Mazon Creek-type Lagerstätten in Pennsylvanian Coal Basins of the US Midcontinent: Biotic Associations and Depositional Frameworks

Gordon Baird; James Schiffbauer; Gigi Feltmann; Casey Bennett; Natalia Bykova

doi:10.57035/journals/sdk.2026.e42.2455

Authors

Gordon C. Baird Department of Geology and Environmental Sciences, SUNY Fredonia, Fredonia, New York, U.S.A.
James Schiffbauer Department of Geological Sciences, University of Missouri, Columbia, Missouri, U.S.A; X-ray Microanalysis Laboratory, University of Missouri, Columbia, Missouri, U.S.A https://orcid.org/0000-0003-4726-0355
Gigi Feltmann Department of Geological Sciences, University of Missouri, Columbia, Missouri, U.S.A https://orcid.org/0009-0003-9666-4740
Casey C. Bennett Department of Geological Sciences, University of Missouri, Columbia, Missouri, U.S.A
Natalia Bykova X-ray Microanalysis Laboratory, University of Missouri, Columbia, Missouri, U.S.A https://orcid.org/0000-0003-4875-5261

DOI:

https://doi.org/10.57035/journals/sdk.2026.e42.2455

Keywords:

Mazon Creek, Carboniferous Period, Coal roof shales, Siderite concretions, Fossils, Paleoecology, Braidwood-Essex associations, estuarine coal-swap sedimentation

Abstract

The Mazon Creek fossil beds of northern Illinois are famous for producing some of the world’s best-preserved fossils from the Pennsylvanian Period, about 307 million years ago. These fossils are found inside ironstone (siderite) concretions that formed rapidly around plants and animals as they were buried. Because of this unusual style of preservation, even soft-bodied creatures—like jellyfish, worms, and the mysterious “Tully Monster”—survive in remarkable detail. For decades, the Mazon Creek has been known for two distinct fossil communities: the plant-rich Braidwood association from nearshore environments, and the more marine Essex association full of jellyfish, shrimps, and other sea life.
This study brings together the largest systematic sampling effort ever conducted at Mazon Creek—over 300 000 concretions collected from more than 350 sites—and compares them with new fossil discoveries from similar rocks across Illinois, Indiana, Missouri, and Oklahoma. We combine detailed descriptions of the preserved fossil assemblages, and show them to represent real, repetitive ecological groupings shaped by ancient environmental gradients.
Our work also demonstrates that Mazon Creek-style fossil deposits are not unique to Illinois: similar environments and fossil communities occurred widely across the ancient U.S. Midcontinent as well as in Europe. These findings help us understand how rapid sea-level rise, coastal flooding, and tidal currents created the conditions that allowed such exceptional fossil preservation, offering a window into ancient ecosystems rarely preserved in the rock record.

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