TESTING MORPHOLOGIC STASIS IN CHIONE EVELATA IN THE CONTEXT OF RAPID EARLY PLEISTOCENE SEA-LEVEL CHANGE IN FLORIDA

Phyletic gradualism and punctuated equilibrium are two hypotheses to explain the pattern of species evolution. The former describes a gradual change, whereas the latter suggests that most of the record is characterized by morphologic stasis with periodic, abrupt change. The genus Chione has undergone species-level changes through the Plio-Pliestocene interval. Here, we examine a section exposed at the Longan Lake quarry of the early Pleistocene Bermont “Formation” located northeast of Naples, Florida. This locality contains six lithologic cycles that are interpreted as repeated sea-level fluctuations. Based on their lithologies, the cycles were divided into three layers. From each layer, five-gallon bulk samples were collected, sieved, sorted, and then the best-preserved Chione elevata specimens were selected for analysis. Following Roopnarine's (1995) earlier morphologic analysis of the genus, complete left valves with visible internal landmarks were selected for morphometric analysis to investigate the degree of morphologic change through a sequence with pronounced environmental flucutuations. Based on abundance and suitable preservation, from 21 to 52 valves of C. elevata were examined from ten horizons within the sequence . All individuals were digitally photographed and analyzed using an image analysis program to identify the positions of nine landmarks. TpsDig, CoordGen6f, and TwoGroup6h were used to eliminate size differences and apply landmarks, compare individuals of an individual layer, and compare the various layers to one another, respectively. Individuals occurring in the same layer exhibited limited morphologic variation; in addition, a comparison of the mean values from each of the ten layers showed no significant change. Despite the differences in time and interpreted environments, C. elevata appears to be morphologically static during Bermont deposition. A possible explanation for this trend is that given the rapidity of environmental change as compared to average bivalve speciation rates, no directional morphologic trends could be maintained.