On the presence of the genus Rhadinosuchus in the Late Middle−Early Late Triassic Chañares Formation of Northwestern Argentina.

TROTTEYN, M.J; EZCURRA, M.D.; VON BACZKO, M. B.; DESOJO, J. B.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

Proterochampsians are a group of quadruped, predatory archosauriforms usually considered as one of the potential successive sister-taxa of the crown group Archosauria. This clade is endemic to the late Middle and early Late Triassic of South America, with a total of eight nominal species recovered in the Ischigualasto-Villa Unión Basin of Argentina and the Paraná Basin of Brazil. We report here a new partial proterochampsian skeleton collected in the ?Campo El Torcido? locality of the Chañares Formation (late Ladinian?earliest Carnian). This locality is situated approximately 4 km east from the ?Chañares type? (= Far East Pocket) locality, which has yielded most of the fossil tetrapod specimens known so far from the formation. The new specimen was collected in the lower member of the unit and in older levels than those of the ?Chañares type? locality. The new specimen (CRILAR PV 488) is very well preserved and includes a right maxilla, distal end of right quadrate, partial left dentary, multiple vertebrae from all the vertebral regions, left scapula, right ulna and radius, right ischium, partial right pubis, distal half of left femur, right tibia and fibula, and possible left metatarsal V. CRILAR PV 488 was associated with an isolated cynodont right ilium (CRILAR PV 489) and a partial postcranium of Gracilisuchus stipanicicorum (CRILAR PV 490). A quantitative phylogenetic analysis found CRILAR PV 488 more closely related to Rhadinosuchus gracilis, from the late Carnian of Brazil, than to other proterochampsians because of the presence of an antorbital fossa immediately below the antorbital fenestra on the horizontal process of the maxilla. In addition, CRILAR PV 488 plus Rhadinosuchus gracilis were found as the sister taxon of Chanaresuchus bonapartei because of the presence of an anteroposteriorly broad antorbital fossa on the ventral process of the lacrimal. Unfortunately, there are only a few overlapping bones between CRILAR PV 488 and the holotype of Rhadinosuchus gracilis (e.g. maxilla, dentary, cervical vertebrae), and we were not able to recognize taxonomically informative differences between both specimens. As a result, we assign CRILAR PV 488 to Rhadinosuchus sp. The new specimen reported here improves considerably the anatomical knowledge of Rhadinosuchus. CRILAR PV 488 also adds a new faunistic component to the vertebrate assemblage of the Chañares Formation and further increases the taxonomic richness of proterochampsians in the late Ladinian?earliest Carnian of South America.