Tullimonstrum gregarium - Illinois' "state fossil", as well as being one of the weirdest, most perplexing creatures known to paleontology.

In 1958, a Mr. Francis Tully brought a siderite (ironstone) nodule into Chicago's Field Museum of Natural History. He had found the nodule in a pile of coal mine tailings in the Mazon Creek area of central Illinois. Since the mid-1800's, these tailings piles have been a treasure trove of fossils from the Late Carboniferous (Pennsylvanian) Period, about 300 million years ago.

The ironstone had preserved the impression of a soft-bodied invertebrate marine creature, previously unknown to paleontologists. This creature lived in the shallow seas that covered the American Midwest at the time, and was preserved because of a unique combination of quick burial in mud and chemical reactions. Its body is approximately 19 cm (8 in) long, and appears to have been round in cross section. Some specimens exhibit a Michelin Man-type segmentation. At the rear are two triangular fins which, one would assume, helped propel it through the water. At the front end was a flexible proboscis tipped with an eight-part "claw". Towards the foward end of the creature are two long stalks with sensory organs, probably eyes, on the end.

The claw at the end of the proboscis leads paleontologists to suspect the Tully Monster was a predator on the shrimp and small fish whose fossils are also found in the Mazon Creek area. The creature's mouth does not appear to be anywhere near the claw or along the proboscis; instead, the creature probably grasped its prey with the claw and fed it to the mouth further back.

No other fossil even vaguely resembed the specimen Mr. Tully brought in. For lack of a better name, staff at the Field Museum began calling it the "Tully Monster". The nickname became official in 1966 when the Field Museum's curator of fossils, Dr. Eugene Richardson, formally described it and gave it the name Tullimonstrum gregarium ("gregarium" meaning "common", as many other specimens had been found since).

To this day, paleontologists are unsure where the Tully Monster belongs in the evolutionary tree. Most suspect it is some sort of mollusk (perhaps a gastropod) or an annelid. The Tully monster's proboscis and tail fin occasionally bring the body plan of the Cambrian arthropod Anomalocaris to mind, but the two are extremely unlikely to be related, as Anomalocaris disappeared hundreds of millions of years before the Carboniferous.

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