ENERGETICS OF THE MOLLUSCAN RECOVERY FROM THE END-CRETACEOUS MASS EXTINCTION

The ecological recovery of shallow marine molluscs following the end-Cretaceous mass extinction is spatially variable. Ecological trends documented from low and high latitude Cretaceous-Paleogene boundary (KPB) sites include an increase in the proportion of carnivores and infaunal molluscs, increased motility, and more deposit feeders. The ecological escalation of shallow marine assemblages in the recovery could be the result of a combination of increased primary productivity or lower assemblage abundance and decreased body volume to compensate for the higher metabolic requirements of a more active fauna. To address energy trade-offs in KPB extinction recovery, we perform estimates of assemblage-level energy required to support Late Cretaceous and Early Danian level-bottom fauna.

Bulk samples collected at half meter intervals before and after the KPB at a U.S. Gulf Coastal Plain site near Malvern, AR allowed us to differentiate intraformational variability from taxonomic, ecological, and energetic shifts associated with the KPB. Molluscs were identified to the species level when possible, functional ecological assignments were made at the family level, and linear shell measurements were used to approximate the body volume of each specimen. Mean metabolic energy of each assemblage was calculated from estimated body mass and a standard metabolic rate adjusted by the relative metabolism of modern representatives of each mollusc family or order.

In the stratigraphically lowest Paleogene samples, high richness and dominance of suspension-feeding oysters is similar to the composition of “disaster” opportunists observed at other KPB sites. Assemblage energy increases substantially two meters above the KPB. This change is associated with a change in fossil preservation and sediment type. The increase in assemblage energy is accompanied by a simultaneous compositional shift from oyster-dominated to deposit-feeding, infaunal clam-dominated ecosystems, increased diversity and abundance, and larger body size, which we interpret as increased primary productivity in the water column leading to more food available on the seafloor.The significantly larger body volume of post-extinction molluscs could be due to selection against small species observed at mass extinction events.