FEEDING MECHANISM OF LARGE MESOZOIC FOSSIL CHIMAEROIDS (CHONDRICHTHYES, HOLOCEPHALI): HOW LARGE A SHELL COULD THEY CRUSH?

Holocephalans are well adapted to durophagy, feeding upon hard prey, with their massive toothplates equipped with hypermineralized pads providing a molariform-crushing surface called tritors. Some fossil chimaeroids possessed the largest toothplates (up to 20cm long) known in holocephalans, suggesting they could have consumed larger and stronger prey than their living relatives. Edaphodon commonly co-occurs in the Cretaceous with inoceramids, which could reach more than 1m in length and up to 1cm in shell thickness. So it is important to test if the genus could crush some of these large bivalves. I estimated the maximum theoretical bite force in two fossil chimaeroid genera, Ischyodus and Edaphodon, to determine their feeding performance. Bite force was calculated using lever mechanics and estimated cross-sectional areas of adductor mandibulae muscles. A preliminary theoretical bite force analysis suggested a range of 210-866N from anteriorly to posteriorly in the jaw of Edaphodon (estimated total length of 305cm) and 232-939N for Ischyodus (333cm). Published data on the extant chimaeroid, Hydrolagus colliei (body size range of 21-44cm) show that bite force ranges from 11-191N including both anterior and posterior values. I also measured shell strength of the extant bivalve Ruditapes philippinarum and the gastropod Tegula funebralis to compare the values with the calculated theoretical bite forces. During the shell strength analysis, metal casts of Edaphodon toothplates were attached to a stress analyzer to investigate the crushing performance of the tritors. The crushing test showed that tritors not only provided crushing surface but also held and stabilized the prey effectively. The measured shell strength ranged from 52-253N for bivalves with shell lengths 3.2-5.3cm and 107-908N for gastropods with shell lengths 1.1-2.7cm. Bite force estimates and extrapolations from the shell strength measurements suggest that Ischyodus and Edaphodon were capable of generating sufficient bite force to consume bivalves within gape size limitations while crushing rigid gastropod shells were mechanically challenging.