Lips in archosauromorph reptiles: the evidence from the Triassic period of Brazil
A new study suggests that the presence of extraoral soft tissues — labial scales or "lips" for short — was not unique to extant lepidosaur reptiles (lizards, snakes, and tuataras), but rather represents the primitive condition for all reptiles, including archosauromorphs that lived during the Triassic period more than 240 million years ago
"The Triassic period represents a period of major speciation and diversification for reptiles, and to work with different archosauromorph clades allowed us to expand the lips hypothesis"
An international team of researchers from the Laboratory of Osteoarchaeology and Palaeoanthropology (Bones Lab) at the University of Bologna, the Santa Catarina West Memorial Center at Community University of the Chapecó Region (CEOM Unochapecó) and the Paleontological Research Center at Federal University of Santa Maria (CAPPA/UFSM), both in Brazil, analyzed the cranial and dental fossils of 15 archosauromorph specimens from the paleovertebrate collection of CAPPA/UFSM.
"Since the 1990’s we’ve been used to the image of dinosaurs and other extinct reptiles with long exposed teeth in crocodile-like fashion", the researchers explain. "While in scientific literature there were attempts to challenge this view, most efforts were directed towards theropod dinosaurs: in our work, we replicated some of these previously established methods used to infer the presence of extraoral tissues in theropods, but for the first time in Triassic archosauromorphs".
Archosauromorpha is the clade that includes the Triassic reptiles of the study and is the sister clade to Lepidosauromorpha, the clade that includes today’s lizards, snakes and tuataras. The archosauromorphs studied were: rhynchosaurs, proterosuchids, pseudosuchians (the clade that includes extant crocodilians), lagerptids (pterosaur precursors), and dinosaurs (the clade that includes birds). These archosauromorphs date back to at least 240 million years ago, from the Triassic period of the Mesozoic Era.
The study relied on three main independent lines of evidence: the distribution of neurovascular foramina along the rostrum (snout), enamel thickness assessed through dental histological thin sections, and a statistical analysis of the relationship between skull length and maxillary tooth height.
The preparation and analysis of the dental thin sections were carried out at the Bones Lab on the Ravenna Campus of the University of Bologna. The results show that Triassic saurischian dinosaurs and pseudosuchians display a distribution and count of rostral neurovascular foramina comparable to that of extant lepidosaurs, such as the Komodo dragon, rather than to that of crocodilians, which have exposed teeth and specialized facial tissues for a semiaquatic lifestyle.
"The Triassic period represents a period of major speciation and diversification for reptiles, and to work with different archosauromorph clades allowed us to expand the lips hypothesis", the researchers confirm. "We hope that our article inspires other researchers into the realm of paleobiology and influences the way the extinct reptiles are presented in both scientific and popular media".
The fossil teeth show no evidence of enamel wear typically observed in crocodilian teeth, also suggesting that they were protected from physical damage and desiccation by lips. The statistical analysis further confirms that maxillary tooth height scales allometrically with skull length in the same fashion as in living varanid lizards, thus challenging the hypothesis that teeth of Triassic archosauromorphs were too large to be covered by lips.
The study, published in Palaeontology, proposes that the absence of lips in extant crocodilians is a derived condition linked to their semiaquatic lifestyle, rather than the plesiomorphic (primitive/ancestral) state for extraoral tissues in Archosauromorpha. The authors argue that if extinct archosauromorphs have traits that support the presence of lips, as in extant lepidosaurs, the probable ancestral condition for all reptiles (the clade Sauropsida) is the presence of lips. This study reshapes how we envision the appearance of prehistoric reptiles and contributes to a better understanding of the evolution of soft tissues in Sauropsida.