SUMMERS, A.P.; Univ. Of California, Irvine: Felling trees with a sock full of custard ? the evolution of durophagy in cartilaginous fishes

Chimaeras, sharks and rays abandoned a bony skeleton in favor of one of mineralized and unmineralized cartilage. Though reports on the material properties of shark cartilage are rare it is clear that mammalian cartilage is far less stiff and strong than bone. In spite of a putatively ‘soft’ skeleton the cartilaginous fishes occupy many of the same functional niches as bony fishes including crushing and consuming hard prey. Durophagy has evolved at least 5 times in the Chondrichthyes and is the basal feeding mode for the group. Taxa that feed nearly exclusively on hard prey include all the chimaeras, the horn sharks, one species of hammerhead shark, several dogfish and a clade of stingrays. Other taxa may opportunistically feed on hard prey and have one or more apparent adaptations for crushing. Possible adaptations include pavement-like dentition with broad contact zones between teeth, stiff dental ligaments, a jaw cross section that resists deformation, hypermineralization of the periphery of the skeletal elements, internal mineralization of the skeletal elements, modified biochemical composition of unmineralized cartilage and short, stiff intermandibular ligaments. Each of the radiations of hard prey crushing fishes exhibits a mosaic of these traits. Modification of the teeth is universal, while structural and material modifications are seen only in some taxa.