Essexella
Essexella is an extinct genus of cnidarian known from Late Carboniferous fossils; it contains a single species, E. asherae. 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. Essexella was originally described as a jellyfish, but was recently redescribed as a sea anemone.
Another alleged jellyfish, Reticulomedusa, is likely Essexella preserved from different angles. Essexella may have produced the common trace fossil Conostichus.