A NEW MEDIUM-SIZED BASAL CROCODYLOMORPH WITH A LIGHTLY BUILT AXIAL SKELETON FROM THE LATE TRIASSIC ISCHIGUALASTO FORMATION, SAN JUAN, ARGENTINA

EZCURRA, M. D.; DESOJO, J. B.; NOVAS, F. E.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontologia de Verterbados; 2011

Crocodylomorpha is the only pseudosuchian clade with extant representatives and their oldest known members are the small-sized Trialestes romeri Reig 1963 and Barberenasuchus brasiliensis Mattar 1987 from the early Late Triassic (late Carnian-earliest Norian) of the South American Ischigualasto and Santa María formations, respectively. Both species are based on poorly preserved materials and several aspects about the early evolution of the group remain enigmatic. We describe a new early crocodylomorph species represented by two specimens coming from the Cancha de Bochas Member of the Ischigualasto Formation, Argentina. The more complete and smaller specimen (PVSJ 846) is represented by several cranial and postcranial bones, whereas the larger specimen (PVSJ 890) corresponds to an isolated posterior cervical vertebra with a maximum height of 13.2 cm. Both specimens are assigned to the same species due to a unique combination of features associated with a lightly built axial skeleton, including the presence of a very deep fossa occupying most of the lateral surface of medium-posterior cervical centra. A preliminar phylogenetic analysis based on the matrix of Nesbitt (2011) nested the new species within Crocodylomorpha and as the sister-taxon of Trialestes, but outside a paraphyletic ?Sphenosuchia?. The synapomorphies that support its assignment to Crocodylomorpha are maxillary palatal processes forming a secondary bony plate and ilium with well-developed preacetabular process and a concave acetabular ventral margin, among others. The new species is found more closely related to Trialestes than to other suchians due to the presence of a hypertrophied sub-acromial process and elongated anterior cervical vertebrae. However, the new taxon differs from Trialestes in several aspects, including a longer anterior maxillary ramus and scapula with a large process above the glenoid fossa. The most conspicuous features of the new crocodylomorph are related with a lightly built cervical series, absent in Trialestes and other crocodylomorphs. The anterior cervical vertebrae present a deep and blind longitudinal fossa dorsolateral to the posterior exit of the neural canal and a deep longitudinal depression along the lateroventral border of the neural arch, which opens into a dorsally oriented blind fossa. The posterior cervical vertebrae present a series of thin laminae in the neural arch and a large and very deep lateral fossa that strongly compresses the centrum. These features resemble the condition of basal theropods and poposauroids (Nesbitt, 2007). However, we could not found unambiguous evidence for the presence of postcranial pneumaticity (O?Connor, 2006), but it constitutes a plausible explanation for the origin of these structures. The new species documents a novel axial morphology present in Crocodylomorpha, represents one of the largest known non-crocodyliform crocodylomorphs, and enlarges our knowledge about the early evolution of the group.