The shallow shelf sediments of the U.S. Gulf Coastal Plain record the geochemical, sedimentological, and paleontological record of multiple Cenozoic climatic events. The end-Cretaceous mass extinction (KPg) (66 Mya) and the Paleocene-Eocene Thermal Maximum (PETM) (~55.9Mya) each perturbed the global carbon cycle and left evidence of their biological impacts in the fossil record of the shallow shelf biota. At the KPg, a bolide impact led to a 30 year “nuclear winter” causing the extinction of primary producers disturbing the biological pump that connects surface ocean primary productivity to the deep sea. It took 1.8 My for carbon cycling to return to pre-extinction export levels and ~4 million years until diversity, size, and other disparate ecological traits were re-evolved in the open-ocean fauna. At the PETM, just 10 million years later, a 5 to 8°C hyperthermal warming event altered terrestrial weathering and resulted in migrations, extinctions, and originations in both terrestrial and marine ecosystems. Our understanding of the biologic impact of the KPg mass extinction and PETM are focused on deep-sea cores and terrestrial vertebrate records while high-resolution records of shallow shelf marine invertebrate fauna (mollusks, echinoderms, etc) remain understudied.

We use fossil deposits from the U.S. Gulf Coastal Plain that bracket the KPg and PETM events to compare the effects of carbon cycle perturbation on invertebrate species richness, proportional extinction and origination, ecological composition, shell size, growth rate, and inferred metabolic energy requirements. Across the KPg boundary, we see preliminary support for an increase in fossil assemblage energy, the result of ecological escalation (increased proportion of actively-mobile species) and a simultaneous increase in shell size. We interpret this increase in metabolic energy as a signal of increased primary productivity in the water column leading to more food availability to support an active shelf ecosystem in the post-extinction ecosystem. The significantly larger body volume of post-KPg molluscs could be due to selection against small species observed at mass extinction events. The extinction trigger and mechanism for KPg mass extinction are not an analog for modern climate change but can still provide insight into how long it takes the Earth System to recover from a major carbon cycle disturbance.

In contrast, across the boundary of the PETM hyperthermal event we observe little impact of the extreme warming event on shell size, functional ecology, or diversity of shallow marine mollusk communities. Limited extinction selectivity for mollusc shell size and contradictory trends in growth rates do not support a directional response of shelf faunas to this extreme warning event. The PETM severely underestimates projections for modern climate change; our anthropogenic input of carbon is happening 10x faster than most models of PETM carbon cycle perturbation. Our results of limited impact on shelf ecology therefore only represents a best-case scenario for predicting how shallow shelf faunas might respond to present-day climate change.