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.