REVISITING COPANO BAY, TEXAS: AN EXCEPTIONAL LONG-TERM RECORD OF ECOLOGICAL COMMUNITIES AND THEIR ASSOCIATED MOLLUSCAN DEATH ASSEMBLAGES

Changes in the diversity of fossil assemblages throughout the Phanerozoic influence how paleobiologists interpret the history of global biodiversity and the evolution of ecological interactions. Understanding the processes that influence death assemblage diversity and species abundances, such as time averaging (the mixing of noncontemporaneous remains) and differential rates of loss among species, is critical to understanding the fossil record. Generally, the taxonomic composition and rank abundance of dead benthic assemblages have been found to reflect the living community from which they were derived. However, time averaging has been hypothesized to cause the evenness (uniformity of abundance) and richness (number of species) of death assemblages to differ significantly from that of the corresponding living community due to both volatility of the current living community and mixing of different community states. Thanks to previous work by Staff et al. (1986), Copano Bay, Texas presents an exceptional research area for studying 1) the effect of living volatility on death assemblage diversity and 2) the stability of death assemblage diversity at a decadal time scale. Staff et al. revisited one site in Copano Bay every six weeks for 18 months in 1981-1982; to acquire a comparable time series, we have re-established a similar sampling program starting in October of 2004. Preliminary results indicate that current molluscan death assemblages are less even (using Hurlbert's PIE) than those collected 23 years ago. Both previous and current findings indicate that death assemblages are stable over a two-year sampling period, yet after 23 years, evenness has changed considerably. These results have significant implications for interpretation of the reliability of evenness and richness of local fossil assemblages. If death assemblage diversity varies significantly through time and also differs considerably from corresponding living communities, then perhaps local fossil assemblages are being interpreted beyond the limits of accuracy.