A MULTI-SCALE BIOTIC ANALYSIS OF THE PALEOCENE-EOCENE THERMAL MAXIMUM SHOWS LITTLE IMPACT ON SHALLOW MARINE MOLLUSKS

The Paleocene-Eocene Thermal Maximum (PETM) (~56Ma) is associated with taxonomic and ecological crisis in benthic foraminifera and terrestrial ecosystems but the shallow marine fauna is understudied. We used mollusk-rich deposits from the Paleogene Gulf Coastal Plain (GCP) to examine the impact of the PETM on richness, extinction and origination, and ecological composition.

We performed a multi-scale evaluation including taxonomic and ecological analyses of database and museum collections and a clade-level study examining body size, growth rate, and morphology in two families; turritellid gastropods and venericard bivalves. A ~1My hiatus in the GCP record precludes a high resolution study directly at the boundary.

Rarefied richness shows no change across the boundary and time series analysis (PaleoTS) on 5 bins (3 Paleocene, 2 Eocene) supports an unbiased random walk. Change point analysis of the abundance distribution of ecological guilds shows evidence for significant differences between each geologic horizon. NMDS and K-means clustering indicate bivalve and gastropod groups were ecologically similar across the boundary.

At the clade level, turritellid gastropods experience a loss of 5 of 7 lineages across the PETM including a complete loss of larger taxa. Seasonal variation in oxygen isotopes shows that Paleocene turritellids were 4x larger than Eocene shells despite the same maximum age of 1.5-2 years. In contrast to turritellids, venericard bivalves show an increase in taxonomic diversity across the boundary and reach their maximum body size in the Late Paleocene but do not show a significant decrease in body size across the boundary.

In summary, our multi-scale analysis of mollusk diversity and ecology shows no substantial change across the PETM, barely different from the other geologic boundaries included in the study. Possible explanations include pre-conditioning by surviving the end-Cretaceous mass extinction, adaptation to warm, shallow-water environments of the Late Paleocene, or the transport of dissolved CO₂ into the deep sea reservoir sparing the shallow marine environment. While the PETM underestimates projections for anthropogenic climate change, results from ecological studies can represent a “best case scenario” for predicting the modern impacts of global warming.