Taxonomy of basal dinosauriforms: Evidence provided by a new specimen from the Triassic Chañares Formation, NW Argentina

NOVAS, F. E.; AGNOLIN, F. L.; EZCURRA, M. D.

Colonia

Congreso; V Congreso Latinoamericano de Paleontología de Vertebrados; 2015

The Middle-Late Triassic (Ladinian-earliest Carnian) Chañares Formation, cropping out at theTalampaya National Park, La Rioja Province, NW Argentina, has yielded the remains of a varietyof early dinosauriforms. Among them, Lewisuchus and Pseudolagosuchus are known by specimenswhich overlap by a single tibia. Although the knowledge of their anatomy is far from satisfactory,two main hypotheses gained acceptance in recent years: 1) Pseudolagosuchus is a junior synonymof Lewisuchus; and 2) it belongs to Silesauridae. Field exploration resulted in the discovery of aspecimen represented by articulated left postorbital, squamosal, and quadrate, an articulated seriesof cervical and dorsal vertebrae, pelvic girdle, and partial left hindlimb (lacking foot). Similar toLewisuchus, the postorbital possesses a well-developed and strongly upturned frontal process, thecervical and anterior dorsal vertebrae show a well-developed system of laminae below thediapophyses, and the vertebral centra are deeply excavated laterally. Besides, the femur, tibia andastragalus of the new dinosauriform specimen match well the morphology described forPseudolagosuchus. Information provided by the new dinosauriform specimen lends support to thesynonymy of Pseudolagosuchus with Lewisuchus, but also sheds light about the anatomicalcharacteristics of pelvic elements and dorsal vertebrae of Lewisuchus: the iliac portion ofacetabulum is medially closed, the ischium and pubis are elongate, representing at least 2/3 offemur length, and the puboischiadic lamina is dorventrally deep below the acetabulum. The dorsalneural spines are fan-shaped, and exhibit a spine table. This peculiar morphology is present inposterior cervicals up to posterior dorsals, probably representing a diagnostic feature of Lewisuchus.No signs of dermal scutes are documented in the new specimen. Furthermore, the specimen lacks afibular crest on the tibia, it shows poorly developed transverse groove on proximal articular surfaceof femur, and the femoral head lacks a deep ventral notch. In these features the new specimendiffers from Silesaurus and its kin, and consequently, weakens previous hypotheses sustainingsilesaurid affinities for Lewisuchs. Known members of Silesauridae possess multiple apomorphiesthat obscure the reconstruction of the plesiomorphic states of the nodes immediately outsideDinosauria. As a result, the dinosauriforms from the Chañares Formation fill a morphological gapbetween lagerpetids and silesaurids/dinosaurs. Therefore, the improved understanding of theanatomy and taxonomy of the Chañares dinosauriforms that is provided by this new specimen willbe very important to shed light on the phylogenetic relationships among non-dinosaurdinosauriforms and the dawn of dinosaur evolution.