In spite of having a skeleton composed entirely of cartilage the chondrichthian fishes evolved durophagy (eating hard prey) four times. These four lineages include the outgroup to all elasmobranchs, the holocephalans, the horn sharks (Heterodontids), one species of hammerhead shark (Sphyrna tiburo), and most of the members of the myliobatid stingray clade. Adaptations for eating hard prey include molariform, or even pavement-like, dentition, large jaw adductors and specialized jaw cartilage.
Trabecular cartilage, a form of strut reinforced mineralized cartilage found in the myliobatid stingrays (cownose ray, eagle ray, bat ray, etc.) is the most obvious adaptation to heard prey crushing. The struts in the jaws are aligned with the forces that are generated during crushing, and they are present even in early embryos. This argues that they are not induced by crushing but are instead a genetically determined trait.
The architecture of the jaws of the durophagous stingrays and horn shark are very different. We have been looking at second moment of area as a structural descrptor for the jaws.