A new specimen of Velocisaurus unicus (Theropoda, Abelisauroidea) from the Paso Córdoba locality (Santonian), Río Negro, Argentina

Abstract

Abelisauroids are the most abundant theropods in the Cretaceous beds of Patagonia. They are traditionally subdivided into large-sized Abelisauridae and smaller Noasauridae. Here, we describe a new specimen of the small enigmatic abelisauroid Velocisaurus unicus Bonaparte, 1991, which was previously known from a single incomplete specimen from Neuquén City, Neuquén Province, Patagonia. The new material comes from the Santonian Bajo de la Carpa Formation (Late Cretaceous) at the Paso Córdova locality, Río Negro Province. It comprises an almost complete left hind limb and offers novel information about the anatomy of this poorly known abelisauroid. The new material shows that Velocisaurus is remarkable in having a very short, stout, and anteriorly bowed femur, which has a notably subtriangular cross-section at its proximal end. The tibia is long and slender, and the anterior surface of the distal end is anteroposteriorly flat and transversely expanded, with an enlarged surface for the ascending process of the astragalus. The pes has a stout third metatarsal, rod-like metatarsals II and IV, and highly modified phalanges of digit IV. The unique combination of characters of Velocisaurus indicates that this taxon belongs to a still poorly understood radiation of gracile-limbed abelisauroids. The inclusion of Velocisaurus in a comprehensive phylogenetic analysis recovers a monophyletic Noasauridae, but with only very weak support. Detailed analysis of features supporting the inclusion of Velocisaurus within Noasauridae is discussed, and their implications for abelisauroid phylogeny are revisited.