Jellyfish are extremely rare in the fossil record. This is hardly surprising, considering they completely lack hard parts, and the tissues they do possess are gelatinous.
One of the most famous deposits containing fossil jellyfish is the Pennsylvanian-aged Mazon Creek Lagerstätte near Chicago, Illinois. In the Mazon Creek area, the Francis Creek Shale consists of concretionary gray shales. The Francis Creek concretions are sideritic, and can be fossiliferous or unfossiliferous. The fossiliferous concretions contain land plants and terrestrial & marine animals, including nonmineralizing organisms.
The most abundant marine organisms in the Mazon Creek biota are jellyfish. Many Mazon Creek jellyfish specimens do not have many diagnostic features, and are affectionately referred to as "blobs" by local fossil collectors ("blob A", "blob B", "blob with character", etc.).
Stratigraphy: Francis Creek Shale Member, Carbondale Formation, Desmoinesian Stage (= Westphalian D), upper Middle Pennsylvanian
4.3 inches long
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