LACK OF MAJOR FAUNAL CHANGE IN SHALLOW MARINE ASSEMBLAGES ACROSS THE PALEOCENE-EOCENE BOUNDARY IN THE U.S. GULF COASTAL PLAIN

The interval surrounding the Paleocene-Eocene (P-E) boundary was a time of major global environmental change and, in many habitats, significant evolutionary turnover. A long term temperature rise was interrupted by several geologically brief global warming events, the best known of which is the Paleocene Eocene Thermal Maximum (PETM, ~55.5Ma). At this time, warming of 5-8°C resulted in massive extinction of benthic foraminifera, increased turnover and morphologic change in planktonic taxa, and wholesale reorganization of terrestrial ecosystems. Relatively little is known, however, about the response of shallow marine macrofaunas to either the long term temperature rise or to the PETM. We have attempted the first comprehensive analysis of one of the world's best-preserved fossil sequences for this interval: the mollusc fauna of the U.S. Gulf Coastal Plain (GCP).

Perhaps surprisingly, the PETM appears to have caused few notable effects on the richness, ecology, and morphology of GCP faunas. At the species level, both synoptic taxonomic lists and sample-standardized abundance data show a significant but only slight decrease in richness that began in the late Paleocene and continued into the earliest Eocene. Origination rate also dropped below extinction rate across the P-E boundary, the only time in the early Paleogene to do so after the end-Cretaceous mass extinction. The ecological composition of faunas, however, showed no significant variation across the P-E boundary. Although the body size of some important gastropod clades declined across the boundary, this does not appear to be a general pattern, as an analysis of an abundant and diverse bivalve clade failed to find significant size differences.

Why might these subtropical shelf faunas be less sensitive to PETM environmental change? The early Paleogene was a time of global warmth. GCP mollusks were apparently able to adapt to rising background temperatures such that a spike to even more extreme conditions during the PETM had little effect. These results highlight the role of historical context in determining the severity of response to perturbation. As such, we hypothesize that PETM-like events during cooler periods in Earth’s history would be far more severe for shallow marine faunas, especially those in higher latitudes.