COMPARING PRESERVATION OF DIFFERENT HISTOLOGIES: MAZON CREEK-TYPE FOSSILS OF THE SUN SPOT MINE, ILLINOIS, USA

The localities of Mazon Creek Lagerstätte, Francis Creek Shale Member of the Carbondale Formation, provide a unique preservational window, with soft-bodied organisms preserved as compressions encased in siderite nodules. This taphonomic style captures a wide range of terrestrial, shallow marine, fresh, and brackish water flora and fauna. Our samples were collected from the now defunct Sun Spot Mine of Fulton County, Illinois, and obtained from the collections of the Illinois Natural History Survey at the University of Illinois at Urbana-Champaign. Analyses were conducted on 50 Sun Spot specimens from 6 phyla, including: Annelida, Arthropoda, Chordata, Cnidaria, Hemichordata, and Mollusca. The Sun Spot locality is representative of a lower paleoestuarine (brackish) setting, and houses highly comparable genera to those known from the Essex fauna. These samples were compared both within and across phyla using microchemical data from scanning electron microscopy energy dispersive X-ray spectrometry. Results were additionally compared to specimens from the other published Mazon localities, including both the Braidwood (terrestrial and freshwater) and Essex biotas. Specifically, areas of compositional distinction were a focus in this study. Several samples of Tullimonstrum gregarium hosted definitive mineralogical differences in the gut tract, with higher concentrations of iron and sulfur (but no observable pyrite) compared to the rest of the fossilized organism. Interestingly, a similar preservational pattern can be observed in cnidarian samples, suggesting these organisms may have shared comparable internal microenvironments during decomposition and early diagenetic preservation. This study provides an overall description of the organisms from the Sun Spot locality, and novel data on the taphonomy of this site, in addition to assessing the preservational potential between organisms of differing tissue types within the Mazon Creek-type taphonomic pathway.