Name: Tullimonstrum gregarium
Name meaning: Tully’s Common Monster
Age: Late Carboniferous (309 – 307 million years ago)
Size: 25 cm in length approx.
Location: Illinois, United States
If you are ever asked what the strangest creature nature has ever produced is, the Tully Monster (Tullimonstrum gregarium) would make a good answer. For decades, this strange animal has baffled palaeontologists as to just where it lies in the phylogenetic tree, and continues to push our imagination of what animals can be.
Tullimonstrum is known from the Late Carboniferous Mazon Creek biota of Illinois in the United States and became the state’s official fossil in 1989. It was discovered in 1958 by Francis Tully, and when first described in 1966, it was originally thought to be a worm-like invertebrate that lived in shallow, coastal waters. Hundreds of Tullimonstrum fossils have been found, usually as stained impressions in siderite nodules. Therefore, palaeontologists agree on what its general, strange anatomy looked like. With a body similar to a squid, its eyes were positioned at each end of a horizontal bar, and at the end of a long proboscis appendage (like an elephant’s trunk) was a strange claw-like structure containing teeth, which may have acted as a very strange grabber arm it would have used to grasp food.
But despite this agreement on what it looked like, palaeontologists have struggled to figure out Tullimonstrum’s role in its ecosystem and what it was related to. Even a relatively basic question like ‘is it a vertebrate?’ seemed to have complicated answers. In 2016, it was suggested that Tullimonstrum was indeed a vertebrate because the melanosomes preserved at the end of its eyestalks were most similar to the melanosomes found in vertebrates. This, along with other characteristics, suggested Tullimonstrum was a vertebrate in the lineage of Petromyzontida (lampreys) due to similar traits that both share, such as a single nostril.
However, this conclusion has been questioned, with other scientists arguing that the vertebrate-like traits found in Tullimonstrum could be the result of convergent evolution, the same as other traits like complex eyes and ‘teeth’. Furthermore, in 2019, a different group of palaeontologists also looked at the melanosomes within the fossilised remains of Tullimonstrum’s eyes and found that the chemical signatures more closely resembled those of cephalopods than vertebrates, suggesting that Tulliomonstrum could be an invertebrate.
Tullimonstrum remains one of the most infamously problematic fossils, along with many others, and we aren’t definitively sure where it belongs in the history of life on Earth. One thing I think all palaeontologists can agree on though, is that this fascinating, peculiar creature has certainly earned the name ‘monster’.
 An artist’s illustration of Tullimonstrum gregarium. Taken from LiveScience.
 T. gregarium fossil from the Mazon Creek Lagerstätte. Scale bar: 40 mm. Taken from Clements et al. (2016).
Information References and Further Sources
 Clements, T., Dolocan, A., Martin, P., Purnell, M. A., Vinther, J., and Gabbott, S. E. (2016). ‘The eyes of Tullimonstrum reveal a vertebrate affinity’, Nature, 532, pp. 500-503. Accessed 8th October 2020. Click Here.
 Kuratani, S., and Hirasawa, T. (2016). ‘Getting the measure of a monster’, Nature, 532, pp. 447-448. Accessed 8th October 2020. Click Here.
 McCoy, V. E., Saupe, E. E., Lamsdell, J. C., Tarhan, L. G., McMahon, S., Lidgard, S., Mayer, P., Whalen, C. D., Soriano, C., Finney, L., Vogt, S., Clark, E. G., Anderson, R. P., Petermann, H., Locatelli, E. R., and Briggs, D. E. G. (2016). ‘The ‘Tully monster’ is a vertebrate’, Nature, 532, pp. 496-499. Accessed 8th October 2020. Click Here.
 Richardson Jr., E. S. (1966). ‘Wormlike Fossil from the Pennsylvanian of Illinois’, Science, 151 (3706), pp. 75-76. Accessed 8th October 2020. Click Here.
 Rogers, C. S., Astrop, T. I., Webb, S. M., Ito, S., Wakamatsu, K., and McNamara, M. E. (2019). ‘Synchrotron X-ray absorption spectroscopy of melanosomes in vertebrates and cephalopods: implications for the affinity of Tullimonstrum’, Proceedings of the Royal Society B, 286 (1913). Accessed 8th October 2020. Click Here.
 Sallan, L., Giles, S., Sansom, R. S., Clarke, J. T., Johanson, Z., Sansom, I. J., and Janvier, P. (2017). ‘The ‘Tully Monster’ is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals’, Palaeontology, 60 (2), pp. 149-157. Accessed 8th October 2020. Click Here.