Etacystis is a unique H-shaped animal from the Mazon Creek formation of Illinois.
It is found in the Francis Creek Shale in pit 11 of the Mazon Creek formation.
This odd H-shaped colonial organism is thought to be a hemichordate, distinct yet
somehow related to the pterobranch class of hemichordates, another colonial class
of hemichordates, though its affinities to this class are still uncertain.
Pterobranchs are usually much smaller than Etacystis and rarely
exceed one centimeter in length, while the Etacystis can grow to around 11 centimeters
per individual. It appears to be a filter feeding organism that grows throughout its lifetime,
due to the variety of sizes that have been found of it.
(Above, left) Diagram comparing Etacystis and Rhabdopleura, a living hemichodrate in the class pterobranchia. (Above, ceneter)
Two specimens of Etacystis. The top shows a closeup of the arms showing the papillations on the arms. (Above, right) My Specimen
of Etacystis. It is much smaller and not as detailed as the others. (below) Two other specimens of Etacystis.
All of the photos and information I have written comes from "Etacystis communis, a Fossil of Uncertain Affinities From the Mazon Creek Fauna (Pennsylvanian of Illinois)" by Matthew H. Nitecki and Frederick R. Schram, reprinted from Journal of Paleontology Vol. 50, No.6, November 1976 pp.1157-1161.