Inferred presence of extraoral tissues in Triassic archosauromorphs and the evolutionary implications for the clade Sauropsida

Recent work has suggested that the presence of extraoral soft tissues (‘lips’), in the form of labial scales in theropod dinosaurs, could be inferred based on: anteroposteriorly distributed foramina in the rostral bones, similar to extant lepidosaurs; vertically projected teeth; uniform enamel thickness in maxillary teeth; and an allometric relation between skull length and maxillary crown height. Here, we review the current state of knowledge on the presence of lips in saurischian dinosaurs and further contribute to the debate by inferring and discussing their presence in Triassic archosauromorphs, particularly those from southern Brazil, using the same methodological approach. Our results support the hypothesis that the lipped condition represents the plesiomorphic state in Sauropsida. Non-theropod Triassic saurischians and pseudosuchians exhibit a foraminal distribution similar to that of theropods and extant lepidosaurs. Histological thin sections reveal a uniform enamel thickness on both labial and lingual sides of functional and shed tooth crowns, and the dentine shows no evidence of wear, contrasting with the condition in crocodilian teeth. Additionally, our regression analysis supports an allometric trend in which increased skull length correlates with increased maxillary crown height in Theropoda, Sauropodomorpha, Herrerasauria and Pseudosuchia. We also infer and discuss the presence of a mandibular gap in Prestosuchus, as well as the occurrence of lips in specialized taxa from the clades Theropoda, Ornithosuchidae, Phytosauria and Proterosuchidae, based on convergent rostral features shared with extant lepidosaurs and actinopterygians. Furthermore, we examine the presence of both rhamphotheca and labial scales across the clades Silesauridae, Lagerpetidae and Rhynchosauria.