Paper No. 6
Presentation Time: 10:05 AM
PHOSPHATIC DECAPOD LAGERSTÄTTEN OF THE SOUTHEAST
Repeated assemblages of exquisitely preserved fossil crabs, lobsters, and shrimp occur in the Cretaceous of the Western Interior, Mississippi Embayment, and on the Coastal Plains, all represent Phosphatic Decapod Lagerstätten. The Dakoticancer Lagerstätten from the Pierre Shale of South Dakota, form a prime Phosphatic Lagerstätte Model preserving beautifully preserved, articulated decapods in phosphatic concretions. When oxygenated, the nutrient-rich, muddy bottom supported local population explosions of annelid worms that triggered local population explosions of decapod crustaceans, particularly crabs, which fed on their abundance. As bottom conditions changed, or volcanic ash blew into the waters, and/or as decapods died of normal causes, the abundant crustaceans were killed and became buried within the soft mud bottom where the very worms they had been feeding upon began feeding on their corpses, recycling their bodies, burrowing into legs and body cavities though thin articulating membranes, depositing trails of fecal pellets along their burrows. As the boundary between oxygenated and anoxic mud fluctuated, sulfur and phosphate mediating bacteria, e.g. Thiomargarita namibiensis, described (Schultz et al., 1999) from the Namibian coast, a colossal bacterium (nearly a millimeter in diameter!) accumulated intracellular polyphosphates under aerobic conditions and released phosphate under anoxic conditions, reaching into the substrate 3 cm beneath the water/sediment interface (Schulz & Schulz, 2005: 416). This process created pore water supersaturated in phosphate that precipitated as phosphorite. The release of phosphate by these organisms explains the accumulation of the repeated South Dakota Dakoticancer Assemblages and similar decapod konservat-lagerstätten described from Mississippi, Alabama, Montana, North Dakota, and New Jersey; including the Blue Springs Coon Creek Dakoticancer Lagerstätte of Northern Mississippi, the Ripley Avitelmessus lagerstätten of Northern Mississippi and Tennessee, the Braggs Avitelmessus Lagerstätte of Alabama and possibly an Avitelmessus Lagerstätte from the Pee Dee Formation of North Carolina.