Postcranial anatomy and osteoderm histology of Riojasuchus tenuisceps and a phylogenetic update on Ornithosuchidae (Archosauria, Pseudosuchia)

Abstract

Ornithosuchidae is a group of terrestrial quadrupedal pseudosuchian archosaurs from the Late Triassic of South America and Europe. Riojasuchus tenuisceps is arguably one of the best representative species of this clade because it comprises very well-preserved three-dimensional, almost complete skeletons. However, R. tenuisceps was originally described 50 years ago and compared then only with Ornithosuchus woodwardi and their affinities were discussed in detail. Here, we provide a detailed description of the postcranial skeleton of R. tenuisceps, which exhibits several remarkable features within pseudosuchians. When a wide spectrum of pseudosuchian archosaurs are considered, the alleged character that linked ornithosuchids with dinosaurs resulted in convergences and some were registered in other pseudosuchian groups as well. Riojasuchus tenuisceps also provided crucial information about the ‘crocodile-reversed’ tarsus, which is a unique feature of ornithosuchids, but it is not completely preserved in O. woodwardi and it is unknown in Venaticosuchus rusconii. In addition, the first histological analysis of cervical and dorsal osteoderms of R. tenuisceps was carried out as well, in order to test the utility of these structures as skeletochronological tools. Finally, the phylogenetic context of Ornithosuchidae is discussed based on the latest phylogenetic studies, which show a close affinity with Erpetosuchidae.