[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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




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

_________________________________________________________________
A place for moms to take a break! http://www.reallivemoms.com?ocid=TXT_TAGHM&loc=us