A NEW, ALMOST COMPLETE SPECIMEN OF ALNASHETRI CERROPOLICIENSIS IMPACTS OUR UNDERSTANDING OF ALVAREZSAUROID EVOLUTION

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

Calgary

Congreso; Society of Vertebrate Paleontology 77° Annual Meeting; 2017

A new specimen of the diminutive alvarezsauroid Alnashetri cerropoliciensis (Museo Provincial Carlos Ameghino 377) was collected from the Candeleros Formation near Cerro Policía, Rio Negro Province, Argentina in 2014. The specimen is very complete missing only major sections of the tail, and represents the first Gondwanan alvarezsauroid with cranial remains. Referral to Alnashetri is based on anatomy of the hindlimb and in particular the metatarsus, which is identical to that of the holotype specimen.Derived traits throughout the new skeleton confirm that Alnashetri is a basal alvarezsauroid: 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 bears a reduced and pinched flexor tubercle and its medial claw groove terminates in an enclosed canal; the opisthopubic pubes lack an apron. The new specimen exhibits numerous plesiomorphic traits when compared to Alvarezsauridae. 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 pubicpeduncles but narrow brevis fossae and do not meet above the sacrum.As the earliest and most complete alvarezsauroid in the South American fossil record, the new specimen has important implications for our understanding of phylogenetic relationships, biogeography and body size evolution in this clade. Phylogenetic analysis posits Alnashetri as sister to Alvarezsauridae, but South American alvarezsauroids remain paraphyletic with respect to Laurasian parvicursorine alvarezsaurids. As one of the smallest as well as earliest diverging alvarezsauroids, Alnashetri provides evidence for multiple instances of miniaturization within this clade of unusual theropods.