A NEW, ALMOST COMPLETE SPECIMEN OF ALNASHETRI CERROPOLICIENSIS (DINOSAURIA: THEROPODA) IMPACTS OUR UNDERSTANDING OF ALVAREZSAUROID EVOLUTION

PETER J. MAKOVICKY; SEBASTIÁN APESTEGUÍA; FEDERICO A. GIANECHINI

Buenos Aires

Jornada; XXX Jornadas Argentinas de Paleontología de Vertebrados; 2016

A new specimen of the diminutive alvarezsaurid Alnashetri cerropoliciensis (Museo Provincial Carlos Ameghino 377) was collected from the Upper Cretaceous Candeleros Formation near Cerro Policía, Río Negro Province. The specimen is very complete missing only major sections of the tail, and represents the first Argentinean alvarezsaurid with cranial remains. Referral to Alnashetri is based on anatomy of the hindlimb, which is identical to the holotype. Derived traits throughout the new skeleton confirm that Alnashetri is a basal alvarezsaurid: the cervical vertebrae are incipiently opisthocoelous; the last sacral vertebra has a convex caudal articulation and a small ventral ridge; the coracoid lacks a coracoid tubercle; the distal end of the humerus is expanded and bears an extensor fossa on its caudal face; Metacarpals II and III are extremely slender compared to metacarpal I; the ungual of digit I has a reduced and pinched flexor tubercle and claw grooves terminating in an enclosed canal; the opisthopubic pubes lack an apron. The new specimen exhibits numerous plesiomorphic traits when compared to other alvarezsaurids. The sternal plates are unfused and plate-like. The forelimb is about half the length of the hindlimb, the olecranon process of the ulna is short, and the ilia have broad pubic peduncles, do not meet above the sacrum, and have narrow brevis fossae. As the earliest and most complete alvarezsaurid in the Argentinean fossil record, the new specimen has important implications for the relationships and body size evolution within Alvarezsauridae and for the phylogenetic placement of the clade within Coelurosauria.