The performance of an organism's feeding apparatus has obvious implications for its fitness and survival. However, the majority of studies that focus on chondrichthyan feeding have largely ignored the role of teeth. Studying the functional morphology of shark teeth not only elucidates the biological role that teeth play in feeding, but also provides insight specifically into the evolution of shark feeding because teeth are often the only structures available in the fossil record. In the present study, we investigate the puncture and draw performance of three general categories of extant teeth, tearing-type, cutting-type, and cutting-clutching type, as well as three fossil morphologies, utilizing a universal testing system. Differences in puncturing performance occurred among different prey items, indicating that not all 'soft' prey items are alike. The majority of teeth were able to puncture different prey items, and differences in puncture performance also occurred among tooth types; however, few patterns emerged. In some cases, broader triangular teeth were less effective at puncturing than narrow-cusped teeth. There were no differences between the maximum draw forces and maximum puncture forces. Many of the shark teeth in the present study were not only able to perform draw and puncture equally well, but also many tooth morphologies were functionally equivalent to each other. The findings obtained in the present study lend little support to the belief that shark tooth morphology is a good predictor of biological role.

No abstract

The majority of studies on the evolution and function of feeding in sharks have focused primarily on the movement of cranial components and muscle function, with little integration of tooth properties or function. As teeth are subjected to sometimes extreme loads during feeding, they undergo stress, strain, and potential failure. As attributes related to structural strength such as material properties and overall shape may be subjected to natural selection, both prey processing ability and structural parameters must be considered to understand the evolution of shark teeth. In this study, finite element analysis was used to visualize stress distributions of fossil and extant shark teeth during puncture, unidirectional draw (cutting), and holding. Under the loading and boundary conditions here, which are consistent with bite forces of large sharks, shark teeth are structurally strong. Teeth loaded in puncture have localized stress concentrations at the cusp apex that diminish rapidly away from the apex. When loaded in draw and holding, the majority of the teeth show stress concentrations consistent with well designed cantilever beams. Notches result in stress concentration during draw and may serve as a weak point; however they are functionally important for cutting prey during lateral head shaking behavior. As shark teeth are replaced regularly, it is proposed that the frequency of tooth replacement in sharks is driven by tooth wear, not tooth failure. As the tooth tip and cutting edges are worn, the surface areas of these features increase, decreasing the amount of stress produced by the tooth. While this wear will not affect the general structural strength of the tooth, tooth replacement may also serve to keep ahead of damage caused by fatigue that may lead to eventual tooth failure.

The nurse shark, Ginglymostoma cirratum, is an obligate suction feeder that preys on benthic invertebrates and fish. Its cranial morphology exhibits a suite of structural and functional modifications that facilitate this mode of prey capture. During suction-feeding, subambient pressure is generated by the ventral expansion of the hyoid apparatus and the floor of its buccopharyngeal cavity. As in suction-feeding bony fishes, the nurse shark exhibits expansive, compressive, and recovery kinematic phases that produce posterior-directed water flow through the buccopharyngeal cavity. However, there is generally neither a preparatory phase nor cranial elevation. Suction is generated by the rapid depression of the buccopharyngeal floor by the coracoarcualis, coracohyoideus, and coracobranchiales muscles. Because the hyoid arch of G. cirratum is loosely connected to the mandible, contraction of the rectus cervicis muscle group can greatly depress the floor of the buccopharyngeal cavity below the depressed mandible, resulting in large volumetric expansion. Suction pressures in the nurse shark vary greatly, but include the greatest subambient pressures reported for an aquatic-feeding vertebrate. Maximum suction pressure does not appear to be related to shark size, but is correlated with the rate of buccopharyngeal expansion. As in suction-feeding bony fishes, suction in the nurse shark is only effective within approximately 3 cm in front of the mouth. The foraging behavior of this shark is most likely constrained to ambushing or stalking due to the exponential decay of effective suction in front of the mouth. Prey capture may be facilitated by foraging within reef confines and close to the substrate, which can enhance the effective suction distance, or by foraging at night when it can more closely approach prey.