BREAKING BIVALVES: VALVE DURABILITY AND ITS EFFECT ON THE QUALITY OF THE FOSSIL RECORD

Palaeoecological analyses require high paleontological fidelity - a faithful archive of ancient source biota in resulting fossil assemblages. However, since species vary in size and skeletal durability, biases can be expected due to preferential preservation of certain species and size classes. To evaluate potential biases that may result from differences in size and thickness of mollusk shells, we used samples of surficial shell accumulations from seagrass habitats of the Gulf Coast of Florida. Bivalve mollusks were selected across thirty species that varied in their abundance. A standing crushing device with a flat head was used to create relative force data, measured in Newtons as a relative marker for how easy it is to destroy a shell. Even amongst specimens of the same species, there is considerable variability in shell strength. Environmental factors like encrustation (both chemical and biological) and prior breakage (fractures and/or drill holes) all caused variations in the shell strength of each valve. Twenty-five shells across eight taxa and four size classes were tested and displayed strong correlations between shell thickness and durability, with other morphological features (length, width, and height) having no significant affinities to the valve’s relative sturdiness. However, there was no significant relation between abundance of a given species and mechanical strength of its skeleton, suggesting that shell strength does not play an important role in governing the abundance of specimens in the currently forming fossil record of seagrass-associated mollusks. The lack of any notable correlation between abundance and fragility suggests that relative abundance of fossil mollusks is unlikely to be driven by variation in skeletal durability.