286,953 CONCRETIONS, HOW DO YOU MEASURE THE MAZON CREEK? REVISITING THE VERACITY OF THE ASSEMBLAGE DIVIDE IN A PALEOECOLOGICALLY INFORMED TAPHONOMIC CONTEXT

The Mazon Creek Lagerstätte captures a diverse view of terrestrial, freshwater, brackish, and marine ecosystems through the exceptional preservation of fossils in siderite concretions. This deposit has provided enigmatic icons of evolution like the Tully Monster as well as unique insights into late Paleozoic coastal paleoecology. The accepted paleoecological paradigm of the Mazon Creek biota has been that of a Braidwood–Essex biotic dichotomy, wherein the former represents a freshwater ecosystem with heavy terrestrial input and the latter represents a marine-influenced prodelta preserving fauna like cnidarians, bivalves, worm phyla, and diverse arthropods. Here, we revisit this topic by analyzing nearly 300,000 concretions from 270 locations with complementary hierarchical clustering and multivariate ordinations. Our results support the Braidwood biota as a legitimate shoreward assemblage and provide evidence for further subdivision of the Essex biota into two distinct assemblages. The Will-Essex is a benthic assemblage dominated by clams and traces representing the transition between nearshore and offshore deposits, whereas the Kankakee-Essex is composed mostly of cnidarians and represents a marine ecosystem approaching the margin of the Mazon Creek taphonomic window. These refined insights into the Mazon’s paleoecology and paleoenvironment have enabled us to propose a newly informed taphonomic model. The recalcitrant Braidwood organisms, mainly plants, were subject to rapid burial rates while the animal-dominated Essex biotas typically had more labile tissues and were subject to slower burial rates. Ergo, the Braidwood fossils should record more complete preservation than the Essex, which were exposed for longer periods of aerobic decomposition. This hypothesis is supported by the higher proportion of non-fossiliferous concretions in the Essex localities versus those of the Braidwood. One anticipated test of this model will be a specimen-based search for demonstrable preservation changes in common marker taxa (ferns, cnidarians, shrimp, polychaetes) along the inferred paleoenvironmental gradient and a closer look at recorded categories denoting advanced degradation. Using standing census data, specific localities representing optimal taphonomic conditions—or preservational hot spots—can be identified regionally.