The Mazon Creek H Creature - Problematic Etacystis communis

Description: The Mazon Creek deposits of the region near Braidwood, Illinois rival the other famous Lagerstatten of the Burgess Shale, Solnhofen, and Liaoning for the variety of detailed life preserved. Many exquisitely preserved specimens are found in the ironstone nodules that make up the deposits. The majority of collecting areas are the spoil heaps of abandoned coal mines, the most famous of which is Peabody Coal Pit 11. Pit 11 now serves as a cooling pond for the Braidwood nuclear power plant, but with over 100 other localities, specimens still come to light.

Described variously as a hydrozoan or a hemichordate, this most enigmatic of Mazon Creek taxa is colloquially termed the H animal. At present, it is thought to be a filter-feeding organism that grew throughout its lifetime, achieving a maximum length of 11 cm.