Growth over ontogeny can profoundly affect the form and function of biological structures. Since animal performance is tightly linked to morphological specialization, ontogenetic change in size may help structure that organism's ecological role. One measure of animal performance, bite force generation, has important ramifications in taxa which consume hard-shelled prey (durophagy). Since high bite force generation is presumably selected for in these taxa, durophagous animals provide an excellent study system for investigating ecomorphological and biomechanical consequences of growth in the feeding apparatus. Elasmobranchs (sharks, rays and skates), with relatively few cranial skeletal components make a simplified system in which to study feeding. Durophagous elasmobranchs are particularly fascinating from a material and biomechanical standpoint as their cartilaginous skeleton is typically much more pliable than the hard prey they consume. I examined through morphological dissection functional and evolutionary aspects as well as general trends in the cranial musculature of myliobatiform stingrays. Durophagous stingrays display a suite of specialized morphological characters, including hypertrophied jaw adductor muscles, characteristically deep, broad craniums, flattened, pavement-like teeth, reinforced skeletal structure, and novel tendon/sesamoid structures which redirect muscle forces during feeding. Non-durophagous stingrays are predominantly more dorso-ventrally flattened, have muscles which rely on direct, aponeurotic insertions on the skeletal structure, and display musculoskeletal specialization for jaw protrusion and independent lower jaw cartilage kinesis, relative to durophagous stingrays. In order to understand how growth affects feeding performance, an ontogenetic series of 26 cownose rays, Rhinoptera bonasus, were dissected in order to development a biomechanical model which predicts bite force generation. Muscle masses, orientation, cross-sectional areas and mechanical lever configurations were measured from the ontogenetic series of stingrays. Increases in jaw adductor muscle masses and cross-sectional areas resulted in positive allometry of bite force across ontogeny in R. bonasus. Mechanical advantage of the feeding apparatus was generally conserved throughout ontogeny, leaving increase in the size of the jaw adductors as the major component to allometric gains in bite force generation. Of primary importance to forceful biting in this taxon is the use of sesamoid structure associated with the insertion of the primary jaw adductor division. This fibrocartilaginous sesamoid serves to redirect lateral muscle forces anteriorly, transmitting force within the axial biting plane. However, measured bite forces obtained through electrostimulation of the jaw adductor divisions in live cownose rays were much higher than predicted by my biomechanical model. We believe this may be the result of differences in muscle physiology between selachian and batoid values for muscle fiber maximum tetanic tension. Caution must be prescribed when developing biomechanical models without validation through in vivo testing.
