“GHOST” FACIES: HOW CHANGES IN LIVING COMMUNITIES CAN INFLUENCE THE COMPOSITION AND DIVERSITY OF MOLLUSCAN DEATH ASSEMBLAGES (COPANO BAY, TEXAS)

Although fossil assemblages have proven to be a reliable and powerful tool for discerning depositional environments in Earth's past, they are nevertheless not strictly equivalent to living communities. Changes in living assemblages during time averaging or addition of exotic remains due to postmortem transportation can, in theory, influence the composition and diversity of death assemblages. Evaluating the potential influence of these mixing processes requires data on temporal changes in the composition and spatial distribution of species associations, both live and dead, in a region over the timescale of death assemblage accumulation. Previous studies of molluscs in Copano Bay, Texas provide the necessary baseline information. Ordination of species lists from 93 sites collected throughout the bay as part of a 1976 survey indicates the presence of three primary living species associations corresponding to shell-, sand-, and mud-dominated substrates. Analysis of dead data from the same sites reveals an additional association that is not represented in the living communities of 1976. Species in the mysterious dead association are typical of more saline conditions than were normal in the bay from 1971 to 1976, but are consistent with higher salinities reported between 1948 and 1965, suggesting that the death assemblages retain a “memory” of previous ecological conditions. However, this also implies that biofacies patterns in the rock record may not be strictly reliable recorders of ecological landscapes as living organisms experienced them. Using the 1976 study as a starting point, ongoing field work in Copano Bay is aimed at identifying the processes that cause mixing during death assemblage accumulation. The result will shed light on the degree to which such “ghost” associations may be contributing to the fossil record.