DID AMMONITES SURVIVE FOR A BRIEF TIME AFTER THE K/PG MASS EXTINCTION ON THE U.S. GULF COASTAL PLAIN?

Ammonite survivors following the Chicxulub impact at the K/Pg boundary have now been reported at several sites globally, including New Jersey, USA; Maastricht, Netherlands; and Stevns Klint, Denmark. However, the conditions of survival are poorly characterized (e.g., duration, abundance, paleoenvironmental context of survivors). We document multiple specimens of the genus Eubaculites from early Danian sediments at a newly discovered K/Pg boundary site in northern Mississippi. Here, the boundary is marked by an impact spherule-rich ‘clastic unit’ overlain by burrowed sands that crop out as ‘channels’ intermittently along a creek. This unit is similar to K/Pg ‘channel sands’ documented at the well-known Moscow Landing site in Alabama. The clastic unit is composed of up to 30 cm thick impact ejecta-rich layers and lenses with reworked Cretaceous fossils. It is overlain by a ~1 m thick ‘burrowed unit’ composed of highly bioturbated, lignitic, micaceous, muddy quartz sand that increases in carbonate content up-section. Early Danian planktonic foraminiferal zones P0, Pα and P1a are present within the clastic and burrowed units and provide an estimate for the duration of ammonite survival. Eubaculites are scattered throughout the burrowed unit, with many specimens occurring ~60-80 cm above the base within the Pα biozone. Zone P1a begins ~1 m above the base of the burrowed unit and the Pα/P1a transition occurs ~200,000 years after impact. Eubaculites specimens are frequently infilled with the surrounding burrowed unit matrix. This suggests that they are not reworked from older strata and were most likely living during the earliest Danian. Additionally, Eubaculites co-occurs with several early Danian taxa such as Cucullaea macrodonta and the generalist oyster Pycnodonte pulaskensis. These findings suggest that despite being prominent victims of the K/Pg mass extinction, some ammonite genera survived for a short period of time (10’s – 100’s kyrs) after the Chicxulub impact event.