New anatomical observations on the anatomy and disparity of Bonapartenykus ultimus (Coelurosauria, Alvarezsauria, Patagonykinae) from the Late Cretaceous of Patagonia

Abstract

The Late Cretaceous patagonykine Bonapartenykus ultimus is one of the most enigmatic large alvarezsaurids, known from a single fragmentary specimen. Here, we provide an exhaustive anatomical redescription of the holotype of B. ultimus based on detailed 3D reconstructions and extensive comparisons with other alvarezsaurians. Furthermore, we present an updated phylogenetic assessment and revised diagnosis, incorporating both recently described and newly studied materials. In addition, we employed morphological disparity and phylogenetic generalised least-squares (pGLS) analyses to examine the evolutionary trends in the clade. Our results suggest that, although its holotype is fragmentary and referred materials are scarce, B. ultimus bears nine autapomorphic features, and our phylogenetic analyses place it as a Patagonykinae alvarezsaurid and sister to the Parvicursorinae. The pGLS and multiple disparity metrics indicate that Patagonykinae display a distinct morphological configuration relative to Parvicursorinae, pointing to divergent evolutionary trajectories between Gondwanan and Laurasian forms. This result reinforces the hypothesis that they evolved under distinct selective pressures and adapted to different ecological niches.