A deeper understanding of the ways in which ecosystems respond to climate change on long and short time scales is necessary because our planet is warming and will continue to do so in the near future.  Deep time faunal studies provide a necessary complement to historical research on how climatic change affects organisms. The Paleocene-Eocene Thermal Maximum (PETM, ~55 million years ago) was a rapid greenhouse warming event triggered by the addition of a massive amount of carbon dioxide to the atmosphere, most likely derived from the catastrophic dissociation of methane hydrates in deep-sea sediments.  As such, it represents the best geological analog for current warming.  The impact on global climate was profound, with effects clearly recorded both on land and in the oceans.  Tropical sea-surface temperatures warmed by 5°C and high latitudes by double that.  In the deep sea, warm bottom water, with consequently lower dissolved oxygen, combined with an increase in acidity to produce one of the most severe extinctions in benthic foraminifera in the last 100 million years.  Surviving taxa tended to be small, opportunistic forms characteristic of upwelling regions.  Planktonic assemblages were also affected, with foraminfera showing a spike in speciation. The productivity of open-ocean calcareous nannoplankton temporarily decreased across the PETM, coeval with increased nutrient availability in shelf areas, but recovered during the later stages of the event.
Results/Conclusions

As summarized above, a substantial body of work details the effects of the PETM on oceanic ecosystems, due in large part to high-resolution records from deep-sea sediments.  Almost nothing is known, however, about how shallow-shelf ecosystems responded.  Here we report data on mollusk faunas from the US Gulf Coastal Plain that straddle the PETM as identified with organic biomarkers and biostratigraphy.  The boundary itself is not fossiliferous, but shell beds above and below preserve the long-term biological consequences of this geologically short-lived event. Consistent with earlier work by Dockery, our sample-standardized faunal data show evolutionary turnover across the transition, but turnover is no greater than that seen at other unit boundaries in the Gulf.  In addition, taxonomic diversity, ecological structure, and abundance distributions of earliest Eocene faunas are nearly identical to those of the latest Paleocene.  These results suggest that, like the response seen in nannoplankton biotas, the PETM created only transient disturbances to nearshore assemblages.  Those immediate effects, more relevant to today’s scenario, will remain unknown unless skeletal faunas are recovered at the boundary.