Tullimonstrum gregarium, colloquially known as the Tully Monster, was a 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.
Tullimonstrum gregarium takes its genus name from its discoverer, Francis Tully, 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.
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. 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. Tullimonstrum had gills, and a notochord, which acted as a rudimentary spinal cord which supported the body.
In 2016 a morphological study showed that Tullimonstrum was in fact a stem-lamprey, closely related to modern lampreys and thus a member of the phylum Chordata. 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. It also has many features not found in lampreys due to its specialized ecological niche. The Tully monster was determined to be a vertebrate.
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.
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 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.
Until 2016 the fossil remained “a puzzle” and interpretations likened it to a worm, a mollusc, an arthropod or a conodont. 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. Similarities with Cambrian fossil organisms were noted. Chen, et al., suggested similarities to Vetustovermis planus. Others pointed to a general resemblance between Tullimonstrum and Opabinia regalis, although Cave, et al., noted that they were too morphologically dissimilar to be related.
In popular culture
A 1966 satire pretended that modern representatives were to be found in Africa.
In 1989 Tullimonstrum gregarium was officially designated the State Fossil of Illinois.