4.1 Friday, Jan. 4 Mechanical Properties of a Shark Jaw Support Structure BALABAN, JB*; SUMMERS, AP; WILGA, CAD; University of Rhode Island; University of Washington; University of Rhode Island email@example.com
The upper jaws of elasmobranchs (sharks, skates, and rays) are not fused to the cranium as they are in tetrapods. Instead, they are suspended by 0-3 ligaments (none in skates and rays) anteriorly and a skeletal element, the hyomandibula (HY) posteriorly. The HY connects the cranium to the jaw joint and can have many orientations, shapes, and sizes depending on the clade. We know how the HY moves during feeding and, from bite force estimates and measurements we can estimate the forces acting on the HY. Here we present data on how well these elements withstand the stresses associated with the loads seen during feeding. We determined the mechanical properties of the HY of four species of sharks with different jaw orientations and feeding styles (bamboo shark, a suction feeder; smoothhound shark, a biter; sandbar shark, a biter; and dogfish, both suction feeder and biter. We used sonomicrometry to track local strain in the direction of loading and at 90 degrees to the loading direction. This allowed us to estimate both the stiffness of the material and its Poisson’s ratio. We also measured the cross sectional shape and the percent area of calcified cartilage to predict how well the element handles force in different directions. Our results show that despite large differences in size and shape, there is little difference between the effective mechanical properties of the HY in different species. It appears that to withstand larger forces the HYs increase in size without a changing in mechanical properties. This is in contrast to analogous results from the pelvic girdles of cartilaginous fishes. However, differences in the shape of the cross-sectional area and relative mineralization levels among species may lead to differences in the response among species to bending or tensile load.