Fossils of the "H Animal" and "Y Animal" discovered at Mazon Creek, near Braidwood, Illinois. A 310 million year old mystery.

The “H Animal," Etacystis Communis.
It was a soft-bodied invertebrate that lived in shallow tropical coastal waters of muddy estuaries (where the tide meets the stream) during the Pennsylvanian geological period, about 310 million years ago. 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. Francis Creek shale 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.

It is thought to be a filter-feeding organism that grew throughout its lifetime, achieving a maximum length of 4.3 inches.

A little hard to see, but look at the darker "H" in the center of the fossil.

The classification is uncertain. The animal had a unique H-shaped body ranging from 3/4 inch to 4.3 inches long, and researchers have suggested a hemichordate (wormlike marine invertebrates) or hydrozoan (relatives of jellyfish) affinity. Examples of the H Animal have been found only in the Mazon Creek (River) fossil beds in Illinois.

The “Y Animal," Escumasia Roryi.
It is a puzzling creature. It has only found in the Peabody Coal Pit 11 of the Mazon Creek formation. The only species of the subfamily, it is Y-shaped, bilaterally symmetrical, soft-bodied and is about 6 inches in length.

It appears to have a mouth slit on the top between the two arms and a second opening on one side of the main body, presumably an anus, though there is no proof that it is an anus. It is attached to the sea floor by a stalk attached to a round base. Oddly, a specimen appears to have a short tube-like structure in between the two arms near the mouth.

Because of the lack of complex structures or apparent internal organs, it is believed that it must have been related to coelenerates (cnidarians) and may have used stinging nematocysts on its arms to capture prey. However, because of its bilateral symmetry and second opening, it can’t fit the technical definition of a coelenerate. 

Therefore, it is considered a separate group that may have broken off of the cnidarians and gone extinct as a failed evolutionary experiment. If it were to be classified as a coelenerate, the definition of coelenerate would have to be changed and a new class would have to be added.

Compiled by Neil Gale, Ph.D.