The Tully monster (Tullimonstrum gregarium) was designated the official state fossil of Illinois in 1989. The Tully monster was a soft-bodied prehistoric marine animal which lived 280 to 340 million years ago during the Pennsylvanian geological period. So far Tully monster fossils are unique to Illinois - more than 100 Tully monster fossils have been found in the state (amateur collector Francis Tully found the first in 1958).
Drawing of Tullimonstrum gregarium swimming through seaweed. Below the Tully monster is a string of aitches (Etacystis communis) - above are a pair of crustaceans (Cyclus americanus) - artwork by user Apokryltaros on Wikipedia (GNU Free Documentation License Version 1.2 or later).
