THIRTY MILLION YEARS AND THIRTY MILLION SHELLS: PALEOGENE DIVERSITY DYNAMICS OF U.S. GULF COASTAL PLAIN MARINE SHELF COMMUNITIES

Sediments of the U.S. Gulf Coastal Plain (GCP) provide an ideal setting within which to examine long-term diversity dynamics of marine shallow shelf communities. The Paleogene strata of the GCP represent a long and relatively complete record of marine shelf deposition and are characterized by the presence of rich and generally well-preserved shell beds throughout. Because preservation of original skeletal aragonite is common, an oxygen-isotope-based record of annual and seasonal temperature change may be obtained and refined. Thus, it is possible to track the in situ waxing and waning of diversity and changes in faunal composition over the course of some of the most pronounced environmental changes of the Cenozoic. This depositional setting offers the potential for a detailed comparative study of extinction boundaries, the relative importance of boundary versus ‘background' turnover, and the interplay between climate change, ecology, and evolution. Here we present a preliminary diversity record for the early Paleocene through the earliest Oligocene based on abundance data obtained from bulk collections. We then compare these data with published climate records supplemented by new isotopic data.

Abundance data drawn from the literature supplement our own field collections in fossiliferous strata from Alabama and Mississippi. We largely restrict analyses to glauconitic, sandy marls to control for facies change. Multiple bulk samples are taken when possible to produce replicate samples. Bulk samples are processed by wet sieving and fossils are tabulated.

Preliminary results indicate that overall, diversity remains relatively stable through much of the Paleogene, despite global climate perturbations and changes. One exception to this is a peak in diversity during the upper middle Eocene followed by a decline into the late Eocene. MDS analyses indicate that this decline in diversity is accompanied by a pronounced change in faunal composition. High resolution, isotope-based temperature records allow us to evaluate the role of regional annual and seasonal temperature in shaping diversity dynamics in the GCP. Understanding faunal responses to changing climate will require a closer examination of environmental factors, beta diversity, evenness, and the ecological characteristics of the fauna.