Faunal stability and environmental change in the middle Eocene Gulf Coastal Plain

The pattern of coordinated stasis is characterized by marked stability in the evolutionary and ecological composition of fossil faunas. Intervals of coordinated stasis persist for several million years and exhibit very little speciation, extinction, immigration, or ecological change throughout. Intervals of stability are separated by brief episodes of turnover during which all these parameters are greatly enhanced. Faunal stability often persists despite evidence for environmental change suggesting maintenance of stability by ecological interactions. Conversely, turnovers often occur during times of pronounced environmental change, suggesting that fauna-wide extinctions may be governing pattern. Coordinated stasis is thought to be relatively common in the fossil record, reinforcing claims for stability during most of life's history.; I conducted an interdisciplinary, field-based investigation in the middle Eocene Gulf Coastal Plain (Lisbon Formation, Claiborne Group) in order to test the generality of coordinated stasis. Testing involves a multi-faceted analysis of faunal change through time, combined with an examination of concurrent change in the physical environment. In Coastal Plain deposits, the most readily accessible variables of environmental change are sea level and temperature. An outcrop-based sequence stratigraphic analysis reveals four major third order sea-level cycles within the Claiborne Group supercycle, three of which occur in the Lisbon Formation. The most significant shift in sea level occurs at the middle-upper Lisbon contact. Oxygen isotopic analyses of biogenic carbonate reveal an abrupt positive shift of 1 per mil at or near the middle-upper Lisbon contact, suggesting cooling of about 4{dollar}spcirc{dollar}C.; Taxonomic range zone data at both the species and genus level show an interval of relative faunal stability within the Lisbon Formation that persists for the duration of the upper Lisbon main depositional sequence (roughly 3 million years). Multivariate statistical tests based on relative abundance data reveal four robust groups of samples. Groups consist of samples from similar facies in different stratigraphic intervals, indicating that facies has a stronger control on sample similarity than does time. Analysis of variance tests on three of the four sets of samples (ranging from middle through upper Lisbon in age) show no significant difference through time using rank abundance data; relative abundance data does show evidence for change. Stability of both taxonomic composition and rank abundance structure for at least 3 million years during the middle and upper Lisbon suggests that coordinated stasis is present. The fact that stability persists across the middle-upper Lisbon contact despite evidence for significant temperature and sea-level change tentatively supports claims for internal regulation of coordinated stasis.