Molarization and the evolution of teleost dentitions.

摘要

Teleost dentitions, often polyphyodont and lacking occlusion, provide the potential for tooth form to respond to environmental stimuli. One example is Cichlasoma minckleyi, a cichlid endemic to the Cuatro Cienegas Basin, Mexico. Multivariate analyses of measurements of the neurocranium, jaws, and pharyngeal bones and teeth distinguish two morphs that possess conical or molar teeth. Individuals raised in the laboratory on several different diets indicate that the polymorphism is caused by genetic as well as environmental components. An analysis of body form using geometric morphometrics found that dental morphs did not differ significantly in size, shape, or allometric slopes, but did differ in allometrc intercepts; dental morphs thus exhibit different patterns of body-form variability.; To understand how environmental influences might be transmitted to developing replacement teeth, I surveyed the location of such teeth in 127 fish species (72 families, 23 orders). Replacement teeth in teleosts may develop in the soft tissue outside the bone to which they will attach (“extraosseous”), or in sockets within the bone (“intraosseous”) beneath their functional predecessor. Two changes are necessary for the evolution of this derived state: replacement teeth must move beneath their functional predecessors and must be encased in bone.; Innervation likely provides the impetus for changes in tooth shape in response to environmental influences, and studies have suggested that innervation is essential for odontogenesis in teleosts. I assessed the location of axons innervating pharyngeal teeth in teleosts with extra- and intraosseous development of replacement teeth using traditional histology and immunofluorescence techniques, in addition to whole-mount clearing and staining of bones (with Alizarin Red S) and nerves (with Sudan Black B). The location of innervation (extraosseous or intraosseous) often matches the location of developing replacement teeth.; I also assessed the systematic affinities of the enigmatic Late Cretaceous teleost Platacodon nanus, known only from isolated teeth (some molariform) and pharyngeal bones. I reviewed characters of pharyngeal bones and teeth and use them to contrast three hypotheses of Platacodon 's affinities. Platacodon has most characters in common with cypriniforms. A case can also been made for pycnodontiform affinities, but percoid affinities seem the least likely option.