Essexella asherae
Pennsylvanian
Francis Creek Shale
Pit 11, Will Co.
Illinois, USA.

Although long referred to as a jellyfish, Essexella asherae is now generally interpreted as a cnidarian closely related to modern sea anemones. Its preservation is remarkable because animals lacking hard skeletons rarely survive the fossilization process.
Huge 120mm in 148mm paired concretion. Biggest we have ever seen