HOW DID MARINE INVERTEBRATES IN THE GULF COAST PLAIN (GPC) REBOUND AFTER THE END CRETACEOUS MASS EXTINCTION EVENT?

The Gulf Coast Plain (GCP) marine fossil assemblage at the Ouachita River near Malvern, AR contains mollusks from before and after the end-Cretaceous mass extinction event 66 mya. The Ouachita River section records a nearshore shelf environment of silty marls, fine sands, and indurated fossiliferous limestones interrupted by a coarse-grained sand with impact spherules containing a reworked Cretaceous fauna at the KPG boundary. Previous studies have documented a diverse vertebrate fauna at this locality including fish, sharks, turtles, and plesiosaur remains.

For investigation of the shallow marine invertebrate fauna, one-gallon bulk samples were collected, each at 0.5 m intervals through 7.5 m of Latest Maastrichtian Cretaceous strata and 4m of Danian strata. This high resolution allows for an evaluation of changes in fossil assemblage structure through time. Specimens from before and after the KPg boundary were analyzed for shell volume, taxonomic richness and diversity, functional ecology, activity quotient and metabolic energy.

Relatively high taxonomic diversity is evident in the Cretaceous with dominance of suspension and deposit feeding, infaunal, motile taxa. The loss of the smallest size classes of bivalves across the boundary represents indirect support for extinction selectivity for small body sizes in bivalves. The early Danian is characterized by a low-diversity taxa of predominantly epifaunal, sessile, suspension feeding oysters followed by a shift to larger, motile detritovores with higher metabolic energy demands. The two faunas of the Danian are interpreted as 1. A low-energy demand, opportunistic oyster fauna followed by 2. An increase in ecological complexity and metabolic demand. The oyster fauna does not represent a true “disaster” fauna as they are an abundant constituent of the Cretaceous assemblage, but experience a bloom of abundance in the earliest Danian samples. The later Danian mollusk assemblages are interpreted as indicative of increased availability of export productivity to the shelf, either from marine plankton recovery or combined with terrestrial sources. Our results suggest that ecological function was restored in shallow marine ecosystems before taxonomic diversity, consistent with results from the open-ocean records of the KPg.