Etacystis communis is one of the more unusual animals that can be found in the Essex (marine) portion of the Mazon Creek deposit. Since being described, there has been much debate over what it might be. It has been suggested that it might be a Pterbranch hemichordate, a marine algae or a coelenterate. It is currently assigned as a hydrozoan. Etacystis is commonly referred to by collectors as the “H” animal. Relatively complete specimens bare a resemblance to the letter H