The effect of morphology on postmortem transportation of bivalves and its taphonomic implications

Fossilized shell assemblages are often the result of postmortem transportation. It is therefore crucial to identify biases introduced due to differences in the hydrodynamic properties of shells to ensure the validity of ecological interpretations. In a flow tank study with the bivalve Donax scortum Linnaeus, 1758, we found that shell size, shape, and ornamentation played an important role in dictating the hydrodynamic properties of shells. This study demonstrates that threshold current velocity for the entrainment of a convex-up shell is generally determined by its size, a result corroborating previous findings. We found that a smaller shell is mobilized with a lower velocity compared to a larger one. Additionally, we found ornamentation of a shell also played a significant role in transportation. Unlike previous studies, we have demonstrated that altered smooth shells require a higher velocity for transportation compared to fresh shells with pronounced ornamentation. The movement trajectory of a shell depends on its asymmetry. The right and left valves of a single individual are deflected in distinctly different directions. This study shows that the extent of such deflection is dependent on the size of the valve and the velocity of the flow. Using a simulation based on our experimental data, we have also demonstrated the effect that transportation bias can have in modifying a shell assemblage. The results of this study underscore the concept that the final distribution of shells following a transportation event may yield a bivalve population significantly different from the original one.