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.