- published: 28 Aug 2008
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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.