Tullimonstrum

*Tullimonstrum*, known as the Tully Monster, is an extinct genus of soft-bodied bilaterian that lived in Illinois during the Late Carboniferous.

## History

Amateur collector Francis Tully found fossils in the Mazon Creek beds in 1955, taking them to the Field Museum of Natural History. Experts at the FMNH, however, had no idea which phylum to classify it, but it was named *Tullimonstrum gregarium* some time later. It has often been informally named the "Tully Monster", of which its scientific name means. These remains have since stayed relatively unknown, with some interpreting them as a worm, mollusc, conodont, arthropod or vertebrate in affinities. This is due to how it has an apparent lack of distinguishing features assignable to phyla. It was speculated to be a stem group worm that is poorly represented today. However, similarities with Cambrian organisms have been found. Chen et al. suggested it was similar to *Vetustovermis planus*, with others finding similarities with *Opabinia regalis*. Cave at al., however, noted the latter is too morphologically dissimilar to have relations.

A 1966 through '68 prank by Bryan Patterson suggested modern ancestors of *Tullimonstrum* were found in the remote Kenyan lakes under the local name Ekurut Loedonkakini. As they are called, the "dancing worms of Turkana" could reportedly kill a human with a single bite, produced a sort of milk and were so "common", school-aged children had heard of them. Patterson sent several letters under many pseudonyms to Eugene Richardson. Patterson had been the Field Museum's Curator of Vertebrate Paleontology, and had retained an accomplice who knew of the prank and prevented it from going too far. An expedition was scheduled, but cancelled after a Christmas letter had notified everyone that i was a hoax. Richardson recounted this story, publishing the letters, poems and doctored images under the name E. Scumas Rory.

*T. gregarium* was made the state fossil of Illinois in 1989. Thus, paleoart of *T. gregarium* is featured on U-Haul rental vehicles from Illinois.

## Description

*Tullimonstrum* likely reached 35 centimeters (14 inches) long, with the smallest at 8 centimeters (3.1 inches) long. It bore a pair of vertical, ventral fins that sat at the end of the body, though this is difficult to tell from fossils, typically having a long proboscis with up to 8 teeth on each "jaw". These may have probed for small animals an detritus on the muddy floor. It was part of the ecological community of soft-bodied organisms in the Mazon Creek assemblage in Grundy County, Illinois. The lack of hard tissue in the remains insinuates it did not bear bone, chitin or calcium carbonate organs There is evidence of serially-repeated organs. It's head poorly differentiates from the remaining body. A transverse bar-like structure, being either dorsal or ventral, end in two round organs, associated with a dark material found to be melanosomes (containing melanin). These have been suggested to be camera-like eyes. *Tullimonstrum* also bears structures interpreted as gills and a possible notochord or ramshackle spinal cord.

## Classification

### Vertebrate

A 2016 morphological study shows *Tullimonstrum* may have been a basal vertebrate, so it should be placed into chordata. One study suggested it may have been close to modern lampreys. This was based on the pronounced cartilaginous arcualia, dorsal fin, keratinous teeth, one nostril, tectal cartilages and an asymmetrical caudal fin. McCoy et al. suggested it was ancestral to the lamprey, having many features not in cyclostomes. Another study found it was a stem vertebrate, since it has a camera-like eye with preserved lenses and cyclindrical/spheroid melanosomes in distinct layering, interpreted as retinal pigmented epithilium, which indicates the bar-like organs are eye stalks. Clements et al. chemically found melanin. This may suggest it was a vertebrate since dual melanosome morphology and retinal pigmented epithilium are strictly vertebrate characters. McCoy et al. (2020), using Raman spectroscopy, identified molecular bonds in the organic remains. Based on samples from multiple areas if the body, they find the organic material as proteins and their decay products, which is seen in vertebrate, rather than the polysaccharide-based chitin of arthropods.

### Non-Vertebrate

A 2017 study rejects *Tullimonstrum* as a vertebrate based on how two melanosomes are variable in vertebrates, how hagfish lack them and how sharks and even the contemporaneous *Bandringa* have spheroid melanosomes. What was interpreted as a notochord only extends in front of the eyes, a feature not seen in any vertebrate. If it were a notochord, the presence of such is not limited only to vertebrates. Criticism was drawn towards the identification of the blocks of the body, as the myomeres and gill pouches vary, despite how they lack differentiation. Myomeres are thinner in vertebrates, extending the entire body rather than ending at the head. The gill pouches, rather than being segmented, are typically embedded into a complex skeleton. Both are not in *Tullimonstrum*.

Other identifications, however, are not as clear. The supposed brain bears no nervous tissue and is disconnected from the eyes, with the supposed liver located under the gills rather than farther back, as in vertebrates. The mouth at the end of the proboscis was described as having gnathostome-like tooth rows, despite lampreys having tooth fields. This would have made it have to convergently evolve grasping jaws, which were lost by lampreys. The thin and jointed proboscis is inconsistent with ram and suction feeding, which is typically used by open water vertebrates. The gill pouches would have obstructed water flow.

They noted the stalked eyes, tail fins and brains are also seen in anomalocarids, with *Opabinia* having a similar "feeding" appendage. While it was rejected *Tullimonstrum* was an arthropods (since Mazon Creek arthropods are 3D with carbonized exoskeletons), it was soon found to be a possibility. Though arthropods lack vertebrate-like melanosomes, some convergently evolve spheroid eye cells that may preserve similarly. However, these ommochromes and pterines bear unique chemical signatures, which were not found in *Tullimonstrum*. Sallen et al. suggested molluscs convergently evolve similar eyes with melanosomes, failing to note that no known molluscs have dual melanosome morphology. Similarities such as a lobed brain, muscle bands, tail fin, proboscis and "teeth" could support *Tullimonstrum* as a type of mollusc. If *Tullimonstrum* eyes are homologous with vertebrates, they may be a tunicate, a lancelet or an acorn worm or vetulicolian. One study published in 2023 argues that it may be a non-vertebrate chordate or a protostome invertebrate based on 3D analysis of its anatomy, revealing that the myomeres, tri-lobed brain, tectal cartilage and fin rays are not comparable to vertebrates.

## Paleoecology

*Tullimonstrum* was likely a fast-moving carnivore who inhabited open marine waters, occasionally washed near shore where some individuals fossilized. Mazon Creek fossils are unusual since they were rapidly buried in silty outwash, with bacteria decomposing organic remains sitting in the mud, producing carbon dioxide in the surrounding sediment. This combined with the iron from groundwater around the remains, which encased them in siderite nodules. As it was entombed, it cast an imprint on the shell, since it decayed slower. Still, Mazon beds fossilization is poorly-known. Rapid burial and formation results in excellent preservation, finding many plants and animals entombed in mood. Thus, the Mazon beds are one of the world's major Lagerstätten. Such rapid burial often causes most *Tullimonstrum* carcasses to fold over and contort, which is seen in many Mazon fauna and *Tullimonstrum* specimens. The proboscis is rarely fully-preserved, being complete in ~3% of specimens but present in some capacity in ~50% of specimens.

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 https://en.wikipedia.org/wiki/Tullimonstrum
2. ↑ https://onlinelibrary.wiley.com/doi/10.1111/pala.12646