ENVIRONMENTAL AND ECOLOGICAL CONTROLS ON FAUNAL TURNOVER IN THE PALEOGENE GULF COASTAL PLAIN

Climate change has long been considered a major driver of origination, extinction, and population restructuring in macroinvertebrates, however relatively few studies have tested these relationships quantitatively. The Paleogene represents an ideal system in which to study the effects of climate change on organisms because it contains gradual and rapid increases and decreases in temperature, including a sudden (~ 200 Ky) rise of about 5 degrees Celsius worldwide, the Paleocene Eocene Thermal Maximum (PETM). Superimposed on the response of communities to temperature change is recovery from the Cretaceous-Paleocene (K-P) extinction. This project aims to categorize and separate the ecological effects of these perturbations on shallow shelf molluscan communities preserved in the eastern portion of the Paleogene Gulf Coastal Plain (Mississippi and Alabama). In particular, the relationship between assemblage level properties, such as diversity, evenness, and species dominance, and temperature is explored in comparison to the time it takes communities to recover from the K-P extinction. The rapidity and magnitude of the PETM exceeds all other known climatic changes during this time period. Thus, initial analyses focused on quantifying faunal turnover and population structure changes resulting from the PETM and comparing these effects to ‘background' times of less severe environmental change and to the interval immediately following the K-P.

The data used in this study include both field and literature based abundance collections ranging from the earliest Paleocene through middle Eocene. Approximately 15,000 individuals of about 300 species from seven formations were identified to species level when possible. While the PETM appears to have caused substantial species level turnover when compared to background times, higher taxonomic levels were unaffected. Similarly, the diversity, abundance structure, and ecological composition of communities remained unchanged before and after the PETM. Preliminary analyses indicate that early Paleocene communities were fundamentally different from those of the rest of the Paleogene, implying that while climate change certainly affected communities, other factors, most likely the recovery from the Cretaceous-Paleocene extinction, were of greater importance.