THE HOLOCENE FOSSIL RECORD OF CERION LAND SNAILS ALONG EASTERN LONG ISLAND, BAHAMAS: EVIDENCE FOR RAPID FAUNAL CHANGE

The land snail genus Cerion is known for its extreme geographic variation in shell morphology. Very different forms occur adjacent to each other, usually with steep character gradients between them. The central east coast of Long Island, Bahamas has the greatest known diversity within the genus Cerion, with the entire, non-overlapping ranges of three endemic species flanked by a broader ranging species (4 transition zones on the order of 20-100 m) all contained within a 12 km region of coastline.

In addition to our studies of the genetic variability of these taxa throughout their ranges and across these transition zones, we have sought to follow these geographic patterns back in time through excavation and analysis of the Holocene fossil record of these snails. We have excavated pits (to 2.35 m in depth) throughout this 12 km region of coastline, as well as sampled from sand quarry walls and erosion cuts. Cerion shells were plentiful throughout most levels in these sands.

Cerion shells were individually dated by amino acid racemization analysis, using the D-alloisoleucine/L-isoleucine (A/I) ratio. The in situ rate of racemization was determined by calibration against a series of radiocarbon-dated shells. This, in turn, allowed estimation of individual shells from their A/I ratios. Corrections for radiocarbon age anomalies for the shells (due to ingestion of old carbonates) was based on radiocarbon analysis of live-collected, pre-bomb shells as well as by regression analysis of A/I values against uncorrected radiocarbon ages of fossils. These analyses showed that the oldest samples dated to ca. 4000 yr BP, and that complete temporal sequences up to the present were recovered from some of our excavation sites. In every case, a substantial mixture of shell ages was found at each level, with average age increasing with depth. Very substantial changes in shell morphology are documented in the temporal sequences constructed for most of the sites excavated. At every site, shells older than ca. 3000 yr are substantially different from living forms. More recent changes are also evident at most sites, with living individuals significantly larger than all but the most recent (<200 yrs) fossil shells. We evaluate several possible scenarios for the origin of the observed pattern of spatial and temporal variation in Cerion distribution.