The Tully Monster (Tullimonstrum gregarium), so far apparently unique to Illinois, was a soft-bodied invertebrate that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 mya. The Tully Monster had fins not unlike a cuttlefish and a long proboscis with eight small sharp teeth, with which it may have probed actively for small creatures and edible detritus in the muddy bottom. It was part of the ecological community represented in the unusually rich group of soft-bodied organisms represented among the assemblage called the 'Mazon Creek fossils' from their site in Grundy County, Illinois.
The formation of the Mazon Creek fossils is unusual. When the creatures died, they were rapidly buried in silty outwash. The bacteria that began to decompose the plant and animal remains in the mud produced carbon dioxide in the sediments around the remains. The carbon dioxide combined with iron from the groundwater around the remains, forming encrusting nodules of siderite ('ironstone'), which created a hard permanent 'cast' of the animal which slowly further decayed, leaving a carbon film on the cast.
The combination of rapid burial and rapid formation of siderite resulted in excellent preservation of the many animals and plants that ended up in the mud. As a result, the Mazon Creek fossils are one of the world's major Lagerstätten, or concentrated fossil assemblages.
Amateur collector Francis Tully found the first of these fossils in 1958. He took the strange creature to the Field Museum, but paleontologists remain stumped as to what phylum Tullimonstrum belongs. In 1989 Tullimonstrum gregarium was officially designated the State Fossil of illinois.
