A new small-sized Coelophysoid (Dinosauria: Theropoda) from the Upper Triassic Los Colorados Formation, NW Argentina

Abstract

Coelophysoids represent the oldest radiation of Neotheropoda and were broadly geographically distributed during the Late Triassic−Early Jurassic. The Triassic theropod record of South America is restricted to non-neotheropod species (e.g. Eodromaeus, probably herrerasaurids), with the exception of the medium-sized Zupaysaurus from the Los Colorados Formation and a putative coelophysoid from the Quebrada del Barro Formation. Here the South American Triassic record of neotheropods is increased with the description of a partial hindlimb (PVL 4414-1−4) collected in the 1960?s in the middle Norian Los Colorados Formation (La Rioja Province) and in association with the hypodigm of the pseudosuchian Riojasuchus. The specimen is composed of a partial femur, tibia and fibula, complete proximal and distal tarsals, and partial metatarsals II−V. PVL 4414-1−4 is recovered in a cladistic analysis as a coelophysoid coelophysid more closely related to other species (e.g. Panguraptor, Coelophysis) than to ?Syntarsus? kayentakatae based on the presence of a distal femur with a shallow extensor fossa, a tibiofibular crest more posteriorly projected than the tibial condyle and a squared anteromedial corner, tibia lacking a groove separating the proximal posterior condyles, and co-ossifi ed proximal metatarsals II−III. A unique combination of character-states distinguishes this specimen from other known theropods (e.g. Zupaysaurus), including an extensor fossa in the femur and an autapomorphic anterior margin of the calcaneum with a laterally projected and posteriorly curved flange. PVL 4414-1−4 is a very small-sized theropod (astragalar width 7 mm) and the fusion between metatarsals might indicate that it was not a very early juvenile.