Since 1955, when amateur fossil hunter Francis Tully discovered the unlikely prehistoric creature in a coal mining area near Morris, the thing that would be named the Tully monster has presented one of the great puzzles in paleontology. The Tully's renown stretched even to the Illinois state legislature, which named it the official state fossil in 1989, some 308 million years after it inhabited the shallow salty waters that turned into the state's Mazon Creek geological deposits, in Grundy County, one of the richest fossil troves on Earth. Utilizing the synchrotron X-ray machine at Argonne and the Field Museum's collection of 2,000 Tully specimens, a team from those two institutions, Yale University and the American Museum of Natural History announced in a paper published in the journal Nature Wednesday that "The Tully monster is a vertebrate." Later papers proposed that it was a "free-swimming shell-less snail," he said, and then a conodont, extinct eel-like creatures very rare in the fossil record. Advertisement Continue reading the main story 
 
 “The frustrating thing is that these morphological features are not typical of any group,” said Victoria McCoy, a paleontologist and lead author on the paper. They closely examined the creature’s features, like its torpedo-shaped body and triangular tail, the proboscis that looks like an elephant’s trunk with sharp teeth, and the eyes on the side of its head, which resemble a hammerhead, but are similar to eye stalks found in crabs and insects. After finding that the creature had a primitive backbone, they could classify it as a chordate, which is a family of species that includes all vertebrates. He does not recall a moment where somebody said, "Hey, lamprey!
– It's been more than half a century since Francis Tully found the monster that has since defied classification. Now, scientists say they know where the prehistoric oddball that lived some 308 million years ago fits on the Tree of Life: "The Tully monster is a vertebrate," according to research published Wednesday in Nature. That's a big step for the creature Tully discovered when he was hunting for fossils in the Mazon Creek geological deposits southeast of Chicago in 1955. Previously the Tully monster, with its torpedo body, hammerhead-like eyes, and long proboscis filled with sharp teeth, had been categorized as "problematica"—"creatures that defied ready classification," the Chicago Tribune reports. Some have speculated that Tullimonstrum gregarium—which, despite its name, is only about a foot long—was related to snails, worms, or insects and crabs, reports the New York Times. "If you put in a box a worm, a mollusk, an arthropod, and a fish, and you shake," one paleontologist tells the Tribune, "then what you have at the end is a Tully monster," Some have even floated the idea that the Tully monster was a tiny version of the Loch Ness Monster, per Smithsonian. However, the researchers found that Tully is related to the lamprey, an "underwater bloodsucker," as the Times puts it. Using a synchrotron X-ray machine, researchers were able to determine that what was previously thought to be the creature's gut was actually a notochord, "the primitive backbone," study lead Victoria McCoy tells the Times. "The coolest thing is finding out that as weird as it looks it is part of a familiar group of animals." Check out a graphic of the Tully monster. (A 90 million-year-old fossil indicates T. rex got smart before it got big.)