Temporal range: 311–306Ma Middle Pennsylvanian
|fossil of Tullimonstrum gregarium
(part and counterpart)
Eugene S. Richardson, Jr., 1966
The Tully Monster (Tullimonstrum gregarium), so far apparently unique to Illinois, USA, was a soft-bodied invertebrate that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago.
Tullimonstrum gregarium takes its genus name from its discoverer, Francis Tully. Appropriately, "tull" is also Norwegian for "nonsense", whereas the species name, gregarium, means "common" and reflects its abundance. The term 'monster' relates to the creature's outlandish appearance and strange body plan.
Tullimonstrum probably reached lengths of up to 35 centimetres (14 in); the smallest individuals are about 8 cm (3.1 in) long.
The Tully Monster had a pair of fins not unlike a cuttlefish at the tail end of its body, and possibly vertical fins as well (though the fidelity of preservation of fossils of its soft body makes this difficult to determine), 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 found among the assemblage called the Mazon Creek fossils from their site in Grundy County, Illinois. The absence of hard parts in the fossil implies that the animal did not possess organs composed of bone, chitin or calcium carbonate. There is evidence of serially-repeated internal structures. Its head is poorly differentiated. A transverse bar-shaped structure, which was either dorsal or ventral, terminates in two round organs which are associated with dark material similar to the pigmentation often found in eyes. Their form and structure is suggestive of a camera-type construction.
There is insufficient evidence to align Tullimonstrum to the crown or even stem group of any phylum. Since it lacks any characters of the well-known modern phyla, it has been speculated that it is a stem group to one of the many phyla of worms that are poorly represented today. Similarities with Cambrian fossil organisms such as Vetustovermis and Opabinia have also been noted.
Tullimonstrum was probably a free-swimming carnivore that dwelt in open marine water, and was occasionally washed to the near-shore setting in which it was preserved.
 Taphonomic setting
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 slowed further decay, 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.
The proboscis is rarely preserved in its entirety; it is complete in around 3% of specimens. However some part of the organ is preserved in about 50% of cases.
Amateur collector Francis Tully found the first of these fossils in 1958. He took the strange creature to the Field Museum of Natural History, but paleontologists remain stumped as to what phylum Tullimonstrum belongs to.
 See also
- Johnson, Ralph Gordon; Richardson, Eugene Stanley, Jr (March 24, 1969). "Pennsylvanian Invertebrates of the Mazon Creek Area, Illinois: The Morphology and Affinities of Tullimonstrum". Fieldiana: Geology (Chicago: Field Museum of Natural History) 12 (8): 119–149. OCLC 86328.
- Mikulic, Donald G; Kluessendorf, J (April 19, 2010). "Illinois' State Fossil Tullimonstrum gregarium". Illinois State Geological Survey. Retrieved March 31, 2012.
- Richardson, Eugene Stanley, Jr (January 7, 1966). "Wormlike Fossil from the Pennsylvanian of Illinois". Science (American Association for the Advancement of Science) 151 (3706): 75–76. Bibcode:1966Sci...151...75R. doi:10.1126/science.151.3706.75-a. PMID 17842092.
- Chen, Jun-yuan; Huang, Di-ying; Bottjer, David J (October 2005). "An Early Cambrian problematic fossil: Vetustovermis and its possible affinities". Proceedings of the Royal Society: Biological Sciences (London: The Royal Society) 272 (1576): 2003–2007. doi:10.1098/rspb.2005.3159. OCLC 112007302. PMC 1559895. PMID 16191609.
- Kloss, Gerald (June 18, 1968). "The Great Dancing Worm Hoax". The Milwaukee Journal. Retrieved March 31, 2012.
- Rory, E Scumas (1969). The Dancing Worm of Turkana. Vanishing Press. OCLC 191964063.
- "State Symbol: Illinois State Fossil — Tully Monster (Tullimonstrum gregarium)". Illinois State Museum. Retrieved March 31, 2012.
|Wikimedia Commons has media related to: Tullimonstrum|
|Wikispecies has information related to: Tullimonstrum|