Tully Monster

Tullimonstrum gregarium fossilized in an ironstone concretion, photographed by Argonne National Laboratory

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.

In March 2015, Argonne National Laboratory used the Department of Energy’s Advanced Photon Source to study a Tullymonstrum gregarium fossil from the Chicago Field Museum with X-rays.

Illustration of Tullimonstrum gregarium by Nobu Tamura

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.