176-5 Seasons of Preservation: Mapping Faunal Gradients and Constraints on Fossilization in the Mazon Creek and Pennsylvanian Midcontinent

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record

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Abstract:

The Mazon Creek Lagerstätte of Illinois remains one of the most significant Carboniferous fossil sites globally, yielding an exceptional diversity of soft-bodied taxa, plants, insects, and more—and offering unparalleled insight into a tropical coastal delta ecosystem. Its paleobiological record has traditionally been interpreted through two faunal associations—the nearshore Braidwood and the estuarine-marine Essex—originally distinguished based on assemblage-level taxonomic and environmental differences. Building on the foundational census of ~300,000 concretions from northeastern Illinois by Baird and colleagues in the 1980s, recent multivariate analyses statistically validate these associations and reveal finer-scale ecological gradients within the Essex fauna.

Here, we synthesize those analyses with data from other midcontinent Pennsylvanian concretionary sites to evaluate stratigraphic controls and faunal distributions across both space and time. Our results confirm the ecological distinctiveness of the Braidwood and Essex associations and further resolve two sub-associations within the Essex fauna that reflect salinity-driven ecological gradients. When integrated with data from Francis Creek-equivalent units in Missouri, Indiana, and Oklahoma, these spatial patterns gain broader context: Essex-type soft-bodied marine communities appear stratigraphically constrained to narrower Francis Creek-equivalent units, whereas Braidwood-type associations recur more broadly in nonmarine-to-brackish facies across Pennsylvanian successions. Together, these findings refine paleoenvironmental models of tidally influenced estuarine systems and suggest that the exceptional preservation seen in the Essex fauna was tightly coupled to transgressive events and specific aggradational taphonomic conditions within a limited stratigraphic window.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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