Since it was first uncovered more than a half a century ago, this kooky-looking creature known as the “Tully monster” has puzzled paleontologists who, frankly, could not make heads, tails or claws of its fossilized remains.

The creature was named after Francis Tully, the amateur who discovered it in 1958 in the Mazon Creek in Illinois. The state has designated the monster as its official fossil.

Some thought the 300-million-year-old creature was a mollusk, like a snail. Others assumed it was an arthropod like an insect or crab. And others believed it was some sort of worm.

Now, a team of researchers from Yale University say they have figured out the monster’s identity: It’s a vertebrate most closely related to the lamprey, an underwater bloodsucker.