New abelisaurid material from the Anacleto Formation (Campanian, Upper Cretaceous) of Patagonia, Argentina, shed light on the diagnosis of the Abelisauridae (Theropoda, Ceratosauria)

Abstract

Abelisauridae is a theropod clade with a wide distribution in the Late Cretaceous of Gondwana. Some of the best preserved abelisaurid specimens were recovered from Patagonia (Argentina) such as Skorpiovenator, Ilokelesia, Carnotaurus and Aucasaurus. Here we describe a dorsal part of a neural spine; a middle caudal vertebra; a distal part of a left metatarsal IV; a complete right phalanx IV-1; left phalanges IV-1, 2 and 3; and a pedal ungual phalanx. These materials were recovered from the same quarry of a recently published indeterminate abelisaurid specimen (MPCN-PV-69). The most distinctive characters are a triangular shape of the distal end of metatarsal IV (present in other abelisauroids); phalanx IV-1 with the proximal surface dorsoventrally tall and the ventral surface wider than the dorsal one causing a medial tilting of bone (set of features considered autapomorphic in Velocisaurus, although is also present in different abelisaurids); phalanx IV-1 and 2 with a ridge which spans from the proximodorsal projection, splits in two branch and surrounds a laterally displaced and obliquely oriented oval hyperextensor pit; a pedal ungual phalanx with two medial and lateral vascular grooves, and lacking a flexor tubercle (abelisauroids synapomorphies). The mentioned feature of phalanges IV-1 and 2 is only found among abelisaurids and is here considered as a possible new synapomorphy of Abelisauridae. Thus, besides they constitute new abelisaurid remains from Patagonia, the new materials provide valuable morphological data that could expand the diagnosis of Abelisauridae.