This is Mazon Monday post #285.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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One of the more problematic animals from Mazon Creek is Etacystis communis, known as the Aitch or “H” animal by amateur collectors. It was described by Matthew Nitecki and Frederick Schram in “Etacystis communis, a fossil of uncertain affinities from the Mazon Creek fauna (Pennsylvanian of Illinois)”, which was published in the Journal of Paleontology. The authors did not assign it to a phylum. Researchers have suggested a hemichordate or hydrozoan affinity, however the animal is missing a stomochord. E. communis, a soft-bodied animal, is only known from Mazon Creek.

Matthew H. Nitecki was a long-time curator of fossil invertebrates at the Field Museum of Natural History. He is the editor of “Mazon Creek Fossils”, which was the proceedings of a Symposium on Mazon Creek fossils held at the University of Michigan in May, 1978.

Frederick R. Schram is an American paleontologist known mostly for his work on crustacean biology, taxonomy, and systematics.

From “Etacystis communis, a fossil of uncertain affinities from the Mazon Creek fauna (Pennsylvanian of Illinois)”

ABSTRACT-Etacystis communis n gen, n. sp. is a strange organism preserved in the siderite concretions of the Middle Pennsylvanian Carbondale Formation. It has the shape of the letter “H” with a saclike structure connected to the cross bar, and ranges in size from 20 to 95 mm. There was apparently no coenoecium or mineralized covering.

Jack Wittry addresses Etacystis communis on pages 155 and 156 of his book “The Fossil Fauna of Mazon Creek”.

Etacystis communis Nitecki and Schram, 1976

Etacystis communis is commonly called the Aitch or “H” because fossilized remains seem to resemble this letter in a lop-sided form. This animal is large, possibly a hydrozoan in the phylum Coelenterata from the Pennsylvanian Essex Fauna. Due to the animal’s size, concretions often contain only incomplete remains. In more complete specimens, a heart-shaped sac is connected by a short stalk to a cross-bar, which connects to a longer leg-like appendage (stolen) that probably was flexible in life. E. communis may have been free-floating or perhaps anchored in the substrate by its stolen.

Some well-preserved examples show what has been interpreted as a mouth near the base of the sac. Wart-like papillations that are difficult to interpret cover the arms and sometimes the sac. These could be suckers or clumps of stinging cells, or possibly even bases for lost tentacles.

E. communis appears twice in “Richardson’s Guide to the Fossil Fauna of Mazon Creek” as a Cnidarian in Chapter 6 by Steven D. Sroka and again in Chapter 20 under “Problematica”.

Chapter 6 – Cnidaria

Etacystis communis Nitecki and Schram, 1976 Figures 6.14, 6.15

Known to collectors as “the H,” Etacystis is a relatively large unsymmetrical H-shaped fossil animal or part of an animal with two unequal arms and a sac connected to a crossbar (peduncle). The longest leg of the H, termed the stolon (Nitecki and Schram, 1976), was apparently flexible. This structure, which varies in length, almost always extends off the edge of the concretion. Within some specimens there is a ca-nallike area down the middle of the stolon (Nitecki and Schram, 1976; Foster, 1979). The stolon may have been both an anchoring organ and a device to connect adjoining animals, or a site of asexual budding (Nitecki and Schram, 1976). There is a heart-shaped saclike structure connected to the peduncle by a stalk with a small aperture located at the point where the stalk joins the sac. This aperture may represent the mouth (Nitecki and Schram, 1976).

Numerous specimens are found with papillations on the arms, peduncle, and sac. These papillations might be suckers, sticky pads, clumps of cnidocytes, remnants of lost tentacles, or even diagenetic alterations (Nitecki and Schram, 1976).

The natural affinities of E. communis have been debated since it was first described. Nitecki and Schram (1976), with some uncertainty, considered it to be similar to pterobranch hemichordates such as Rhabdopleura, which contains a pair of tentaculate lophophore arms. Foster (1979, p. 219) thought it possible that Etacystis is not related to hemichordates. He assigned Etacystis to the Hydrozoa (order Siphonophora) because of its planktic nature, its morphology, and its great range in size without appreciable change in form of the peduncle, sac, and arms.

Chapter 20 – Problematica

“H” Animal Etacystis communis Nitecki and Schram, 1976 See Figures 6.14, 6.15

Known to collectors as “the H,” Etacystis is a relatively large asymmetrical H-shaped animal or part of an animal with two unequal arms and a sac connected to a crossbar (peduncle). For a more detailed description of E. communis, see Chapter 6. Unfortunately, detailed internal morphology is largely lacking.

The affinities of E. communis have been debated since its discovery. Nitecki and Schram (1976), with some uncertainty, considered it to be similar to pterobranch hemichordates such as Rhabdopleura, which has paired tentaculate lophophore arms. Foster (1979b) assigned Etacystis to the Hydrozoa (order Siphonophora) because of its possible planktic nature, morphology, and the great range in size in specimens lacking appreciable variation in form of the peduncle, sac, and arms.

Alternatively, Schram (1991), using cladistic analysis, argued that Etacystis is closely allied with the Ectoprocta, Phoronida, and the higher deuterostome/lophophorates.

Specimens

From Wittry

ESCONI member Rich Holm