SUMMERS, A.P.; ROWE, T.; KETCHAM, R.A.: Ontogeny of the hornshark chondrocranium and the evolution of trabecular cartilage

The skeleton of the sharks, rays, and chimaeroids is composed of cartilage, some of which is calcified. The vertebrae are completely calcified, while the remainder of the skeleton has one or more thin layers of calcified blocks (tesserae) covering a core of uncalcified cartilage. In a clade of stingrays containing hard prey specialists an internal network of calcified struts augments the surface calcification. This strut-reinforced tissue is called trabecular cartilage. Some sharks, most notably the heterodontids, also eat hard prey. We examined a size series of Heterodontus francisci with a high resolution CT scanner for evidence of trabecular cartilage and to examine the morphology of the jaw joint. The CT scans and dissection revealed no trace of trabeculae. The jaw mechanism is the mechanical antithesis of that of the hard prey crushing stingrays. The rays have a well-calcified symphysis but the upper and lower jaws meet at a joint that has a large uncalcified region. In contrast heterodontids have an extremely loose, uncalcified, ligamentous symphysis and the upper and lower jaw joints form a well-calcified hinge joint. This raises the possibility that the left and right side of the shark’s jaws work as levers with the fulcrum at the joint. Dissection of other shark taxa that are reported to eat hard prey, Sphyrna tiburo and Orectolobus ornatus, did not reveal any trabecular cartilage. Trabecular cartilage appears to be an evolutionary innovation, confined to the myliobatid stingrays that allows them to eat harder prey than other cartilaginous fishes.