For a creature accused of being “problematic” and a “monster,” Tully’s monster was actually quite delicious. Its body was long and soft like an overripe cucumber, tapering to a spade-like caudal fin at its rear end. At the front of the creature, where one would expect a head, its body tapers instead into an appendage similar to an elephant’s trunk and ends in a claw with gripping teeth. And as a final disconcerting garnish, a pair of eyes at each end of the horizontal rod rest on the creature like an eerie oar.

This 300 million year old creature has been mysterious since its discovery decades ago. It’s an animal with no obvious ancestors or descendants, an organism seemingly out of place in any place or time or, well, situation. It’s the kind of fossil that makes me wonder if it could ever be exposed as a hoax; the creature’s neck-like snout even convinced a Loch Ness truther that Tully was Nessie’s ancestor.

For decades, scientists have tried to embed Tully’s monster in the tree of life. People have repeatedly suggested that the creature was a worm, or maybe a lobster, or some kind of slug, or maybe a fish? Different scientists have switched between calling the creature a vertebrate or an invertebrate, and each new theory inevitably leads to triumphant headlines declaring that, finally, the mystery of Tully’s Monster has been “solved”. The latest of these headlines arrived this week, accompanying a new article published in the journal Paleontology which aims to tip the vertebrate-invertebrate pendulum in favor of invertebrates.

Over the years I have followed the mystery of the Tully Monster and other “problematic” fossils as they have ostensibly been solved. Last year, the mystery of the “alien goldfish” was solved, the mystery of the “skeleton tubes” was solved, and the mystery of the “half-billion-year-old creature without an anus” was solved . With so many of these stories floating around, it’s easy to lose sight of what, exactly, is “solved” here. The obvious and most common answer is taxonomy, where each of these creatures can be classified on the sprawling tree of life. The alien goldfish was a mollusc; the skeletal tubes belonged to an ancient tube-making jellyfish; and the creature without an anus was an ancestor of spiders and insects. This classification question forms the titular mystery of Tully’s Monster: What kind of animal was it?

Tully’s monster roamed the tropical coastal waters of what is now Illinois, with more readable relatives of modern shrimp, jellyfish, sharks, and sea cucumbers. The species was discovered in 1958 when Francis Tully, a retired pipefitter and amateur paleontologist, found an ironstone nodule that had cracked in two. Nodules are commonplace at Mazon Creek, where millions of years ago they hardened by chemical reactions around dead animals and plants and were then dumped by coal miners into heaps laden with fossils. This particular cracked iron stone contained the impression of an airship-like creature that Tully had never seen before. He took his find to the Field Museum, where it also baffled paleontologists; he was soon dubbed Tully’s Monster. In 1966, Eugene Richardson, the museum’s curator of fossil invertebrates, described the fossil in Science and doubled it Tullimonstrum Gregariumsimply Latinizing his nickname.

Years of collecting have unearthed thousands of Tully’s monsters, ranging in size from a few centimeters to nearly a foot long. The creature has become the state fossil of Illinois, as well as the face of many U-Haul trucks. As the fame of Tully’s Monster grew, so did attempts to classify it. In his description of the fossil in 1966, Richardson went a little further than “worm-like”. In 1979, a geologist suggested that it was an “aberrant member” of molluscs, possibly something like a sea snail. About a decade later, a new analysis suggested that the parents most relatives of Tully’s Monster were either sea snails or extinct, oddly toothed eel-like creatures called conodonts. Somewhere along the way, another paleontologist concluded that the fossil must be one of two unrelated types of worms.

All of these identifications seemed plausible, but somewhat impossible to prove. The iron stones of Mazon Creek have preserved beautiful silhouettes of even the most diaphanous creatures, but no organic material from the animal itself. But in 2016, two independent articles, both coincidentally published in the journal Nature, came out with new evidence suggesting Tully’s monster was a vertebrate. A team of researchers discovered microscopic pigments in the eyes of the Tully fossils, examined them under a scanning electron microscope and found a carpet of pigment granules in the shape of “meatballs” and “sausage”, a characteristic unique among vertebrates. The other team re-examined a white streak down the middle of the Tully’s Monster body that was previously interpreted as the creature’s digestive tract – odd, given that other fossils found at Mazon Creek had black digestive tracts and some fossils of Tully’s monsters had a black line. in addition to a white. The researchers therefore identified the linea alba as a notochord, a flexible rod found only in vertebrates.

This latter group suggested that the closest living relative of the Tully Monster was a lamprey, the noodle-like fish with concentric rings of nightmarish teeth. Renowned Papers—The New York Times, The Atlantic, And American Scientist– said the Tully mystery was finally solved, and people flocked to the streets to blow up festive pots and pans and party vuvuzelas.

But nothing from 300 million years ago is ever really settled, and a year later a group of totally unconvinced paleontologists hit back in an article in Paleontology titled in part “The ‘Tully Monster’ is not a vertebrate.” He pushed back on the claims in the previous two articles, noting that lampreys have been found at Mazon Creek that don’t look like Tully’s Monster, and citing the invertebrate kingdom’s many beautifully complex eyes – think octopus! “If you’re going to make extraordinary claims, you need extraordinary evidence,” Lauren Sallan, a paleobiologist at the University of Pennsylvania and author of the Paleontology paper, said in a press release.

In the years that followed, the newspapers kept coming. A 2019 analysis of Tully’s monster eyes in the newspaper Proceedings of the Royal Society B threw a hat in the invertebrate ring, and a soft tissue analysis in 2020 in Geobiology tossed a coin into the vertebrate fountain. And now a 2023 article creating a 3D model of Tully’s monster in Paleontology joined the ranks of would-be mystery solvers, arguing that the creature was an invertebrate due to segmentation in its head region.

In the opening of the beloved musical fiddler on the roof, the villagers of the Russian Anatevka shtetl sing a song about their traditions, aptly called “Tradition”. The song reaches an iconic climax when the villagers, divided over the identity of an ungulate recently sold at the market, begin to shout “Horse! Mule! Horse! Mule!” before returning to sing together once more on the tradition. That’s how I feel every time the mystery of Tully’s Monster is breathlessly announced to have been solved once again. The headlines read like facts, as if the debate was indeed settled. Tully’s Monster didn’t have a backbone after all! Tully’s monster eyes prove it’s a vertebrate! The true nature of Tully’s monster revealed! Of course, I want more, endless articles about this unusual creature. Finding the true role of Tully’s Monster in the history of life on Earth is an important question, one that could eventually be answered with new tools, new fossils, or even new researchers. It’s the great tradition of science: to get closer and closer to our best approximation of the truth.

But in my view, taxonomy is not the only, or even the most interesting, issue at stake here. Whether or not Tully’s monster had a notochord does not mean solve the mysteries of a truly bizarre creature that once invaded Chicago’s submerged canyons like Cubs fans in 2016 (a huge year for Illinois creatures past and present). I could spend hours speculating on how the creature’s dumbbell-shaped eyes may have scanned the tropical waters for prey, what colors adorned its soft and possibly slimy body and, most importantly again, about all the ways it used its curved mouth-snout – for food, sure, but any elongated snout raises a host of evolutionary questions. The existence of a living being is not limited to knowing who its relatives are. And the closer we get to the truth, the more we remember the unfathomable interval of time.

Either way, see you next year when the mystery of Tully’s monster is solved again and BYOV (bring your own vuvuzela)!