Anatomy ofPatagonykus puertai(Theropoda, Avialae, Alvarezsauridae), from the Late Cretaceous of Patagonia

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Abstract
The fragmentary skeleton of a new Late Cretaceous avialan theropod from Patagonia is described. Patagonykus puertai possesses caudal trunk vertebrae that are slightly convex caudally. The last sacral centrum is ball-shaped caudally, a condition that is also found in the caudal vertebrae. Caudal synsacral vertebrae are keeled ventrally. The humérus is craniocaudally compressed, and the internal tuberosity is unusually robust. Distally, the humerus has only one large subspherical condyle for articulation with the ulna and radius. The ulnar olecranon is strongly developed. The carpometacarpal is dorsoventrally compressed and quadrangular in dorsal view. Phalanx 1 of digit I has a pair of strong proximoventral hooklike processes. The ilium lacks the cranioventral fossa for the origin of the femoral protractor M. cuppedicus. The pubic pedicel of the ilium is cranioventrally oriented. The long and slender pubis is caudoventrally directed and ends distally in a massive pubic “foot”. The presence of a deep proximal socket on the calcaneum suggests that the fibula was in articulation with the tarsus. Metatarsal III is proximally narrower than metatarsals II and IV. Patagonykus puertai is closely related to another Patagonian taxon, Alvarezsaurus calvoi, and also to the Mongolian Mononykus olecranus. Alvarezsauridae encompass these three taxa, sharing outstanding synapomorphies such as procoelous sacral and caudal vertebrae, and a stout ungual in the first digit of the manus. The Patagonian taxa are geologically older and morphologically more primitive than Mononykus olecranus. Alvarezsauridae documents a group of theropods widely distributed in late Mesozoic times, relevant to discussions of bird origins.