Tullimonstrum is a common fossil found in Illinois. It lived in the Pennsylvanian epoch of the Carboniferous period, about 300 million years ago. The smallest individual Tullimonstrum fossils are 8 centimeters long, but scientists think it could probably grow up to 35 centimeters in length. Tullimonstrum is very mysterious, because no one knows exactly what it is related to. Maybe it's related to modern day mollusks, or maybe it's related to something totally different.
Tullimonstrum is also called the Tully Monster, because the person who first discovered it was Francis Tully. Francis Tully took the fossil to the Field Museum in Chicago, where they named the fossil the Tully Monster. The name stuck. Then later, someone described it and named it Tullimonstrum gregarium. Its species name, gregarium, means common. Tullimonstrum is the state fossil of Illinois.
Tullimonstrum lived in the open ocean and may have hunted small soft-bodied creatures, such as trilobites that had recently molted. Tullimonstrum had a long proboscis with a claw at the end, which had eight teeth, but the proboscis probably was not the mouth. The mouth was probably located under the head, like Opabinia's mouth. At the posterior end, Tullimonstrum had something that looked remarkably like the posterior end of a squid. Tullimonstrum had a bar that went through its head with a swelling on each end. These swellings may have been eyes, but they could have been another sensory organ. No one actually knows what they were for.