EFFECT OF ENVIRONMENT ON SHELL PRESERVATION OF AN INTERTIDAL BIVALVE: IMPLICATIONS FOR TAPHONOMY

Examination of modern organisms can provided insight into fossil preservation in subtly different depositional environments.  Inspection of *Donax gouldii*, an intertidal bivalve, reveals numerous post-mortem shells with the outer prismatic layer removed along concentric growth bands.  The intertidal zone where *D. gouldii* is found experiences high wave-energies and periods of desiccation; the post-mortem band stripping observed may be caused by mechanical (breakage) or chemical (dissolution) processes.  Chi-square tests were performed between five San Diego county beaches on post-mortem *D. gouldii* shells.  A difference between the proportions of stripped versus non-stripped shells was observed (p < 0.001), which may be a function of the beach being open or protected from strong wave action.

*Donax* has a poor fossil record but resurgent populations have been observed in some species > 20,000 individuals/sq. m (Coe, 1953).  Field (1974) suggested that the absence of *D. variabilis* deposits along the Florida coastline may be due to shell destruction from reworking.  *Donax* deposits have been assumed to represent open shorelines.  Statistical comparison performed on the shells, however, suggests that preservation of *D. gouldii* shells is not along open shorelines but within surf protected zones.

This study further investigates the differences between open shorelines and surf-protected beaches by 1) establishing counts for live specimens/sq. m along a trend perpendicular to the surf.  Establishing live counts for each beach will identify if differences are being influenced by low population numbers.  Heavy surf can influence populations by removing them from the beach or burying them too deep (Coe, 1953); 2) analyzing and comparing sediment samples with chi-square.  Sediment samples generally reflect environmental conditions with coarser sediments in higher energy environments; and 3) determining by taphonomic experiments if band stripping is a result of chemical dissolution or mechanical wear.  One half of the valves from paired shells were placed into acidic acid to simulate chemical dissolution.  The other valves were placed on a stirring plate with sand and water to mimic the mechanical actions typical of wave-action.