Wikipedia
Essexella
Essexella is a genus of scyphozoan jellyfish known from Late Carboniferous fossils containing the species Essexella asherae. See list of prehistoric medusozoans. It is one of the most recurrent organisms in the Mazon Creek fossil beds of Illinois. In the Essex biota of Mazon Creek, it consists of 42% of all fossil finds. Its behavior is speculated to be similar to that of modern-day jellyfish.