A new proterochampsid (Archosauriformes: Proterochampsia) from the Late Triassic of southern Brazil and the emergence of archosaurian hind limb traits

Abstract

Characterized by an elongated snout, proterochampsids are carnivorous non-archosaur archosauriforms. The clade is endemic to South America and its fossil record extends from the early Carnian to the late Carnian/early Norian. Nesting close to Archosauria, it is a key clade for understanding the origin and evolution of archosaurian traits. Unfortunately, hind limb elements are usually poorly preserved for the group. Therefore, the hind limb anatomy of proterochampsids still lacks detailed descriptions. In the present study, we partially fill this gap with the description of a new proterochampsid represented by a complete and well-preserved hind limb. Stenoscelida aurantiacus gen. et sp. nov. was excavated from the late Carnian/early Norian (Late Triassic) beds of southern Brazil. A phylogenetic investigation recovers the new taxon as a non-rhadinosuchine proterochampsid. The species bears an unusual set of traits for the group, which provides clues on the evolutionary origins of some muscle attachment structures. For instance, the femur of Stenoscelida aurantiacus gen. et sp. nov. possesses an anterior trochanter and an anterolateral scar. So far, these features have not been reported in other non-archosaurian archosauriforms. Therefore, the new specimen indicates that some typical archosaurian features evolved earlier than previously thought. The taxon also carries additional uncommon features for proterochampsids, such as an iliofibularis tubercle on the anterior margin of the fibula and a vestigial phalanx in digit V. In sum, Stenoscelida aurantiacus has one of the best-preserved hind limbs within Proterochampsidae and sheds light on the polarization of important traits regarding the evolutionary context of Archosauria.

http://zoobank.org/urn:lsid:zoobank.org:pub:C78B7CE3-AB9B-4543-833E-B2A3FEA957D9

Keywords:

• Archosauria
• Archosauromorpha
• Carnian
• Gondwana
• Proterochampsidae
• South America

Acknowledgements

We thank Martin D. Ezcurra for photographs of some proterochampsids. We thank the Willi Hennig Society for supporting the free use of TNT software. We extend our gratitude to Martin D. Ezcurra and an anonymous reviewer for corrections and comments that greatly improved this manuscript. We are grateful to Caio Fantini for the digital reconstruction of Stenoscelida aurantiacus depicted in Figure 7. This work was carried out with aid of the Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS 21/2551-0000680-3) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq 404095/2021-6) to Rodrigo Temp Müller, and a Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES 88887.608076/2021-00) scholarship for Mauricio Silva Garcia.

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2022.2128913.

Associate Editor: Richard Butler