In the summer of 1955, when Francis Tully set off on a fossil-collecting jaunt some 50 miles south of Chicago, he had no idea that his name was about to go down in paleontological history. A pipe-fitter by trade, Tully was just an amateur collector, but a skilled one. He knew that the coal miners of Mazon Creek had discarded vast piles of shale that contained fossils galore. And as he sifted through the fragments, he found two rocks that had cracked open and that held something incredible between them.

It looked like … an obese foot-long earthworm with a trunk and a spade-shaped tail? “I knew right away. I’d never seen anything like it,” Tully later said. “None of the books had it. I’d never seen it in museums or at rock clubs.” He took it to the Field Museum in Chicago, where it came to the attention of Eugene Richardson, curator of fossil invertebrates. He had no idea, either. He couldn’t work out what the creature was related to, and he couldn’t assign it to any of the major animal groups—a “serious and embarrassing matter,” he wrote.

Whatever it was, it was clearly an abundant part of its ancient ecosystem, some 300 million years ago. Collectors quickly found one specimen after another. Hundreds ended up at the Field Museum. Many more lined shelves in rock shops and private collections. The creature became affectionately known as the Tully monster, and the name became so popular that Richardson felt compelled to formally describe it in 1966. For lack of a better alternative, he simply Latinized the nickname into Tullimonstrum gregarium. (“Darned right,” said Tully.)

Fifty years after Tully’s discovery, he and Richardson have both passed away, and the Tully Monster is the official state fossil of Illinois. And finally, a team of scientists led by Victoria McCoy at Yale University have solved the mystery of the strange beast, and assigned it a spot on the tree of life.

It turns out to be a close relative of modern lampreys—those nightmarish, blood-sucking fish that are essentially toothed suction cups propelled by sinuous, eel-like bodies. But although the Tully monster is a lamprey at heart, it looks nothing like one from the outside. Its body is short and stout. Its eyes sit at the end of a rigid bar. And instead of the distinctive sucker, its mouth is a long, triple-jointed claw. It looks like the rejected doodle of a drunk fantasy artist.

The monster was one of McCoy’s favorite fossils when she was a small child. When she finally decided to study it, she found plenty of kindred spirits. “I approached the Field Museum and even before I told them my project, they said: We’d love for someone to look at the Tully Monster,” she recalls. With fifteen colleagues, a patient photographer, and some state-of-the-art equipment, she catalogued more than 1,200 specimens and spent months poring over them.

When she started, she figured that the Tully monster might have been a mollusk—a kind of shell-less, swimming snail. There are even living sea snails called heteropods, or elephant snails, that look superficially similar. They also have tail fins and long, trunk-like mouths. But soon after studying the specimens, McCoy defected from Team Mollusk and realized that the animal was almost certainly a vertebrate—a back-boned animal, just like us.

There were two critical clues. First, almost all the specimens had a light line running down its middle, from the eyes to the tip of the tail. Other scientists had interpreted this as a gut, but the animal’s actual gut ends further up its body and appears black in the fossils. The light line is actually a notochord—a flexible rod that forms the basis of our backbones. That’s the defining feature of the chordates, the group to which we and other vertebrates belong.

McCoy’s team also found a few specimens with flaps on their sides that would have housed gills. These gill pouches are obvious in many fossil fish, which tend to land on their side when they die. But for some reason, dead Tully monsters typically landed on their fronts or backs—a position that obscured their gills as they turned to stone. Thankfully, the team had so many specimens that at least some were preserved side-on.

Based on the notochord, gill pouches, and other features, the team concluded that the Tully monster was a vertebrate, and specifically an early member of the lamprey family. “The evidence seems secure and clear,” says Mike Benton from the University of Bristol. “It puts to rest a long-running conundrum over the identity of Tullimonstrum.” Indeed, Philippe Janvier from the National Museum of Natural History in Paris says that another team is about to publish a paper that makes different arguments but comes to the same conclusions.

But the Tully monster is extravagantly different from other lampreys, both living and extinct.“Living lampreys are often perceived as 'primitive'—a good proxy for the form of the earliest fish,” says Martin Smith from Durham University. “Tullimonstrum provides a powerful antidote to this misconception: the lamprey lineage has undergone its own 500 million years of evolutionary experimentation, producing a great diversity of complex, unusual, and sometimes unexpected body plans.”  

Unexpected, indeed! Why, for example, were Tullimonstrum’s eyes at the end of a rigid bar? “We think that the best comparison is to the hammerhead shark,” says McCoy. Their wide-set eyes give them exceptional binocular vision. “We think the eyebar allowed Tully monster to see the things it was grabbing with the mouth at the end of the proboscis.”

Oh yes. There’s that. Other scientists had interpreted that weird mouth as a flexible trunk. But since it usually had sharp bends in the same three places, McCoy’s team think it was jointed. It ended in a claw-like mouth, which contained two rows of teeth and could probably open and close. The mouth also contains something that looks like a tongue.

Perhaps the whole proboscis is an extremely extended version of the lamprey’s sucker. Lampreys stick to passing fish with their mouths and teeth, while rasping off bits of flesh with their tongues. Perhaps the Tully monster did the same, but at a distance. “It might have been a sort of protrusile, lamprey-like feeding apparatus, like the jawed tongue of the monster in Alien,” says Janvier.

“It could have been sticking this thing down worm burrows or grabbing things that pass by,” adds McCoy. “But we don’t know a ton about it.”

That will come with time. New specimens and powerful scanning techniques are helping scientists to study charismatic fossils that have resisted classification. Take Hallucigenia—an acid dream of an animal complete with spines, tentacles, and flexible legs. It was discovered in 1977, but it took a while for scientists to work out which way up it stood, and it took till last year for them to conclusively agree on which end is the head. “Forty years ago, the fossil record seemed to be replete with 'problematic' fossils,” says Smith, who worked on Hallucigenia. “One by one, these have been brought into the fold and found places on the tree of life.”