COMMUNITY CHANGES IN SHALLOW BENTHIC INVERTEBRATE ECOLOGY AT THE K/PG BOUNDARY SITE AT MALVERN, ARKANSAS

The geologic record documents two major disturbances to the Earth System near the Cretaceous-Paleogene (K/Pg) boundary 66 Mya: volcanic outgassing from the Deccan Traps and the impact of a bolide at Chicxulub. Changes in benthic macroinvertebrate assemblages after mass extinction events commonly include reduction in body size of surviving organisms and preferential survival based on ecospace occupation. This study examines patterns of ecological change throughout the post-extinction Danian recovery at a K/Pg boundary section in Malvern, Arkansas USA.

The Ouachita River outcrop has been identified as a shallow marine site within the Mississippi embayment of the Gulf Coastal Plain (GCP). Evidence for the K/Pg boundary includes ammonite and dinoflagellate biostratigraphy as well as a sandy event bed with impact spherules. This study collected bulk samples starting at the K/Pg event bed and continuing every half meter through 4 meters of Danian outcrop. Ecospace occupation (motility, feeding, and tiering) were determined for macroinvertebrate mollusk specimens. Linear shell measurements were converted to shell volume with specimens modeled as ellipsoids and changes in body size distribution for each sample were compared using Wilcoxon rank.

The proportional abundance of each life mode shows a distinctive shift from an Ostreidae oyster-dominated assemblage in the first 1.5 meters to a less taxonomically rich community composed of abundant Nuculana and other mobile, infaunal, suspension and deposit-feeding bivalves. This assemblage persists for the remainder of the sampled interval. The shift in functional ecology is matched by a significant increase in bivalve body size. The oyster-dominated interval is similar to disaster assemblages documented elsewhere in the GCP. )The development of a more mobile and infaunal Danian mollusk assemblage at Malvern, AR is consistent with previous community ecology studies in the recovery from the end-Cretaceous mass extinction. Body size increases throughout the Danian along with more active life modes is interpreted as increased energy availability in the benthic ecosystem at this site.