AMMONITE FAUNA AND SHORT-TERM SURVIVAL ACROSS THE K-PG BOUNDARY FROM A NEW SITE IN THE US GULF COASTAL PLAIN

Ammonoids are among the most prominent victims of the Cretaceous-Paleogene (K-Pg) mass extinction event (66 Ma). Studies from Europe and the Atlantic Coastal Plain, USA, suggest some ammonites may have survived briefly into the early Danian. A new K-Pg boundary site near Starkville, Mississippi, contains latest Maastrichtian to earliest Paleogene ammonites. The K-Pg boundary is marked by the contact between the uppermost Maastrichtian Prairie Bluff Chalk and the Danian Clayton Formation. Generally, the boundary is unconformable, undulating, and heavily bioturbated with Thallasinoides burrows. However, the stratigraphic succession across the K-Pg varies laterally. The Clayton Formation consists of silty marls with Pycnodonte pulaskensis, a generalist oyster that may represent a return to stable marine conditions during the ecological recovery phase following the K-Pg mass extinction. Two distinct units composed of sand rich in impact debris (spherules, tektites) crop out intermittently along the creek bed as channel-cut infill, most likely representing rapid deposition associated with the Chicxulub impact event. Reworked specimens of Eubaculites occur within both units of the event deposit. The event horizons are overlain by heavily bioturbated, lignite-rich, micaceous, muddy fine sands of the Clayton Formation. This unit appears to represent normal, post-impact sedimentation during the earliest Danian, most likely prior to the ecological recovery phase as defined by Hansen et al (2004). A well-preserved specimen of Eubaculites carinatus was recovered from this unit. The fragile shell is infilled with the surrounding matrix, indicating that it was not reworked from older units and may represent an early Danian survivor. This is the first evidence for short-term ammonite survival following the K-Pg extinction in the Gulf Coastal Plain. A record of ammonites across the K-Pg interval sheds light on the mechanics of ammonoid extinction, particularly the occurrence of Eubaculites carinatus in Danian strata above the facies widely interpreted as event deposits. This suggests that some ammonites in the Gulf Coastal Plain region may have survived the initial effects of the Chicxulub impact event.