Tullimonstrum gregarium is an enigmatic soft-bodied fossil from the Pennsylvanian Mazon Creek Lagerstätte of Northern Illinois. It was originally described by Richardson (1966) as problematica, but is listed as a worm-like animal in the title. Since this initial description, researchers have struggled to classify the 30 cm long fossil. Cladistic analyses have placed it within the clade uniting vertebrates, molluscs, annelids, and nemerteas.

Recently, these fossils have been subject to renewed testing and analysis. McCoy et al. (2016) assigned it to the Agnathans based on identifying a notochord, gill pouches, and w-shaped myomeres. They further supported this assignment with synchrotron analysis of trace element enrichment in the fossil. A vertebrate assignment was indicated by the discovery of melanosome structures in the eyes using SEM (Clements et al. 2016). In situ Raman microspectroscopy showed evidence that the structural tissue was made of proteins and not chitin, providing strong evidence for a vertebrate assignment (McCoy et al. 2020).

Mikami et al. (2023) examined surface features on the fossils using a 3D laser scanner, conducted X-ray microcomputed tomography scans of the teeth-like structures, and suggested that some features in the head represent preserved segments; they argued that the combination of features they observed is most consistent with a non-vertebrate chordate classification.

The use of new technologies and innovative methods to classify Tullimonstrum gregarium has resulted in new ideas about its origin, but researchers still disagree on its identity and even on its morphology. However, research continues, including a current investigation of its structural tissues using scanning electron microscopy, and each project provides new data that brings us closer to a final answer to this mystery.

References

Clements T, Dolocan A, Martin P, Purnell MA, Vinther J, Gabbott SE. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature. 2016 Apr 28;532(7600):500-3. doi: 10.1038/nature17647.

McCoy VE, Saupe EE, Lamsdell JC, Tarhan LG, McMahon S, Lidgard S, Mayer P, Whalen CD, Soriano C, Finney L, Vogt S, Clark EG, Anderson RP, Petermann H, Locatelli ER, Briggs DE. The 'Tully monster' is a vertebrate. Nature. 2016 Apr 28;532(7600):496-9. doi:10.1038/nature16992. Epub 2016 Mar 16. PMID: 26982721.

McCoy VE, Wiemann J, Lamsdell JC, Whalen CD, Lidgard S, Mayer P, Petermann H, Briggs DEG. Chemical signatures of soft tissues distinguish between vertebrates and invertebrates from the Carboniferous Mazon Creek Lagerstätte of Illinois. Geobiology. 2020 Sep;18(5):560-565. doi:10.1111/gbi.12397. Epub 2020 Apr 28. PMID: 32347003.

Mikami T, Ikeda T, Muramiya Y, Hirasawa T. Iwasaki W.Three-dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities. Palaeontology. 2023 66: e12646. doi.org/10.1111/pala.12646

Richardson, ES Jr. Wormlike Fossil from the Pennsylvanian of Illinois. Science. 1966 151, 75-76. doi:10.1126/science.151.3706.75.b