A medium-sized basal abelisauroid with well-developed manus from the Late Cretaceous of southern Patagonia

EZCURRA, M. D.; AGNOLIN, F. L.; NOVAS, F. E.

San Luis

Jornada; XXV JORNADAS ARGENTINAS DE PALEONTOLOGÍA DE VERTEBRADOS; 2010

Universidad Nacional de San Luis

Late Cretaceous Gondwanan outcrops provide abundant abelisauroid theropods. Medium- to large-sized members of thegroup represent them as characterized by reduced forelimbs supplied with highly atrophied hands (e.g. CarnotaurusBonaparte, Aucasaurus Coria et al., Majungasaurus Lavocat). This panorama changes with the discovery of a new mediumsizedbasal abelisauroid (MPM-PV 10003) coming from the latest Cretaceous (Maastrichtian) Pari Aike Formation, SantaCruz Province, Argentina. MPM-PV 10003 preserves a metacarpal III, manual non-ungual phalanx, mid-caudal vertebrae,metatarsals II and IV, distal end of tibia, and pedal phalanges. Although fragmentary, the new specimen preserves an autapomorphicmetacarpal III with a dorso-ventrally compressed shaft and posteriorly displaced collateral fossae, and pedalphalanges with a conspicuous longitudinal crest delimitating the dorsal margin of the collateral fossae, allowing distinctionof MPM-PV 10003 from other neotheropods. Cladistic analysis recovered the new species as more derived thanCeratosaurus Marsh and Berberosaurus Allain et al., but within a polytomy at the base of Abelisauroidea. This assignmentis supported by synapomorphies (distal end of tibia with a vertical facet for reception of the astragalar ascending processand a scar of the ascending process bearing a median vertical ridge). The manus of MPM-PV 10003 presents a metacarpalwith a long shaft and well-developed distal pulley and a phalanx with well-developed distal articulation. Informationprovided by MPM-PV 10003 suggests that the strong reduction of the forelimb recorded in derived abelisaurids is not directlycorrelated with body-size increase; it seems to be an evolutionary event exclusive of this lineage withinCeratosauria.