PRELIMINARY RESULTS OF MICROCYSTIN (MT) AND SAXITOXIN (SXT) PRESERVATION IN FOSSIL MOLLUSKS OF THE LATE CRETACEOUS COON CREEK FORMATION LAGERSTÄTTE: IMPLICATIONS FOR A KILL MECHANISM PRODUCING POSSIBLE MARINE REPTILE DEADFALLS

The Upper Cretaceous Coon Creek Formation (CCF) lagerstätte of Western Tennessee is recognized for its pristine preservation of an abundant and diverse biota of invertebrates, vertebrates and rare plants. The overall environment of deposition for the CCF was a level-bottom, clastic-dominated reentrant depocenter within the Late Cretaceous Seaway with a high sedimentation rate. One of the taphonomic mysteries of the CCF type section (the “Old Dave Weeks Place” in McNairy County) is how parts of of five different individuals of air-breathing mosasaur, and possibly a newly discovered plesiosaur skeleton, died and were buried within the same small area of seafloor (one adult and one juvenile are located within 30 feet of one another). As these marine reptiles were air-gulpers, anoxia does not seem to be a mechanism of quick kill (and bottom sediments and fauna show no evidence of anoxia). What then is the kill mechanism that led to the “deadfall-like” preservation of these animals? The pristine nature of preservation of the CCF fauna opens the possibility of checking for evidence of other geochemical kill mechanisms, primarily the presence of saxitoxins (SXT) or microcystin (MT). Cyanobacteria and dinoflagellates were common in Cretaceous sediments and are known to produce these toxins that cause red tides and harmful algal blooms. There is a “shallow-time” fossil record of MT and SXT, but herein we report on the oldest identified occurrence. We tested several taxa of mollusks, enclosing sediment, and younger overlying soils to determine the occurrence and survivability of MT and SXT within the fossils of the CCF. We found a strong signature of over 10ug/gram MCT and over 7ug/gram SXT and in the bivalve Pterotrigonia and oyster Exogyra. Sediment levels of MT and SXT show lower abundance. Our preliminary study demonstrates the preservability of MT and SXT in the CCF fossil shell laminae and the potential to identify seawater chemistry kill mechanisms within the CCF.