A new, almost complete specimen of Alnashetri cerropoliciensis (Dinosauria: Theropoda) impacts our understanding of alvarezsauroid evolution.

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

buenos aires

Jornada; 30 JORNADAS ARGENTINAS DE PALEONTOLOGIA DE VERTEBRADOS; 2016

museo argentino de ciencias naturales

A new specimen of the diminutive alvarezsaurid Alnashetri cerropoliciensis (Museo Provincial Carlos Ameghino 377) was co­ llected from the Upper Cretaceous Candeleros Formation near Cerro Policía, Río Negro Province. The specimen is very com­ plete missing only majar sections of the tail, and represents the first Argentinean alvarezsaurid with cranial remains. Re­ ferral to Alnashetri i s 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 11 and 111 are extremely slender com­ pared to metacarpal 1; the ungual of digit 1 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 fassae. As the earliest and most complete alvarezsaurid in the Argentinean fossil record, the new specimen has important implications far the relationships and body size evolution within Alvarezsauridae and far the phylogenetic placement of the clade within Coelurosauria.