Tullymonstrum gregarium became the Illinois state fossil in 1989. Discovered in 1958 by Frances Tully, it became known as the "Tully Monster". Fossils have only been found in Illinois in ironstone concretions, the reddish-brown rounded stones found in rock removed from coal mines. It swam in the tropical ocean that covered Illinois about 300 million years ago.
Only about a foot in length, the soft-bodied invertebrate had a tapered, segmented body and large tail fins. Two eye-like projections on stalks extended on either side near the front of its body. A long, thin “arm” at the front of its body ended in a mouth-like structure with a number of sharp projections that may have been used for catching prey and feeding. An evolutionary mystery, it may be related to mollusks like snails and slugs.