Postcranial anatomy and osteoderm histology ofRiojasuchus tenuiscepsand 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.

Relationships of mass properties and body proportions to locomotor habit in terrestrial Archosauria

“…The most frequently misclassified training dataset taxa were the pseudosuchian Riojasuchus (misclassified as bipedal, 12 out of 22 times) and the sauropod Neuquensaurus (misclassified as bipedal, 11 out of 22 times), which had COM X residuals among the lowest, if not the lowest, values for their respective clades (i.e., caudal COM X location), potentially explaining their relatively frequent misclassifications. Both taxa are known from the majority of the skeleton (Otero 2010; von Baczko et al 2019), suggesting underlying causes other than reconstruction error, such as apomorphic anatomies. For instance, Neuquensaurus was quite small for a sauropod, and moreover it possessed a very broad pelvis, potentially resulting in a more caudal COM X location.…”

3D hindlimb joint mobility of the stem-archosaur Euparkeria capensis with implications for postural evolution within Archosauria

“…This condition appears to be present in the basal avemetatarsallian Teleocrater rhadinus 38 , 61 (ilium NHMUK PV R.6795 and second sacral vertebra NMT RB519) and the Triassic pseudosuchian ornithosuchids, e.g. Riojasuchus tenuisceps 62 , 63 . It thus seems likely that the pillar-erect hip morphology evolved as a precursor to the ‘buttress-erect’ morphology of both dinosauriforms 60 , 64 and early crocodylomorphs 10 , 32 , 65 , and originated once at the base of Eucrocopoda.…”

Femoral specializations to locomotor habits in early archosauriforms

“…However, a lesser trochanter is absent in the bipedal pseudosuchians Shuvosaurus and Poposaurus (Nesbitt, 2007; Schachner et al, 2019), supporting the inference that this feature appeared only in bipedal avemetatarsalians, with parasagittal gait as a prerequisite (Carrano, 2000). Moreover, one specimen of Riojasuchus , perhaps a facultatively bipedal ornithosuchid (Baczko et al, 2020), resembled more quadrupedal pseudosuchians whereas the other specimen was closer to bipedal archosaurs, such as members of Poposauridae, in the morphospace, despite having a proximo‐anteriorly developed lesser trochanter (Figure 3a; “Rio, Pop”).…”

“…Moreover, one specimen of Riojasuchus, perhaps a facultatively bipedal ornithosuchid (Baczko et al, 2020), resembled more quadrupedal pseudosuchians whereas the other specimen was closer to bipedal archosaurs, such as members of Poposauridae, in the morphospace, despite having a proximo-anteriorly developed lesser trochanter (Figure 3a; "Rio, Pop").…”

“…However, this hypothesis of morphofunctional disparity is challenged by the discovery of new species reducing the gap of disparity between pseudosuchians and avemetatarsalians at the end of the Late Triassic, as well as increasing the amount of functional and morphological convergences between Avemetatarsalia and Pseudosuchia in the last four decades (Bates & Schachner, 2012;Brusatte et al, 2008aBrusatte et al, , 2008bCarrano, 2000;Foth et al, 2016Foth et al, , 2021Gatesy, 1991;Grinham, 2019;Nesbitt & Norell, 2006;Novas et al, 2021;Parrish, 1986Parrish, , 1987Singh et al, 2021;Toljagić & Butler, 2013). Indeed, discoveries of small bipedal pseudosuchians such as Shuvosaurus (Chatterjee, 1993;Nesbitt & Norell, 2006) and the larger Poposaurus (Gauthier et al, 2011), and the possibility that larger pseudosuchians like Postosuchus (Weinbaum, 2013) and Riojasuchus (Walker, 1964;Baczko et al, 2020) were facultative or obligatory bipeds, blur the line of morphofunctional distinction between the two clades. The same phenomenon applies to early avemetatarsalians, with the recently discovered early diverging taxon Teleocrater (Nesbitt et al, 2017) being quadrupedal, and some early dinosauriforms such as Silesaurus and Asilisaurus (Nesbitt et al, 2010;Piechowski & Dzik, 2010) perhaps being obligate quadrupeds or only facultatively bipedal.…”