THE ECOLOGICAL DYNAMICS OF THE FIRST THIRTY MILLION YEARS OF THE CENOZOIC IN THE GULF COASTAL PLAIN

The early Cenozoic (Paleocene through early Oligocene) contains several prominent climatic perturbations known to have influenced the evolutionary and ecological trajectories of many organisms. Within the Gulf Coastal Plain (GCP - Mississippi, Alabama, Texas, Louisiana, and Georgia) these climatic changes are one of several factors ascribed to control the diversity and turnover dynamics of marine molluscan communities. We compare the abundance structure and ecological characteristics of assemblages to global and local temperature estimates to determine the role of climate change in shaping the ecological history of these communities. The GCP represents a unique setting in which to study these questions because it contains some of the world's best preserved and most diverse Paleogene assemblages. Fossils are preserved in largely similar environments through time, allowing us to restrict analyses of local assemblages to primarily one facies, that of sandy, glauconitic marls. Our data consist of field and literature counts of species abundance tallied from over 60,000 individuals in twelve formations.

Preliminary analyses discussed in a companion abstract by Baugh et al. show that diversity remained surprisingly constant over much of the thirty million year study interval, despite high levels of turnover between most beds and all formations, indicating a poor correlation between diversity and global temperature. Abundance structure shows a better correlation with temperature, with some warmer intervals, such as the late Paleocene, appearing to have relatively less dominant assemblages than cooler periods, such as the late Eocene and early Oligocene. Trophic guild characteristics of taxa, such as feeding mode, life habitat, and motility are tabulated for assemblages and compared to temperature estimates. Climate change seems to exert a stronger control on the ecological characteristics of local GCP assemblages than it does on their diversity.