EVIDENCE THAT ALGAL TOXINS PLAYED A ROLE IN THE DEATH OF MOSASAURS FOUND IN THE UPPER CRETACEOUS COON CREEK FORMATION, TENNESSEE

The Upper Cretaceous Coon Creek Formation (CCF) of western Tennessee has been designated a lagerstätte because of its exceptionally well-preserved and diverse assemblage of shallow marine fossils. Microfossils have been identified and confirm a Late Cretaceous (Campanian) age for this unit and include a combination of warm and cool-water calcareous nannoplankton. The occurrence of several mosasaur fossils and a plesiosaur fossil in close association at the type locality raises the question of what caused the death of multiple air-breathing marine reptiles in such proximity that they were preserved in the same small area of the seafloor. In modern ecosystems, marine vertebrate mass die-offs are often ascribed to red tide or harmful algal bloom (HAB) events. Mollusk shells, sediments, and microfossils from multiple horizons within the CCF were collected to test whether algal toxins may have played a role in the deaths of marine vertebrates at the site. Forty-two dinoflagellate cysts were identified in the mosasaur stratum and consist primarily of gonyaulacoid-peridiniods. Extant dinoflagellates from these groups are capable of producing several algal toxins. Scanning electron microscope images of the CCF mollusks have confirmed that they are exceptionally well-preserved, consisting of primary aragonite prisms and fiber, which suggests very little diagenesis of the shells has occurred. CCF shells and modern shells with and without known exposure to algal toxins were analyzed for three toxins: microcystin (MC), brevetoxin (BVT), and saxitoxin (SXT). The modern shells raised under clean conditions had no detectable toxins, but the shells exposed to BVT or SXT red tides were positive for their respective toxins. Fossil shells from the mosasaur stratum were positive for all three toxins, and filter feeding mollusks showed higher toxin levels than non-filter feeding taxa. The concentration range of SXT in CCF shells (0.18 to 4.1 ng/gram) was equivalent to modern SXT-exposed shells (0.12 to 5 ng/gram). The concentration range of BVT in CCF shells (0.09 to 0.41 ng/gram) was below the range of modern BVT-exposed shells (0.55 to >3 ng/gram). These preliminary results support the hypothesis that HABs may have played a role in the death of the large marine reptiles at CCF.