THE BIVALVE FAUNA OF FLORIDA'S PLIOCENE PINECREST BEDS: PALEOECOLOGIC IMPLICATIONS OF HIGH-RESOLUTION SAMPLING

Florida's Pinecrest beds contain a virtually unparalleled diverse and abundant bivalve fauna. In general, analysis of this fauna has focused on relatively broad sampling of the stratum. For this study, we sampled a 4 m sequence exposed in Phase 8 of the SMR Quarry consisting of six units based on lithologic differences, largely defined by changes in mud content and the presence of Mercenaria lags. Each unit was subsampled at intervals ≈35 cm, and all samples consist of approximately four liters of material. The overall species richness sampled in the sequence is 115 species, and a preliminary rarefaction curve suggests that there are two components to obtaining this diversity: 1) an initial, steep limb, with a slope of almost 20 species/100 individuals sampled, suggesting that approximately half the total species richness is found in each sample; and 2) a less steeply sloping limb representing the sampling of slightly more than one additional species for each 100 individuals sampled. This suggests that although the common components of the bivalve communities are readily sampled, substantially larger samples are required to more fully capture rarer species. As compared to the Early Pleistocene Bermont and Late Pleistocene Fort Thompson formations, Pinecrest bivalve communities contain higher diversity and greater equitability, as seen in the lack of domination of Chione elevata characteristic of younger units. The Pinecrest data also allow competing hypotheses related to the formation of the sequence to be tested. The pronounced similarity in common taxa among the samples combined with their inferred depth ranges based on closely related, if not identical, modern species suggest that the individual units ‘sample' a virtually identical suite of communities. Thus, these results favor a series of temporally isolated depositional events rather than the filling of accommodation space with shells and sediment indicative of progressively shallower environments. These data also have implications for the nature of the Plio-Pleistocene extinctions. The vast majority of the bivalve species are either morphologically similar or identical to extant taxa. Hence, much of the bivalve extinction across the Plio-Pleistocene boundary represents ‘pseudoextinction' rather than actual lineage termination.