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Development, functional morphology, and evolution of kinethmoid-mediated premaxillary protrusion Open Access

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Feeding success is critical to the health of all animals. Feeding in fishes is especially challenging given the viscous fluid in which they function. Throughout vertebrate history premaxillary protrusion, the ability to push the jaws toward prey thus increasing feeding success, has evolved several times among teleost fishes. While many acanthomorphs protrude the upper jaws by opening the mouth (given ligaments between the upper and lower jaws), cypriniform fishes employ a complex mechanism incorporating multiple ligaments and a novel sesamoid bone, the kinethmoid (Staab and Hernandez, 2010). The kinethmoid is unique to the clade, and a synapomorphy for Cypriniformes, the world's most diverse group of freshwater fishes (Nelson, 2006). Previous work examining the functional mechanisms involved in cypriniform jaw protrusion was hampered by lack of both adequate technology and a proper phylogenetic context, as many studies examined only one species.An integrative approach was employed to improve our understanding of the function, performance implications, and evolution of kinethmoid-mediated premaxillary protrusion. The use of developmental morphology, high-speed videography, x-ray observation of moving morphology (XROMM), and digital particle image velocimetry (DPIV) integrated information on the development, function, and evolution of the cypriniform jaw. This integrative approach has yielded key findings regarding both the development and function of the cypriniform protrusible jaw. Analyzing developmental morphology within a functional context allowed for the proposal of a hypothesis regarding the evolutionary origin of a morphological novelty, the kinethmoid (Staab and Hernandez, 2010). Three functional projects revealed that cypriniform premaxillary protrusion is characterized by substantial variation in feeding mechanics suggesting that this complex morphology is capable of functional flexibility. Finally, using goldfish as a cypriniform representative, and bluegill as an acanthomorph representative, DPIV revealed that even in distantly related species there is a fundamental performance advantage associated with premaxillary protrusion: to increase the hydrodynamic forces exerted on prey during suction feeding.These findings contribute to our knowledge of convergent evolution and subsequent diversification of morphological and functional traits. Given that most research investigating mechanisms of premaxillary protrusion has been done on acanthomorphs, these results will be useful in comparing two independent origins of this complex trait.

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