Tullimonstrum gregarium Richardson, 1966
Probably the most fascinating of all Mazon Creek fossils is T. gregarium, popularly known as the "Tully Monster". Tully Monster fossils are restricted to the Essex fauna of Will and Kankakee Counties, at Chowder Flats near Morris in Grundy County, and from the Sunspot Mine near Astoria, Fulton County. Appreciation of the uniqueness of the Tully Monster goes beyond scientists and collectors because it was recently chosen as the state fossil of Illinois. Johnson and Richardson (1969), Foster (1979a), and Becall (1991) all provide excellent descriptions of T. gregarium, but their interpretation of anatomical details and functional morphologies differ.