WHEN DOES A TAPHOCOENOSE BECOME A BIOCOENOSE? A STORM-GENERATED INLAND MOLLUSCAN ASSEMBLAGE, SAN SALVADOR ISLAND, BAHAMAS

On September 13, 1999, Hurricane Floyd (a Category 4 hurricane) passed approximately 40 km north of San Salvador Island, Bahamas. This study examines how Floyd-generated waves transported and deposited a molluscan assemblage derived from an inland saline lake of San Salvador, as well as how that assemblage changed taphonomically during the succeeding two years. Most significantly, our findings demonstrate how an allochthonous thanatocoenose can change to an autochthonous one over time, effectively blurring distinctions between a taphocoenose and a biocoenose. The initial molluscan assemblage consisted primarily of small, thin-shelled bivalves and gastropods transported from Osprey Pond into a shrubland north of the lake. The deposits, discovered in December 1999 and revisited in January 2002, were 30-cm thick and 10-m wide rippled beds with E-W orientations to ripple crests. Their location and orientations indicate that they were brought ashore by northward-flowing storm waves off Osprey Pond. Most bivalves were still articulated and undissolved in December, 1999, whereas gastropods showed much evidence of dissolution, suggesting that bivalves were still alive but most gastropods were dead when transported. Disarticulation and dissolution increased during the following two years; shells were also moved by bioturbating land crabs and ants. Interestingly, relative proportions of bivalve versus gastropod species nearly reversed during the past two years. In a sample taken in 1999, bivalves Anomalocardia leptalea and Polymesoda maritima comprised 72% of the assemblage, whereas gastropods Batillaria minima and Cerithidea scalariformis made up 27% (n=250). All of these species live in Osprey Pond, thus their presence and quantity in the initial assemblage were not surprising. In contrast, a sample taken from 2002 showed that 62% of the assemblage was composed of gastropods, including a significant increase in mangrove and terrestrial species such as Truncatella pulchella, Melampus coffeus, and Ellobium pellucens (n=1183). Our study thus implies that a storm-generated molluscan assemblage can quickly change from one indicating a saline lake to one suggesting more of a terrestrial assemblage and serves as a cautionary tale for paleontologists interpreting similar assemblages in the fossil record.