| Summary: | The skeletons of chondrichthyans (sharks, skates, rays, and chimeras) are composed entirely of cartilage, yet must still provide the skeletal support that bone does in other vertebrates. There is also an incredible range of diversity in the morphology of the cartilaginous skeleton of the feeding apparatus in Chondrichthyans. The goal of this research is to provide insight into the morphological evolution and biomechanical function of the cranial skeleton in chondrichthyans. Differences in feeding style likely change with the gross morphology of the ten skeletal elements of the shark feeding apparatus, since these elements are common to all shark species. In chapter one, to increase our understanding of how the feeding skeletal morphology has evolved with the of feeding style of sharks, the length, width, and angles of the elements of the feeding apparatus are measured in four species (white-spotted bamboo, Chiloscyllium plagiosum; spiny dogfish, Squalus acanthias; sandbar, Carcharhinus plumbeus; and dusky smoothhound, Mustelus canis). These species encompass a wide phylogenetic range, and include suction and bite feeders as well as two different orientations of the hyomandibula, the major jaw supporting element. A principle components analysis is used to identify relationships among the skeletal elements by species, and linear regressions are then used to test the effect of hyomandibula length on the other morphological variables. Strong relationships were discovered between the length of the hyomandibula and the lengths of all other skeletal elements and the angle of the hyomandibula. The bite feeders have longer elements and appear to maximize the size of the oral cavity, allowing larger prey to be swallowed. Suction feeders have shorter elements, which restrict the size of the oral cavity and mouth opening, but can concentrate suction forces. Based on the strong relationship between hyomandibula length and angle on feeding morphology, the mechanical properties of the hyomandibular cartilages in the same four shark species is investigated in chapter two. Young's modulus, a measure of stiffness, and Poisson's ratio, a measure of three-dimensional shape change, of the hyomandibular cartilages are compared. While Poisson's ratio is similar among the species, Young's modulus increases with mineralization and is larger in the suction feeders. Though sharks have a cartilaginous skeleton, some species have higher mineralization of elements that are under higher stress.
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