Dentition of the smallnose fanskate Sympterygia bonapartii (Rajiformes: Chondrichthyes): A qualitative and...

The present study provides the first qualitative and quantitative characterization of the dentition of the smallnose fanskate Sympterygia bonapartii. An integrative morphological and microscopic approach revealed marked sexual and ontogenetic heterodonty. Mature males had teeth with semi-circular crown bases and sharp cusps arranged in well-defined rows, whereas females had flat, blunt teeth forming a mosaic pattern. Immature individuals of both sexes exhibited flat, blunt teeth resembling those of mature females, although mature females had larger tooth dimensions than immature females. The analyses included 138 jaws from specimens collected by commercial vessels between 2020 and 2025 in the Southwest Atlantic Ocean. The dental formula ranged from 31 to 41 tooth rows in the upper jaw and 28-41 in the lower jaw. Seven tooth rows were selected for detailed analysis, and tooth length and width were measured. The seventh tooth was selected as a standardized position representative of the functional dentition for comparisons among immature females, mature females, immature males and mature males. Generalized linear models revealed that tooth dimensions were significantly influenced by total length, sex, maturity stage and the interaction between sex and maturity. Tooth size increased with total length. Mature females showed wider and longer teeth than immature females, while mature males had longer but narrower teeth compared to immature males. These patterns are consistent with functional differentiation associated with reproductive behaviour, as males grasp females during copulation. The observed dimorphism supports the hypothesis that sexual selection plays a key role in shaping dental morphology in this species, reinforcing patterns reported for other rajiform species. Overall, these findings contribute to a broader understanding of dental morphology in chondrichthyans.