The littleneck clam Protothaca staminea in Argyle Creek and Argyle Lagoon on San Juan Island (Washington, USA) provides an ideal opportunity to test the effect of life habits on the taphonomic signature of shells. This bivalve exhibits two different modes of life in adjacent habitats: infaunal in muds and muddy sands (Argyle Lagoon) and free epifaunal on gravels (Argyle Creek). The mode of life significantly affected the taphonomic signature of both live and dead shells. Epifaunal P. staminea exhibit more damage than infaunal shells, suggesting that the infauna has a greater fossilization potential and may be more heavily affected by time-averaging than the epifauna. Both live infauna and epifauna suffered important taphonomic modifications after death, especially on the internal surface of the shell, but infauna did not reach the high level of damage acquired by the epifauna. In Argyle Creek, taphonomic agents were more effective at the sediment-water interface than within the sediment.
Because mode of life has a significant influence on processes of preservation, different taphonomic patterns in fossil bivalves do not necessarily imply different postmortem histories of shells, even when the taphonomic analysis is restricted to a single species. Some external modifications and internal shell damage cannot be regarded as unambiguously postmortem since edge and color modification, external corrasion and encrustation, and internal bioerosion can occur during the lifetime of the animal. Finally, this paper shows that a single bivalve species can exhibit more than one mode of life even within closely proximate environments. The typical mode of life is reflected in shell morphology while the secondary one is not. Thus, functional-morphology studies of extinct species can lead to incomplete interpretations of the range of a bivalve's life habits. An integrated approach combining functional morphology, comparisons with close relatives, and lithofacies analysis can be useful in paleoecological interpretations of extinct bivalve species.