Key Takeaways on the Tully Monster
The Tully monster lived before the dinosaurs 300 million years ago and had a fish-like body, with a long, slender proboscis that had a toothed claw and eyes on either end of a rigid rod across its back.
The Tully monster was discovered 70 years ago and experts are still puzzled by the creature. They have failed to classify it and it does not have a place in the tree of life.
Some investigators lumped the Tully monster in with segmented worms, others with mollusks, and still others with arthropods, which covers everything from lobsters to insects to millipedes.
One day in elementary school, in the pages of some forgotten book on paleontology, Victoria McCoy stumbled upon a supremely strange, ocean-dwelling animal. Then, as now, it was known only as the Tully monster.
Living 300 million years ago — long before the first dinosaurs — it had a fish-like body, a long, slender proboscis tipped with a toothed claw, and eyes fixed on either end of a rigid rod running across its back.
But what most intrigued McCoy, now a paleobiologist at the University of Wisconsin, Milwaukee, was the fact that no one knew what it was. Experts had failed to classify it even at the most fundamental levels. Was it a worm, a slug, an arthropod, or something else entirely? Where did it belong?
“With the arrogance of a child,” McCoy says, “I thought: Someday I will answer that question.”
The arrogance wasn’t hers alone; many people have tried to assign the Tully monster a place in the tree of life since it was first discovered 70 years ago. Yet its bizarre anatomy leaves plenty of room for interpretation, and consensus remains elusive.
The Tully Monster Discovery
It all began in 1955, in the Mazon Creek fossil beds near Chicago, Illinois, where an amateur fossil collector named Francis Tully found the mineralized imprint of an organism he didn’t recognize.
“None of the books had it,” he told the Chicago Tribune. “I’d never seen it in museums or at rock clubs. So I brought it to the Field Museum to see if they could figure out what the devil it was.”
No one had a clue. Eugene S. Richardson Jr., then a curator at the museum, later wrote that “we could not even decide which phylum to put it in.” Considering a phylum is the highest level of taxonomic classification within the animal kingdom, “that was a serious and embarrassing matter.”
Ideally, formal scientific names reflect an organism’s suspected relationship to other lifeforms. But in 1966, after a decade without progress, Richardson gave one to the mystery fossil anyway. Since people had taken to calling it “Tully’s monster,” and since it had turned out to be rather common, he simply translated all that into Latin: Tullimonstrum gregarium.
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What Is the Tully Monster?
As thousands more specimens emerged from Mazon Creek, speculation about the Tully monster’s identity ran wild. Some investigators lumped it in with segmented worms, others with mollusks, and still others with arthropods, which covers everything from lobsters to insects to millipedes.
More recently, in a 2016 Nature paper, McCoy and her colleagues made a startling assertion: based on several lines of reasoning — including what seemed to be a notochord, a sort of primitive backbone — they concluded the Tully monster was a vertebrate, in the ranks of mammals, reptiles, birds and fish. Its closest living relatives, in that case, would be jawless fish like lampreys and hagfish.
The very next month, in another Nature paper, researchers from the University of Leicester bolstered the case for classifying the Tully monster as a vertebrate. They reported a particular arrangement of pigment granules in its eyes that, up to then, had been found in all vertebrates and nowhere else.
“At the time,” McCoy says, “it was viewed as a smoking gun.” But later research showed that decay processes can give a similar appearance in the eyes of cephalopods, like squid or nautiluses.
Tully Monster - Vertebrate vs. Invertebrate
A few years later, a team of researchers, led by University of Tokyo paleontologist Tomoyuki Mikami, used 3D laser scanning to map the bodies of some 150 Tully monster fossils. In a 2023 paper in the journal Paleontology, they argued that the various structures other researchers had deemed vertebrate-like (including the teeth, brains, ray fins, and muscle segments) were “not comparable with those of vertebrates.”
Instead, they suggested it may have been an invertebrate chordate, meaning it had a notochord but not a true backbone. This small group, closely related to vertebrates, is represented in modern times only by eel-like lancelets and tunicates, barrel-shaped marine organisms more commonly known as sea squirts.
McCoy is open to that possibility, saying, “It would not surprise me at all” if future phylogenetic analysis lends weight to Mikami’s interpretation. But due to the Tully monster’s large size and complex eyes, she adds, “I'm fairly convinced by my conclusions.” She believes the creature is a vertebrate, albeit a singular one.
For now, there’s likely no evidence that everyone will find compelling. McCoy is holding out hope that more Tully monster specimens lie waiting beyond Mazon Creek, perhaps preserved in a different manner.
“If we had a really clear picture of the internal organs and musculature, I think that would clarify a lot of things,” she says. “And you just don’t get that at Mazon Creek, except really rarely.”
How Old was the Tully Monster?
If you go back 540 million years to the Precambrian period, it’s perfectly normal to find fossils out of left field. That’s because it was a particularly innovative era for evolution; the phyla we see today — the basic body plans of all modern animals — were just beginning to differentiate. So, many organisms from that time likely don’t fit into our scheme of categorization.
The Tully monster, by contrast, “is very young to be so mysterious,” McCoy says. An unclassifiable fossil from 600 million years ago, sure. But 300 million years ago? That’s unusual. On the other hand, it leaves McCoy confident that there is, in theory, a taxonomic home for Tullimonstrum gregarium. Even if the placement within existing phyla isn’t obvious, she says, “we wouldn’t expect there to be some additional phylum we don’t know about.”
Exciting as it would be to discover that the Tully monster is a member of an unknown branch of the animal kingdom, the odds are low. But given the creature’s extraordinary, unique appearance, it will undoubtedly reveal much about whichever group it belongs to. When paleontologists finally straighten things out, McCoy says, “learning about it could really tell us a lot more about the tree of life.”
Read More: Newly Identified Monstersaur Lizard Had an Armored Skull and Walked Among Dinosaurs
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Nature. The eyes of Tullimonstrum reveal a vertebrate affinity
Proceedings of the Royal Society B. Synchrotron X-ray absorption spectroscopy of melanosomes in vertebrates and cephalopods: implications for the affinity of Tullimonstrum
Paleontology. Three-dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities
Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
