BREAKING BIVALVES: A CASE STUDY ON THE PRESERVATION OF SEAGRASS MOLLUSKS

Analyses of paleontological data often rely on the assumption that the fossil record has high fidelity, thus representing a faithful archive of the original biological information preserved in fossil assemblages. However, because organisms vary in skeletal durability and size, preservational biases across species and size classes can be expected. In this study, differential preservation was tested directly by integrating empirical and experimental approaches. Samples collected from surficial shell accumulations at 15 sites in seagrass habitats along the Florida Gulf coast were used to evaluate potential biases that may result from differences in size and thickness of mollusk shells. Bivalve mollusks were selected across 21 species that varied in their abundance. Relative force data, measured in Newtons, was created using a flat-headed standing crushing device (SHIMPO Digital Force Gauge, 100 N) to quantify relative shell durability. Species varied in durability, although there was also notable variability within species. Environmental factors like chemical and biological encrustation, as well as prior damage caused by physical or biological processes (e.g., drill holes) may have contributed to intraspecific and interspecific discrepancies in shell strength. Eighty-one shells across 21 taxa and four size classes were subject to crushing experiments. The force at failure (Newtons) displayed strong correlations with two morphological proxies (shell thickness and mass/area ratio). No significant correlation was observed with body size or individual shell dimensions (length, width, and height). There was no significant relation between abundance of a given species in death assemblages and the mechanical strength of its skeleton, suggesting that shell strength does not play an important role in governing species abundance of the currently forming fossil record of seagrass-associated mollusks. The lack of notable correlation between abundance and fragility suggests that relative abundance of fossil mollusks in seagrass habitats may reflect ecological rather than taphonomic processes.