THE EVOLUTIONARY TEMPO OF LUCINID BIVALVES IN FLORIDA DURING THE CONTRASTING NEOGENE AND QUATERNARY CLIMATIC REGIMES

Understanding the evolutionary patterns (i.e., stasis, gradualism) of morphological change is a critical component of modern evolutionary theory. Over the past few decades, various studies have placed substantial focus on understanding the frequency of different evolutionary patterns within various taxonomic groups through the Phanerozoic. These studies have revealed that many lineages show contrasting evolutionary patterns through time; however, the exact cause of this phenomena remains elusive due to a limited understanding of their environmental context. To test for the potential role environmental controls might exert on evolutionary dynamics, this project examined evolutionary changes in two different genera of lucinid bivalves from the relatively more stable mixed-house climate regime of the Neogene and the less stable icehouse climatic regime of the Quaternary. The lucinids chosen for this study include western Atlantic Lucina and Anodontia, which are well-represented in the Neogene to Quaternary record from this region. Morphological change through time was evaluated using both size data and elliptical fourier analysis of outline shape data. Fourier coefficients were analyzed using principle component and canonical variate analyses in R. Lucina show no change in shape from the Miocene to Pleistocene with substantial change in size during the Miocene and limited change in size from the Pliocene to Modern. In contrast, Anodontia show substantial change in size and shape during the late Miocene but limited to no change in size and shape during the middle Miocene and Quaternary. In both cases, most evolutionary change coincided with the stable mixed-house climate regime of the Miocene, whereas stasis was primarily concentrated during the less stable Quaternary icehouse. These cases provide strong support for Sheldon’s (1996) ‘Plus ça change’ model, which predicts that relatively more stable environmental settings, such as during a mixed-house climate regime, will display gradualism, whereas a more frequently changing environment, such as during an icehouse climate regime, will display stasis.