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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteThe Tully monster was an ancient animal, but scientists still disagree about what kind. Its scientific name is Tullimonstrum gregarium, and it lived in what is now Illinois roughly 309–307 million years ago. A 2016 study argued that it belonged among early vertebrates near the lamprey lineage; later studies challenged that reading. A 2025 review still treated its evolutionary relationships as unresolved.
What was the Tully monster?
Tullimonstrum gregarium, commonly called the Tully monster, is a soft-bodied fossil animal found in the Mazon Creek deposits of Illinois. The 2016 study describing the fossil beds dates their biota to approximately 309–307 million years ago, in the late Carboniferous.
Fossil collector Francis Tully discovered the animal in 1958. Illinois designated it the state fossil in 1989, according to the 2016 Nature paper.
Why does it look so unusual?
The fossil preserves an elongated body with fin lobes at the tail, eyes that project sideways from a bar, and a long proboscis ending in a toothed, grasping structure. These impressions do not include the familiar hard skeletons that often make fossil comparisons more straightforward. That makes interpretation difficult, but it does not mean the fossils contain no anatomical evidence.
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In 2016, researchers examined more than 1,200 specimens and interpreted some preserved structures as a notochord and gill pouches—features they considered evidence of vertebrate anatomy. Other researchers have disputed whether those structures are correctly identified or uniquely indicate a vertebrate.
Why did some scientists call it a vertebrate?
In a 2016 paper in Nature, McCoy and colleagues argued that the Tully monster was a vertebrate on the stem lineage leading to lampreys. Their case rested on the interpretation of several structures, including a proposed notochord and gill pouches, alongside other anatomical features.
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That was a scientific hypothesis based on how the team read the preserved impressions, not a settled classification. A 2017 paper in Palaeontology challenged the vertebrate interpretation, arguing that the supposed features did not establish that relationship.
What evidence has challenged the vertebrate hypothesis?
Eye chemistry
A 2019 chemical study compared fossil eye pigments with those of sampled modern animals. The reported chemistry was more similar to the sampled invertebrates than to vertebrates, weighing against the vertebrate interpretation. But that result did not identify which invertebrate group, if any, the Tully monster belonged to. As researcher Chris Rogers put it in Live Science: “So while our work adds weight to the idea that Tully is not a vertebrate, it doesn’t clearly identify it as an invertebrate either.”
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Three-dimensional anatomy
A 2023 study used surface scans from 153 specimens and micro-CT analysis of proboscis stylets to reassess the animal’s anatomy. Its authors cast doubt on features previously taken to support vertebrate affinity. The study in Palaeontology added a different kind of evidence from the large-scale comparative morphology in the 2016 work.
Why is there no definitive answer?
The competing studies ask different questions of fossils whose soft tissues are preserved as impressions. Researchers disagree both about what some structures represent and whether those structures, even if identified correctly, are distinctive enough to show a close evolutionary relationship.
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- Anatomy: The 2016 team read particular impressions as vertebrate features; later researchers questioned those identifications or their diagnostic value.
- Chemistry: The 2019 eye-pigment results weighed against vertebrate affinity in comparison with the animals sampled, but did not settle an alternative classification.
- Different methods: Comparative morphology, chemical analysis, surface scans, and micro-CT examine different evidence. A result from one approach does not automatically resolve what another approach can establish.
A 2025 review in Geology Today still described the Tully monster as an evolutionary enigma and discussed multiple proposed relatives, including a possible molluscan relationship. That is not proof of any one placement: the review reflects that its affinities remain debated.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.So, is the Tully monster a vertebrate or an invertebrate?
Neither label can be presented as settled. The 2016 vertebrate and lamprey-lineage proposal remains contested, while evidence against vertebrate affinity has not established a definitive invertebrate group. The most accurate answer is that Tullimonstrum gregarium is a fossil animal whose evolutionary placement remains uncertain.
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