MODIFICATION AND REDUCTION OF THE FORELIMBS OF THE ALVAREZSAURIDS (THEROPODA, COELUROSAURIA)

MATTIA ANTONIO BAIANO; CHRISTIAN PEREYRA; JORGE GUSTAVO MESO; LEONARDO SALGADO; STEPHEN BRUSATTE; JUAN IGNACIO CANALE; JONAH CHOINIERE

Puerto Madryn

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2018

Asociación Paleontológica Argentina

Alvarezsaurid forelimbs are markedly short and have a robust digit I ending in a stout ungual phalanx. Recently, two new basal alvarezsaurids, Bannykus and Xiyunykus Xu et al., were published, yielding evidence on the evolution of this part of the skeleton in this group of coelurosaurian theropod dinosaurs. The inclusion of Bannykus and Xiyunykus (both from the Lower Cretaceous of China), and the specimen MPCN-PV 738 (from the Upper Cretaceous of Patagonia), in a phylogenetic dataset of 888 characters and 163 avian and non-avian theropod taxa, shed some light on that issue. Our results not only show a better resolution of the internal relationships of Alvarezsauridae with respect to previous analyses; recognizing two different clades of derived alvarezsaurids from the Upper Cretaceous, one integrated by Asian forms, and other formed by Patagonian species, but also that the extreme modifications of the forelimb evolved gradually, leading to different anatomical configurations in both clades. The Patagonian forms have the proximal (preungual) phalanx longer than the ungual, while the derived forms from the Upper Cretaceous of Mongolia have a much shorter proximal phalanx compared with the ungual. The taxa from South American possess an ulnar olecranon process more robust and posteroventrally flexed, while in the forms from Asia, the olecranon is posteriorly projected. This last difference is also related with a variation in the articulation angle of the elbow in both subclades. In this way, these different trends in the forelimb modifications inside Alvarezsauridae may suggest different adaptations between Patagonian and Asian forms.