GERRY, S.P.*; DEAN, M.N.; University of Rhode Island; University of California, Irvine: Symphyseal force transmission in the jaws of Squalus acanthias

Complete fusion of the symphysis stiffens and strengthens the jaws to allow large bite forces to be generated. An unfused symphysis is functionally versatile, however, is likely poor at transmitting forces across the jaws. To investigate force transmission in a jaw with a partially fused symphysis, in vivo muscle stimulation experiments of Squalus acanthias were compared to theoretical estimates based on a two-dimensional model. According to this model, when stimulated unilaterally, a fused symphysis should generate the highest force ipsilaterally and that force will decrease linearly as it approaches the balancing-side. An unfused symphysis will also produce a high ipsilateral force, but that force will rapidly diminish at the symphysis preventing any transmission of forces to the balancing-side. These hypotheses were not supported: unilateral stimulation generated 100% normalized force at the working-side, 50% force at the symphysis and 45% force at the balancing-side. Therefore, a three-dimensional model is required to explain the transmission of forces across the jaws from working-side to balancing-side. In the orbitostylic jaw suspension of Squalus, the long orbital processes of the palatoquadrate articulate with the neurocranium, providing lateral support for the upper jaw. The orbital processes could serve to transmit forces across the neurocranium from the working-side to the balancing-side of the jaws. Additionally, the lateral head-shaking behavior of Squalus during prey capture could position the prey at the jaw joint for maximum bite force as well as increasing the contact between the orbital processes and the neurocranium, thus increasing prey processing efficiency.