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.