MORPHOLOGICAL VARIABILITY IN FOUR BIVALVE GENERA AMONG DIFFERENT TROPHIC LEVELS FROM FLORIDA'S MIOCENE THROUGH PLIO-PLEISTOCENE

Outline shape in bivalves captures a critical set of morphological characteristics that reflect life habit and function. Within a given genus, change in shape can represent evolution or adaptation to an alteration in the broadly defined environment. In this study, morphological analysis has been conducted in order to identify and quantify the evolution of shape through the Neogene in four different bivalve genera. In addition, this study was designed to test the degree of any change in shape or whether there has been morphologic stasis as suggested by Stanley and Yang (1987). The bivalve genera studied here consist of groups that have well-established phylogenies, and that also represent different trophic levels within level-bottom communities. They include: Chione (infaunal suspension feeder), Corbula (infaunal suspension feeder), Divaricella (infaunal chemosymbiotic deposit feeder) and Lucina (chemosymbiotic deposit feeder). The interval investigated ranges from the Miocene through the Late Pleistocene and the specimens investigated are derived from the Chipola, Jackson Bluff, Bermont, and Fort Thompson formations in Florida.

Morphological analysis was conducted using the Fourier shape analysis software of dimensional outlines, which uses digitized XY coordinates of the outlines to produce an average of the morphologies. For comparison, linear measurements of the height and length of right valves supplemented the shape analysis and provided an assessment of morphology independent of overall shape. Preliminary morphological analysis of Chione, Corbula, Divaricella and Lucina suggest that no major changes had occurred in the shape of the shell outline. This morphological stasis is also suggested by the measurements of height and length taken from the samples that showed no statistical change through the interval. This suggests that, at least from a morphologic analysis of shape, that the evolutionary records of these genera largely record stasis despite the documented environmental changes that occurred during the Neogene and independent of trophic group.