Convergent Evolution in Tooth Morphology of Filter Feeding Lamniform Sharks

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The basking shark (Cetorhinus maximus) and megamouth shark (Megachasma pelagios) are two species of filter-feeding sharks, both belonging to the order Lamniformes. There are two conflicting hypotheses regarding the origins of filter feeding in lamniform sharks; that there is a single origin of filter feeding within Lamniformes, or conversely, the filter-feeding adaptations have been developed independently due to different ancestral conditions. Evidence obtained from several morphological and molecular studies strongly supports the latter hypothesis. Because evidence suggests that Megachasma and Cetorhinus have developed their filterfeeding adaptations independently, we hypothesized that convergent evolution in tooth morphology is taking place within these two lineages. Geometric morphometric analyses were performed on fossil and recent teeth of Megachasma and Cetorhinus to determine if there is commonality among tooth shape. Increasingly similar shapes in recent teeth compared with fossil teeth would provide one piece of evidence to support this hypothesis. Relative warp axes were interpreted using percent variation explained and compared by species. Multivariate analyses of variance (MANOVA) were used to test for significant morphological differences in fossil and recent teeth of each species and a series of one-way analyses of variance (ANOVA) with corresponding Tukey intervals (95% CI) was used to test for morphological differences in overall mean tooth shapes between fossil and recent Megachasma and Cetorhinus teeth for each significant relative warp axis. The results of the MANOVAs showed significant differences in fossil and recent Megachasma and Cetorhinus teeth and support independently derived filter-feeding adaptations and the results of the ANOVAs and corresponding Tukey intervals support convergent evolution in tooth morphology within these two species of filter-feeding lamniform sharks.

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