First report of a derived abelisaurid theropod from the Bajo de la Carpa Formation (Late Cretaceous), Patagonia, Argentina

Abstract

An isolated anterior caudal vertebra of a medium-large sized theropod dinosaur is reported here. It comes from the Bajo de la Carpa Formation (Santonian, Late Cretaceous), Río Colorado Subgroup, Río Negro Province, northern Patagonia, Argentina. The specimen, MACN-PV-RN 1012, exhibits several derived characters that are diagnostic among theropods. Specifically, MACN-PV-RN 1012 is referable to Abelisauroidea because it possesses a centrodiapophyseal lamina on the ventral surface of the transverse processes, and to Abelisauridae due to the presence of transverse processes that are 1.4 times longer than the anteroposterior length of the centrum, a large median depression anterior to the base of the neural spine, and the lack of a prespinal fossa. MACN-PV-RN 1012 is included within Carnotaurinae, sharing with Carnotaurus and Aucasaurus transverse processes that are oriented dorsolaterally at an angle of 45° or more with regard to the horizontal axis of the vertebra. Thus, the specimen described here constitutes the first record of a derived abelisaurid from the Bajo de la Carpa Formation.