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

JUAN IGNACIO CANALE; MATTIA ANTONIO BAIANO; RODOLFO ANIBAL CORIA

Neuquén

Jornada; II Jornadas de Paleovertebrados de la Cuenca Neuquina; 2019

Abelisauridae is a theropod clade with a wide distribution in the Upper Cretaceous of Gondwana. The best preserved abelisaurid record is represented by forms such as Skorpiovenator, Majungasaurus, Carnotaurus and Aucasaurus. The most distinguishable features of this family include ornamented snout bones and cranial roof; extremely reduced forelimbs; spherical humeral head; pickaxe-like lateral processes on caudal vertebrae; and axe-shaped cnemial crest. We report new abelisaurid materials found in the same quarry of a recently published abelisaurid specimen (MPCN-PV-69) including: a distal end of a unidentified neural spine; a mid-caudal vertebra; a distal part of a left metatarsal IV articulated with a proximal part of phalanx IV-1; and several phalanx of Digit IV. By reviewing that specimen and the holotypes of Skorpiovenator and Aucasaurus, we identified a new apomorphic feature that is shared with Brachyrostra, increasing the diagnosis of that clade. Abelisauroid features in MPCN-PV-69 include a triangular distal end of metatarsal IV; a phalanx IV-1 with the proximal surface dorsoventrally taller than mediolaterally wide; a ventral surface transversely wider than the dorsal one, and the presence of medial tilting. Interestingly, certain features present in the phalanx IV-1 such as a ridge that spans from the proximodorsal projection and splits in two branches that surround an obliquely oriented and oval hyper-extensor pit, is unquestionably present in all abelisaurids with a known Digit IV (except Majungasaurus), and not observed in any other theropod clade. Hence, here we propose the aforementioned combination of features as a possible synapomorphic character for the clade Brachyrostra.