TAPHONOMIC VARIANCE BETWEEN FERNS AND MEDULLOSAN SEED FERNS FROM THE MAZON CREEK LAGERSTATTEN

Exceptional fossils from the Pennsylvanian-aged (307 Ma) Mazon Creek (Francis Creek Shale) region of northeastern Illinois represent a rare window into late Paleozoic ecosystems. These fossils record a complex deltaic depositional environment with fluctuating salinities in response to cyclic sea-level changes. The interplay between saline and brackish waters were key in the formation of highly resistant siderite concretion nodules that nucleated around decaying plants and animals, faithfully preserving soft-tissues. An extensive concretion collection is reposited at the Royal Ontario Museum (ROM) in Toronto, Canada, allowing for comprehensive studies of the preservational variance occurring within, and between, species of ferns and Medullosan seed ferns. Several hundred individuals from these two clades were subdivided into morphologically-similar cohorts and compared from a taphonomic perspective. Detailed matrices were constructed to evaluate the preservational variance on a character-by-character basis, in addition to evaluating the overall preservational quality of the two groups. Datasets were statistically compared through rigorous multivariate techniques including Principal Coordinate Analysis (PCO) and non-metric Multidimensional Scaling (NMDS) to find relative preservational trends. Ultimately, the construction of qualitative and quantitative taphonomic series of numerous anatomical structures between a large number of individuals may help constrain relative rates of decay, sequences of character loss, and allow inference of original tissue labilities and taphonomic trajectories in extinct organisms.