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This specimen is an ichnofossil of an Essexella, an extinct genus of scyphozoan jellyfish dating to the Late Carboniferous Period (c. 300,000,000 years ago).

Such soft body structures are usually lost to time, but at Mazon Creek, Illinois, a unique fossil record captures cycles of rapid coastal flooding that combined layers of ocean sediment with iron-rich groundwater to produce tough ironstone nodules. This makes it possible for incredibly rare soft fossils like jellyfish to be found inside concretions.