RECOVERY OF METABOLIC ENERGY DEMANDS OF A SHALLOW MARINE INVERTEBRATE ASSEMBLAGE FOLLOWING THE END-CRETACEOUS MASS EXTINCTION ARE DECOUPLED FROM THE RECOVERY OF TAXONOMIC DIVERSITY

An asteroid impact at the end-Cretaceous (66 Mya) resulted in the extinction of up to 70% of Earth’s species. The marine fossil record from the Cretaceous-Paleogene boundary (KPB) can yield insights into how ecosystems respond to rapid environmental change. A KPB shallow shelf marine invertebrate macrofauna from Malvern, AR fills a geographic gap between previously studied KPB sites in the Mississippi Embayment from the western (Texas) and eastern (Mississippi, Alabama) U.S. Gulf Coastal Plain. We use shelf assemblages at Malvern, AR to develop a Cretaceous-aged, pre-extinction baseline for diversity, functional ecology, and body size. In the early Danian recovery, we observe a disaster fauna replaced by an opportunist stage with increased body size.

Bulk samples of shallow marine invertebrates were collected from the upper Maastrichtian and lower Danian including a reworked spherule layer interpreted as the approximate KPB. Taxonomic and ecological similarity were used to compare pre- and post-extinction shelf assemblages. Shell volume and a generalized metabolic rate equation were used to estimate each assemblage’s basic energy requirements.

Facultatively-mobile, infaunal bivalves are abundant in the Cretaceous. Post-impact, suspension-feeding, epifaunal-cemented oysters dominated the shelf system at Malvern. Two meters above the KPB there is a significant increase in shell volume and a fauna dominated by motile, deposit-feeding nuculanids. A lack of diagnostic index fossils precludes more specific temporal constraint for this faunal transition. The initial dominance of suspension-feeding bivalves is similar to the functional composition of “disaster” opportunists documented in the Early Triassic mass extinction recovery and other KPB recovery intervals. The subsequent compositional shift to motile, deposit-feeders of significantly larger size is herein interpreted as a higher metabolic energy demand suggesting increased primary productivity in the water column and/or terrestrial run-off providing more detritus to the shelf system. In the Danian at Malvern, functional ecological guild occupation returns to pre-extinction proportional composition and Cretaceous level of mean metabolic energy; a functional recovery without a return to a pre-extinction levels of diversity.