The effect of jaw suspension on cartilage strength in elasmobranchs

The jaws and their supporting cartilages are tessellated in elasmobranchs and exhibit an abrupt increase in stiffness under compression. The major jaw-supporting cartilage, the hyomandibula, varies widely by shape and size and the extent of the load-bearing role is hypothesized to be inversely related to the number of craniopalatine articulations. Here, we test this hypothesis by evaluating the strength of the hyomandibular cartilage under compression in 13 species that represent all four jaw suspension systems in elasmobranchs (amphistyly, orbitostyly, hyostyly, and euhyostyly). The strength of the hyomandibular cartilages was measured directly using a material testing machine under compressive load, and indirectly by measuring morphological variables putatively associated with strength. The first measure of strength is force to yield (F_y), which was the peak force (N) exerted on the hyomandibula before plastic deformation. The second measure was compressive yield strength (σ_y, also called yield stress), which is calculated as peak force (N) before plastic deformation/cross-sectional area (mm²) of the specimen. Our results show that the load-bearing role of the hyomandibular cartilage, as measured by yield strength, is inversely related to the number of craniopalatine articulations, as predicted. Force to yield was lower for euhyostylic jaw suspensions and similar for the others. We also found that mineralization is associated with greater yield strength, while the second moment of area is associated with greater force to yield.