A COMPARISON OF THE TAPHONOMY OF MODERN AND ANCIENT MOLLUSKS IN SEA GRASS BEDS

Delineation of taphonomic signatures – taphofacies - on fossil material has been shown to be an effective complement to sedimentologic and paleontologic data. Since physical, biological and chemical processes can leave a potentially preservable imprint on shell material, comparative taphonomy of taphofacies can determine the original habitat of fossilized assemblages when existing data are inconclusive. By looking at variations in preservation of specific organisms, inferences about different depositional environments can be made. However, the degree to which taphofacies analysis can indicate a specific depositional environment for a fossil assemblage has not been assessed adequately. In this study, a comparison was made between the taphofacies present on mollusks obtained from a modern sea grass bed and those obtained from a facies interpreted to represent a Pleistocene sea grass environment. If the distribution of taphonomic characters described from the modern assemblage are matched by those obtained from its Pleistocene analogue, then taphofacies for the modern environment have been preserved. Five hundred shells were collected from a Pleistocene sequence exposed in a quarry on San Salvador Island, Bahamas. Each shell was examined under a microscope and the degree of coverage by a variety of taphonomic characters was measured. No significant (a=0.05) difference in the distribution of characters was measured between the fossil and modern assemblages, although differences in the degree of coverage of some of the characters existed. Fragmentation values in the Pleistocene sample were elevated, due in part to laboratory methods: removal of the mollusk shells by chisel may have broken off fragments of previously intact shells. Additionally, dissolution values in the Pleistocene sample may also be elevated because acid was used to free the shells from surrounding matrix. Although these differences in taphonomic averages are noteworthy, the similarity in distribution of taphonomic characters between the modern and ancient assemblages suggest that, for recognizing depositional environments whose dominant organism is inherently non-preservable (sea grass), application of the taphofacies concept is especially useful.