Reig, 1963:15). With the realization that 
_Proterochampsa_ was not closely related to Crocodylomorpha (Walker, 1968), 
there is no reason that the limbs should not be referred to the same animal 
as the skull of _Trialestes_ (Bonaparte, 1972). A third specimen from the 
same horizon (PVL 3889) was subsequently referred to this taxon by Bonaparte 
(1978). It includes portions of the forelimb other than the carpus, most of 
the pelvis and hind-limb, and several vertebrae. The pelvis and hindlimb 
have features considered diagnostic for dinosaurs, including a perforated 
acetabulum with a well-developed supraacetabular crest, an inturned femoral 
head that is more distinct than that in sphenosuchians, a mesotarsal ankle 
joint and a functionally tridactyl pes. The vertebral centra have excavated 
lateral surfaces.

  "Although the forelimbs of both specimens 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. 
However, the limb proportions are indeed striking and deserve attention. In 
particular, the great length of the radius and ulna relative to the humerus 
is very unusual. The radius and ulna are significantly longer than the 
humerus in both specimens (1.15 times in PVL 2559). In no other basal 
archosaur known to us are the radius and ulna significantly longer than the 
humerus. Either the two specimens represent two different taxa, one a basal 
crocodylomorph and the other a basal dinosaur, but both converging in 
forelimb structure, or the two specimens represent the same taxon, which 
combines dinosaurian and crocodylomorph characteristics. Considering the 
latter possibility, it is not clear that this taxon would be referable to 
the Crocodylomorpha. The only apparent crocodylomorph feature is the 
elongation of the radiale and ulnare, whereas at least four different 
dinosaur-like features are present in the hind-limb and vertebrae. The 
simplest solution
would be to refer this taxon to the Dinosauria and to interpret the elongate 
radiate and ulnare as the result of convergent evolution. However, if the 
specimens actually represent two different taxa, then PVL 2561 is a basal 
crocodylomorph and PVL 3889 a dinosaur. The problem posed by these two 
specimens is intriguing, but its resolution must await the discovery of 
additional material including both the carpus and hind-limb."



Reference

Clark, J. M., Sues, H.-D. & Berman, D. S., (2000)  A new specimen of 
_Hesperosuchus agilis_ from the Upper Triassic of New Mexico and the 
interrelationships of basal crocodylomorph archosaurs.
Journal of Vertebrate Paleontology 20: 683-704.



Cheers

Tim

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