After the divergence of chondrichthyans and teleostomes, the structure of the feeding apparatus also diverged. Hyoid morphology varies considerably among fossil and extant elasmobranchs and has been hypothesized to be related to feeding style. How hyomandibular morphology affects function during feeding is of great interest since hyoid movement is a key component in the feeding mechanism. Cranial kinematics and buccal pressure during feeding was quantified in elasmobranchs and teleostomes. The feeding mechanism in elasmobranchs with anterior (AHY) or lateral (LHY) directed hyomandibulae differs from teleostomes primarily in hyoid movements due to morphological constraints. The hyoid expands ventrally during feeding in the species studied; however, the hyoid laterally compresses in AHY and LHY species but not in teleostomes or shark species with posterior directed hyomandibulae (PHY).Anterior and lateral hyomandibular orientation in elasmobranchs constrains them to adduct during feeding. This represents a fundamental difference in the feeding mechanism regardless of feeding style between elasmobranchs with AHY or LHY compared to sharks with PHY and teleostomes as a result of morphological divergence in hyoid morphology. It appears that posterior directed hyomandibulae may have evolved to increase the gape in specialized bite feeding sharks while lateral directed hyomandibulae evolved in specialized suction feeders.
