DIVERSITY AND LIFE MODES OF BIVALVES FROM LA BELLE, FLORIDA, WITH IMPLICATIONS FOR THE PLIO-PLEISTOCENE EXTINCTION

Western Atlantic molluscs suffered a series of Plio-Pleistocene extinctions, but extinction causes and dynamics may have varied geographically. Productivity decline noted in the Caribbean was not evident in the Carolinas based on life modes of Plio-Pleistocene bivalve genera. A graduate paleoecology class at University of North Carolina Wilmington analyzed Pleistocene bulk samples collected 3.5 km west of La Belle, FL (loaned by the American Museum of Natural History). Collectors Squires and Heaslip assigned the samples to the Caloosahatchee Formation; the overlying Bermont Formation had not been distinguished from the Caloosahatchee at time of collection (1955). The samples were collected from spoil banks, and no additional stratigraphic information is available, but the fauna lacks some typical Caloosahatchee taxa (e.g., sinistrally coiled cone shells), suggesting the fauna is at least in part post-extinction (Bermont).

Previous UNCW classes identified ~8000 bivalve specimens, focusing on larger taxa. The 2014 class identified remaining bivalves >5mm, bringing the number of specimens to ~26,000. Richness, dominance, and evenness were calculated using PAST and rarefaction curves were used to compare the assemblage to Plio-Pleistocene assemblages of the Carolinas (data at genus level). The La Belle samples included 49 bivalve genera; dominance of the assemblage was 0.429. The assemblage had a very low evenness (0.094) due to high relative abundance of Transennella (~15,500 specimens) and Chione (~6300 specimens). Evenness calculated without Transennella remained low (0.139). Rarefaction indicates the Florida assemblage is less rich than any assemblages from the Duplin or Waccamaw Formation of the Carolinas, when the dominant genus is omitted. When Transennella is included, La Belle richness is most similar to an upper Waccamaw (post-extinction) assemblage from Myrtle Beach dominated by Mulinia.

Life modes (diet, substrate relations, attachment, mobility) were assigned to La Belle species based on the NMITA database. As in the Carolinas, suspension feeders dominated the fauna (96% of specimens; 75% excluding Transennella and Chione), suggesting lack of a productivity crisis coincident with the La Belle fauna. Unattached, actively mobile, infaunal siphonates represented >96% of specimens.