ICHNOTAXONOMIC ASSESSMENT OF MAZON CREEK AREA TRACE FOSSILS

The Francis Creek Shale Member of the Middle Pennsylvanian Carbondale Formation along Mazon Creek near Morris, Illinois, is a conservation Lagerstätte well known for its diverse flora and fauna, exhibiting exquisite soft-body preservation within siderite concretions. Trace fossils, although less well-known, are also found within concretions. Examination of trace-fossil collections at the Field Museum of Natural History (FMNH), Chicago, Illinois, and the University of Kansas Natural History Museum (KU), Lawrence, Kansas, yielded at least 20 ichnogenera. The FMNH collection consists primarily of Arenicolites, Diplocraterion, Skolithos, Planolites, Cochlichnus, and Chondrites. There are several ichnogenera represented by a few or only a single specimen: Asterophycus, Aulichnites, Beaconites, Bergaueria, Biformites, Cosmorhaphe, cf. Cruziana, Paleomeandron, a new ichnospecies of Pennatulites, Protovirgularia, Rhizocorallium, Taenidium, and Thallasinoides. The KU collection consists primarily of Planolites and Arenicolites. Several specimens misidentified as trace fossils are actually worm body fossils, jellyfish tentacles, or soft-sediment deformation structures. One FMNH specimen contains the most common Mazon worm, Didontogaster cordylina, preserved with the base of the most common trace, Arenicolites. Several traces are associated with plants including Phagophytichnus ekowskii, leaf mines, and possibly the earliest galls in the fossil record. Thin sections of several specimens of Planolites yielded little additional information with the exception of a few possible peloids. The ichnofacies represented by these trace fossils must be inferred because the concretions from the Mazon Creek area were previously collected with no detailed stratigraphic information. The abundance and assemblage of Arenicolites, Diplocraterion, and Skolithos suggest a Skolithos ichnofacies, which indicates a nearshore, shallow marine environment. The abundance and assemblage of Planolites, Cochlichnus, and Chondrites, along with Aulichnites and Rhizocorallium, suggest a Cruziana ichnofacies, which represents a sublittoral environment. These two ichnofacies are consistent with previous interpretations that the Francis Creek Shale was deposited in a shallow marine deltaic environment.