Unquillosaurus ceibali Powell, a giant maniraptoran (Dinosauria, Theropoda) from the Late Cretaceous of Argentina

Abstract

A reviewed anatomy and phylogenetic relationships of the Late Cretaceous theropod Unquillosaurus ceibali is presented. This taxon is represented by an isolated pubis from Los Blanquitos Formation (Maastrichtian), from the province of Salta, NW Argentina. The size of the bone (51 cm long) originally lead to interpret this theropod as a member of the large "Carnosauria". However, the pubic anatomy of Unquillosaurus is more congruent with that present in metornithine coelurosaurians, a group including Alvarezsauridae, Therizinosauroidea, Oviraptorosauria, Deinonychosauria, and Aves. Derived features present in Unquillosaurus include: opisthopubic pelvis, pubic pedicle of ilium craniocaudally wide, ventral margin of pubic pedicle strongly concave, cranial process of pubic foot short, and length of pubic foot less than 30 percent of pubic total length. Moreover, some traits shared with early avians (e.g., pubic foot proximodistally tall and craniocaudally short), suggest that this dinosaur may be more closely related to birds than suspected. The set of autapomorphic features recognized in the pubis indicates that Unquillosaurus may belong to an endemic lineage of large maniraptoran theropods.