Cranial osteology of Neotropical squamate reptiles of the genus Riama (Gymnophthalmidae: Cercosaurinae)

Cranial bones of squamate reptiles diverge in shape and structure across lineages, while preserving the basic skull plan. Therefore, variation in cranial morphology of squamates is key to understanding phylogenetic relationships, species delimitation, and lifestyles, which ultimately contribute to insights into their evolution and ecology. Nevertheless, although Gymnophthalmidae is the most diverse endemic non-ophidian squamate family in the Neotropics, studies on comparative cranial osteology within this lineage remain scarce. Herein, we thoroughly describe and compare the external skull anatomy of six species of the Andean genus Riama from Colombia and Ecuador (R. columbiana, R. simotera, R. striata, and three undescribed taxa), aiming to provide phenotypic evidence for phylogenetic, taxonomic, and functional analyses. We examined 12 cleared-and-stained skulls of adult specimens (one male and one female per species) to preliminarily assess inter- and intraspecific variation within Riama. Interspecific variation occurs in the degree of contact between nasals (and consequently in the articulation of the nasal process of the premaxilla), and in the absence/presence of the posterior process of the jugal, among others. An anterior process of the ventral crest of the vomer in female R. columbiana is seemingly unique among gymnophthalmids. The number of tooth positions and teeth morphology vary both among sexes and species. Preliminary evidence implies sexual dimorphism and bilateral variation in some characters. Our findings suggest that variation in skull anatomy within Riama can inform species delimitation, and phylogenetic and functional analyses. Finally, we discuss variation in cranial morphology among species of Riama within a phylogenetic context.