Evolutionary Stability of the Shark Snout: Geometric Morphometrics of Ventral Facial Openings.

Chondrichthyans are among the most ancient vertebrates, and their morphology has enabled adaptation to diverse marine environments. In sharks, the head is the functional center for feeding, respiration, and sensory perception, reflecting a balance between phylogenetic constraints and ecological demands. Striking specializations, such as the cephalofoil of hammerhead sharks or the elongated rostrum of sawsharks, illustrate remarkable adaptations. Most studies on head shape have focused on single species or restricted clades, whereas broader analyses have focused on parts of the skull. We analyzed the ventral head morphology of 453 shark species (Selachii), focusing on the relative position of the rostrum, mouth, and nostrils (in a subset of 395 species), using standardized illustrations and landmark-based geometric morphometrics. Shape variation was related to ecological and anatomical factors. The morphological space reveals a phylogenetic signal, with reconstructed ancestral shapes close to the overall mean configuration, indicating strong morphological conservatism. Evolutionary model fitting supports Brownian motion dynamics with limited divergence from ancestral forms. Correlations with habitat and body size are weak, whereas stronger associations are observed with the number of olfactory lamellae. Our results provide a large-scale comparative framework of ventral head morphology in sharks, highlighting its evolutionary stability and predominant phylogenetic control.