Their fossils are locally common but have been found nowhere else on Earth. That’s why the Tully Monster (Tullimonstrum gregarium), a slender, soft-bodied creature with a long, narrow snout and sensory organs (primitive eyes) set away from the body on stalks, has come to represent Illinois as the State Fossil. Francis Tully found the first one in 1958. He took it to experts at the Field Museum of Natural History in Chicago for identification, but the scientists were stumped. Museum staff with a sense of humor nicknamed it the “Tully Monster.” In the second picture, you can see a three-dimensional rendering, courtesy of The Field Museum, that gives us a picture of what the Tully Monster might have looked like.
Three hundred million years ago, during the Pennsylvanian Period, Tully Monsters hunted in a marine coastal environment. When a Tully Monster died, its likeness may have been preserved in an “ironstone concretion.” These are hand-sized nodules of rock that when split open, may reveal a fossil inside (image 1). These fossils are unique because of the chance that soft tissue shapes have been preserved, often with many fine details.
While the Tully Monster was originally found among the Mazon Creek Deposits, it has since been found in a few additional locations in central Illinois where surface coal mines have been opened and revealed fossil beds. But the Tully Monster is still all ours, a uniquely Illinois treasure, and a link to Illinois’ tropical past.