Quantifying taphonomic bias in molluscan death assemblages from the upper Chesapeake Bay: Patterns of shell damage

This study focuses on two main questions: (1) what types of shell damage occur in the death assemblage of upper Chesapeake Bay benthic mollusks; and (2) how does shell damage differ according to intrinsic factors such as life habit, shell mineralogy, and shell organic content. Extrinsic and intrinsic factors, ranging from the environment to shell composition, interact to influence the quality of fossil preservation. Our understanding of how extrinsic factors affect shell-damage profiles has improved dramatically with the development of taphofacies models, but the role that intrinsic factors play is still poorly understood. Molluscan death-assemblage material was obtained via box coring, identified, and assigned taphonomic damage states. The most common forms of shell damage were disarticulation, fine-scale surface alteration (FSA), periostracum loss, edge modification, and fragmentation. Four patterns were documented consistently across habitat types when shell damage was examined according to life habit and shell composition. Infaunal specimens exhibit significantly more severe damage due to internal FSA than epifaunal specimens. Calcitic specimens experience higher levels of external encrustation than noncalcitic specimens. Specimens with high levels of shell organics experience significantly more fragmentation and edge modification than specimens with low levels of shell organic content. The direction and degree to which other damage variables differ among intrinsic categories vary according to the variable assessed or the habitat type sampled. In the northern sites (i.e., deeper water), all the variables that recorded differential damage according to shell-organic content recorded greater damage in specimens with high shell organic content, suggesting that the latter may be experiencing selective removal from the death assemblage.