Illinois: Tullimonstrum gregarium (state fossil)
Illinois: Tullimonstrum gregarium (state fossil)
The state fossil of Illinois is an enigmatic, wormlike creature called the Tully Monster (Tullimonstrum gregarium). It was a predatory creature that inhabited the lowland swamps that covered the state during the Pennsylvanian Period, around 300 million years ago. This member of the world-famous Mazon Creek fauna still presents science with a mystery regarding its relationships to any modern creatures.
Mountains to the east of Illinois at the time created abundant rainfall, allowing the formation of a delta and swamp system across the state, which at the time lay nearly on the equator. Lush forests ("coal swamps") were dissected by sluggish rivers. Decay of organic matter created conditions of low oxygen that resulted in the many fossil-bearing concretions ("coal balls") of the Mazon Creek Formation. Ferns, horsetails, insects, soft-bodied invertebrates, and of course, the Tully Monster are all known from these concretions.
The first Tully Monster was discovered by amateur collector Francis Tully in 1958. He took the strange creature to the Field Museum, where none of the staff could identify it. Curator Eugene Richardson gave it a proper scientific name in 1966, dubbing it Tullimonstrum gregarium, meaning "Tully's common monster."
A committee of paleontologists from the Illinois State Geological Survey lobbied the State Legislature for designation of the Tully Monster as the state fossil, and a bill to this effect was passed in 1989.
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