A review of the archosaur Trialestes romeri (Suchia: Crocodylomorpha) from the Upper Triassic Ischigualasto Formation of Argentina

EZCURRA, M.D.; LECUONA, A.; IRMIS, R.B.

Neuquén

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Universidad Nacional del Comahue

The Ischigualasto Formation (San Juan and La Rioja Provinces, NW Argentina, Carnian-Norian) yields on of the most diverse and abundant Late Triassic terrestrial tetrapod assemblages. Dinosaur and crocodylomorph specimens from this assemblage are particularly remarkable because they represent the oldest records of their respective clades. Trialestes romeri is the only putative crocodylomorph known from this unit. Reig (1963) originally considered Trialestes a theropod dinosaur, based on the holotype (PVL 2561: fragmentary skull, scapula, forelimb elements and a metacarpus) and a referred specimen (PVL 2559: axial and hind limb elements), both from the lower third of the formation. Later on, Bonaparte (1978, 1982) referred a third specimen to Trialestes romeri (PVL 3889: based on several postcranial bones) from the middle of the unit, and he was the first to assign Trialestes to Crocodylomorpha. Benton and Clark (1988) suggested that Trialestes was the sister group of all other crocodylomorphs. Novas (1993) reassigned PVL 2559 to an indeterminate dinosaur, and more recently, Clark et al. (2000) reiterated a crocodylomorph assignment for PVL 2561 and questioned the referral of PVL 3889 to Trialestes. We have re-examined all three specimens previously assigned to Trialestes romeri. Regarding PVL 2559, we partially agree with Novas (1993) in considering part of the specimen as a basal dinosaur, but some larger bones can be re-assigned to an indeterminate suchian. The holotype of Trialestes is assignable to Crocodylomorpha because of the presence of a triangular deltopectoral crest, a humerus shorter than radius/ulna and an elongated radiale and ulnare. Referred specimen PVL 3889 also exhibits apomorphies shared with basal crocodylomorphs such as enlarged, robust, and dorsally directed prezygapophyses on cervical vertebrae, elongate cervical vertebrae, and a pre-acetabular process of ilium that extends beyond the anterior margin of the pubic peduncle. Although these characters are present in other archosaur groups, they are only all found together in Crocodylomorpha. Though elements shared by PVL 2561 and in PVL 3889 (scapulae, humeri, ulnae, and radi) are similarly shaped, they do not share any apomorphies to the exclusión of other basal crocodylomorphs. Thus, on the basis of the available bones, there is no evidence that PVL 3889 can be referred to Trialestes, and we prefer to be cautious in considering PVL 3889 an indeterminate crocodylomorph.Trialestes can be differentiated from other suchians by the following unique combination of character states: ulna with distinct and sharp ridge along the posterolateral border, scapular blade with rounded posterodistal corner, humerus with deltopectoral crest running along 30% of the shaft, and radiale with a proximal width only two times greater than its length. Trialestes romeri and PVL 3889 are the geologically oldest unambiguous crocodylomorph specimens. The complete and detailed reappraisal of the anatomy of these early crocodylomorphs helps elucidate the relationships among basal Crocodylomorpha and consequently the character polarities in the evolutionary lineage leading to Crocodyliformes.