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Not to be confused with Telly Monster.
Temporal range: Middle Pennsylvanian
Tullimonstrum gregarium 343.JPG
Fossil of Tullimonstrum gregarium
(part and counterpart)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Hyperoartia
Genus: Tullimonstrum
Richardson, 1966
Type species
Tullimonstrum gregarium
Richardson, 1966

Tullimonstrum gregarium, colloquially known as the Tully Monster, is an extinct, soft-bodied vertebrate that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago. Examples of Tullimonstrum have been found only in the Mazon Creek fossil beds of Illinois, United States. Until 2016, its classification was uncertain, and interpretations of the fossil likened it to a mollusc, an arthropod, a conodont, or to one of the many phyla of worms.


Life reconstruction of Tullimonstrum gregarium based on studies by McCoy et al. 2016

Tullimonstrum probably reached lengths of up to 35 centimetres (14 in); the smallest individuals are about 8 cm (3.1 in) long.[1]

Tullimonstrum had a pair of fins like those of a cuttlefish, which were situated at the tail end of its body. The organism also possibly featured vertical, ventral fins (though the fidelity of preservation of fossils of its soft body makes this difficult to determine), and typically featured a long proboscis with up to eight small sharp teeth on each "jaw", with which it may have actively probed 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.[2] The absence of hard parts in the fossil implies that the animal did not possess organs composed of bone, chitin or calcium carbonate.[1] There is evidence of serially repeated internal structures.[1] Its head is poorly differentiated.[1] 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.[1] Tullimonstrum had gills, and a notochord, which acted as a rudimentary spinal cord which supported the body.[3]

History of discovery[edit]

Outdated restoration showing T. gregarium as an invertebrate

Amateur collector Francis Tully found the first of these fossils in 1955 in a fossil bed known as the Mazon Creek formation. He took the strange creature to the Field Museum of Natural History, but paleontologists were stumped as to which phylum Tullimonstrum belonged.[4] The species Tullimonstrum gregarium, as these fossils later were named, takes its genus name from Tully,[1] whereas the species name, gregarium, means "common", and reflects its abundance.[5] The term monstrum, "monster", relates to the creature's outlandish appearance and strange body plan.

Until 2016, the fossil remained "a puzzle", and interpretations likened it to a worm, a mollusc, an arthropod or a conodont.[6] Since it appeared to lack characteristics of the well-known modern phyla, it was speculated that it was representative of a stem group to one of the many phyla of worms that are poorly represented today.[1] Similarities with Cambrian fossil organisms were noted. Chen et al. suggested similarities to Vetustovermis planus.[7] Others pointed to a general resemblance between Tullimonstrum and Opabinia regalis,[8] although Cave et al. noted that they were too morphologically dissimilar to be related.[9]


In 2016 a morphological study showed that Tullimonstrum was in fact a basal vertebrate, and thus a member of the phylum Chordata,[10][11][12] closely related to modern lampreys. This affinity was attributed based on pronounced cartilaginous arcualia, a dorsal fin and asymmetric caudal fin, keratinous teeth, a single nostril, and tectal cartilages like in lampreys. McCoy et al. raise the possibility that Tullimonstrum belongs to the ancestral group of lamprey,[10][12] but it also has many features not found in Cyclostomes (lampreys + hagfishes) and thus its exact phylogenetic position is still controversial.[13][4] Researchers at the University of Leicester found further evidence that Tullimonstrum was a vertebrate: two types of melanosomes in the creature's eyes, a feature found only in vertebrates.[11]


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.[1]


Fossil in the Museo Civico di Storia Naturale di Milano. This specimen clearly shows the eye bar structure.

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.[1]

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.[1]

In popular culture[edit]

A 1966 satire pretended that modern representatives were to be found in Africa.[14][15]

In 1989, Tullimonstrum gregarium was officially designated the State Fossil of Illinois.[5][16]

See also[edit]


  1. ^ a b c d e f g h i j 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. 12 (8): 119–149. OCLC 86328. 
  2. ^ Richardson, Eugene Stanley, Jr. (January 7, 1966). "Wormlike Fossil from the Pennsylvanian of Illinois". Science. 151 (3706): 75–76. Bibcode:1966Sci...151...75R. doi:10.1126/science.151.3706.75-a. PMID 17842092. 
  3. ^ Dunham, Will (March 16, 2016). "Tully Monster Mystery Solved, Scientists Say". Scientific American. Reuters. Retrieved March 18, 2016. 
  4. ^ a b Greshko, Michael (March 16, 2016). "Scientists Finally Know What Kind of Monster a Tully Monster Was". National Geographic. Retrieved March 17, 2016. 
  5. ^ a b Mikulic, Donald G.; Kluessendorf, Joanne (1997). "Illinois' State Fossil—Tullimonstrum gregarium" (PDF). Geobit. 5. OCLC 38563956. Archived from the original (PDF) on February 22, 2014. 
  6. ^ Briggs, Helen (March 16, 2016). "Fishy origin of bizarre fossil 'monster'". BBC News. 
  7. ^ 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 B. 272 (1576): 2003–2007. doi:10.1098/rspb.2005.3159. OCLC 112007302. PMC 1559895Freely accessible. PMID 16191609. 
  8. ^ Switek, Brian (January 27, 2011). "Tully's Mystery Monster". Wired. Laelaps. Retrieved February 5, 2014. 
  9. ^ Cave, Laura Delle; Insom, Emilio; Simonetta, Alberto Mario (1998). "Advances, diversions, possible relapses and additional problems in understanding the early evolution of the Articulata". Italian Journal of Zoology. 65 (1): 19–38. doi:10.1080/11250009809386724. 
  10. ^ a b McCoy, Victoria E.; Saupe, Erin E.; Lamsdell, James C.; Tarhan, Lidya G.; McMahon, Sean; et al. (April 28, 2016). "The 'Tully monster' is a vertebrate". Nature. 532 (7600): 496–499. doi:10.1038/nature16992. 
  11. ^ a b Clements, Thomas; Dolocan, Andrei; Martin, Peter; Purnell, Mark A.; Vinther, Jakob; Gabbott, Sarah E. (April 28, 2016). "The eyes of Tullimonstrum reveal a vertebrate affinity". Nature. 532 (7600): 500–503. doi:10.1038/nature17647. 
  12. ^ a b St. Fleur, Nicholas (March 16, 2016). "Solving the Tully Monster's Cold Case". The New York Times. Retrieved March 16, 2016. 
  13. ^ Kuratani, Shigeru; Hirasawa, Tatsuya (April 28, 2016). "Palaeontology: Getting the measure of a monster". Nature. 532 (7600): 447–448. doi:10.1038/nature17885. 
  14. ^ Kloss, Gerald (June 18, 1968). "The Great Dancing Worm Hoax". The Milwaukee Journal. Retrieved March 31, 2012. 
  15. ^ Rory, E. Scumas (1969). The Dancing Worm of Turkana. Vanishing Press. OCLC 191964063. 
  16. ^ "State Symbol: Illinois State Fossil — Tully Monster (Tullimonstrum gregarium)". Illinois State Museum. Retrieved March 31, 2012. 

External links[edit]