Taphonomic processes in modern freshwater molluscan death assemblages: Implications for the freshwater fossil record

Abstract

Unionid life and death assemblages in several streams and reservoirs in east-central Ohio were investigated to examine taphonomic processes in freshwater environments. Twenty-six species were collected. As in most marine environments, most living molluscan species were represented in the death assemblage. Fourteen species were found in both the life and death assemblage at at least one study site, one species was found only in the living community, and one species was found live at one site, dead at another Ten species were found only in the death assemblage. With one exception, the time-averaged death assemblage was more species rich than the living molluscan community based on single-sample census data. The death assemblage preserves the rank orders (live/dead) of abundance and biomass of the preservable molluscan components in some environments but not in those the most environmentally disturbed. Unionid live/dead fidelity is high and compares favorably with live/dead fidelity in marine and estuarine environments. In nine of thirteen within-study site comparisons of life and death assemblages, there were no significant size differences between the single-census life and time-averaged death assemblages. These results are in contrast to estuarine assemblages along the Texas coast where 13 of 15 comparisons of the death assemblage and the estimated mortality from the living community indicated little similarity between life and death assemblage size distributions. It is likely that death assemblage formation and associated taphonomic processes, coupled with the unique life-cycle of unionid molluscs, are distinctive in freshwater environments. Recognition of the unique taphonomic characteristics of unionids should prove useful in freshwater paleocommunity analyses.