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.

Neuquen

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

The Ischigualasto Formation (San Juan and La Rioja Provinces, NW Argentina, Carnian-Norian) yieldsone of the most diverse and abundant Late Triassic terrestrial tetrapod assemblages. Dinosaur andcrocodylomorph specimens from this assemblage are particularly remarkable because they represent the oldestrecords 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: fragmentaryskull, scapula, forelimb elements and a metacarpus) and a referred specimen (PVL 2559: axial and hind limbelements), both from the lower third of the formation. Later on, Bonaparte (1978, 1982) referred a third specimento Trialestes romeri (PVL 3889: based on several postcranial bones) from the middle of the unit, and he was thefirst to assign Trialestes to Crocodylomorpha. Benton and Clark (1988) suggested that Trialestes was the sistergroup of all other crocodylomorphs. Novas (1993) reassigned PVL 2559 to an indeterminate dinosaur, and morerecently, Clark et al. (2000) reiterated a crocodylomorph assignment for PVL 2561 and questioned the referral ofPVL 3889 to Trialestes.We have re-examined all three specimens previously assigned to Trialestes romeri. Regarding PVL2559, we partially agree with Novas (1993) in considering part of the specimen as a basal dinosaur, but somelarger bones can be re-assigned to an indeterminate suchian. The holotype of Trialestes is assignable toCrocodylomorpha because of the presence of a triangular deltopectoral crest, a humerus shorter than radius/ulnaand an elongated radiale and ulnare. Referred specimen PVL 3889 also exhibits apomorphies shared with basalcrocodylomorphs such as enlarged, robust, and dorsally directed prezygapophyses on cervical vertebrae, elongatecervical vertebrae, and a pre-acetabular process of ilium that extends beyond the anterior margin of the pubicpeduncle. Although these characters are present in other archosaur groups, they are only all found together inCrocodylomorpha. 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 exclusion 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 weprefer to be cautious in considering PVL 3889 an indeterminate crocodylomorph.Trialestes can be differentiated from other suchians by the following unique combination of characterstates: ulna with distinct and sharp ridge along the posterolateral border, scapular blade with roundedposterodistal corner, humerus with deltopectoral crest running along 30% of the shaft, and radiale with aproximal 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 therelationships among basal Crocodylomorpha and consequently the character polarities in the evolutionary lineageleading to Crocodyliformes.