English: Tullimonstrum gregarium Richardson, 1966 - odd fossil in an ironstone concretion from the Pennsylvanian of Illinois, USA. (Orton Geology Museum, Ohio State University, Columbus, Ohio, USA)

One of the most remarkable soft-bodied fossil deposits (lagerstätten) on Earth is the Pennsylvanian-aged Mazon Creek Lagerstätte near Chicago, Illinois. In the Mazon Creek area, the Francis Creek Shale consists of concretionary gray shales. The Francis Creek concretions are composed of argillaceous ironstone, and can be fossiliferous or nonfossiliferous. The fossiliferous concretions contain land plants and terrestrial & marine animals, including nonmineralizing organisms.

Seen here is a "Tully Monster", the state fossil of Illinois. This is a famous fossil oddball, or problematicum, meaning it's classification and taxonomic identity are uncertain. The genus name is actually Tullimonstrum. It is a bilaterally symmetrical, soft-bodied animal with an elongated body. A pair of fins is present near the posterior end, resulting in a kite-shaped tail area. An inferred eye bar is present in the mid-regions of the body. The anterior end has a single, narrow appendage ending in a claw-like structure.

This specimen is a partial - this is the back end of the animal. The kite-shaped tail is at right.

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From exhibit signage:

Mazon Creek, Illinois

About 15 miles southwest of Chicago is a window on life as it was 300 million years ago. In layers of shale between beds of coal are iron carbonate concretions that have produced over 350 species of ancient plants and over 340 species of animals, including 140 kinds of insects, 166 arachnids and myriapods (spiders, scorpions, centipedes, millipedes), 25 kinds of fish, and 9 amphibians, plus crabs, shrimp, jellyfish, worms, molluscs, and the Tully Monster.

Tully Monster

In 1955, Francis Tully, an avid collector of Mazon Creek fossils, discovered the fossil of an animal no one had ever seen before. Up to 13 inches long, it had a long "arm" with "teeth" at the front and a squid-like tail. Found nowhere else, it has been called "an orphan in search of a phylum" because, although over 1500 specimens have been discovered, paleontologists still don't know to what group of organisms it belongs.

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Classification: Incertae Sedis

Stratigraphy: Mazon Creek Lagerstätte, Francis Creek Shale Member, Carbondale Formation, Desmoinesian Stage (= Westphalian D), upper Middle Pennsylvanian

Locality: unrecorded/undisclosed site in the Mazon Creek area, Illinois, USA

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See info. at:

en.wikipedia.org/wiki/Tullimonstrum