Forelimb information on basal crurotarsal phylogeny and the position of Ornithosuchidae

Abstract

The objective of the present contribution is to restudy the forelimb anatomy of basal crurotarsals and to evaluate the phylogenetic information of this anatomical region. The position of Ornithosuchidae is one of the topics of particular interest, given the ongoing debate about the affinities of this clade of Triassic archosaurs. This clade of basal archosaurs was originally depicted as the sister group of the Ornithodira, but recent studies agreed on its crurotarsal affinities, depicting it either as a basal member of this clade or as the sister group of the rauisuchians within Suchia. This last hypothesis is supported by forelimb characters in this study, particularly by synapomorphies shared with some members of the Rauisuchia, such as: a deep groove ventral to the glenoid; presence of an ectepicondylar groove; and the pressence of an ossified intermedium. Most of these traits are only shared with Postosuchus and Batrachostomus, because of the lack of complete forelimb material in other rauisuchians. Other analyzed forelimb characters have a broader distribution within Archosauria, such as the presence of an ossified olecranon process on the ulna, being a synapomorphy of Archosauria; the structure of the medial tuberosity, that forms a distinctive separate structure from the medial margin of the humeral head in most crurotarsals (being this condition strongly developed in Riojasuchus). Finally, this preliminary study shows the phylogenetic informativeness of the forelimb in basal archosaurians, which provides valuable information that has been previously unexplored.