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. 
 
 Much as the people of Metropolis wondered whether Superman flying overhead was a bird or a plane, scientists have struggled to classify these fossils that showed traits associated with several disparate animal types and such abnormalities as eyes mounted on an external bar and a long, toothy proboscis. 
 
 "If you put in a box a worm, a mollusk, an arthropod and a fish, and you shake, then what you have at the end is a Tully monster," said Carmen Soriano, a paleontologist at Argonne National Laboratory. 
 
 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. 
 
 Now, though, Tullimonstrum gregarium has a home on the Tree of Life rather than in the biological category known as the "problematica." 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." 
 
 Below that headline, the paper describes Tully as belonging "on the stem lineage to lampreys," a find that "resolves the nature of a soft-bodied fossil which has been debated for more than 50 years." 
 
 "This is one of the mysteries that I heard about since I was a kid," said Soriano. "To be able to study, to basically 'unmonsterize' the monster, is really exciting." 
 
 "Resolving this is a big deal," said Scott Lidgard, the Field's associate curator of fossil invertebrates and another of the paper's authors. "It's one of the examples used in textbooks around the world as what are called 'problematica,' " creatures that defied ready classification and were sometimes thought to be examples of extinct phyla, or animal categories. 
 
 "This is kind of a poster child for that sort of evolutionary puzzle," Lidgard said. The finding "changes it from a mystery to a fishlike organism that is probably on the lineage leading to what we would recognize as lampreys." 
 
 It's also a big moment for those who study lesser prehistoric animals and realize, said Lidgard, that "we're never going to be as popular as dinosaurs and fossil birds." 
 
 The Tully monster is named for its assemblage of features, not for any sort of fearsome size. The biggest of the many, many specimens that have been found suggested a maximum length of about 18 inches and typical length of 12. 
 
 But because Mazon Creek fossils are so well preserved, there is a lot of Tully to study. Skeletons have not survived, but detailed impressions in stone have. 
 
 "If you see the specimens, they are typically well preserved," Soriano said. "It's not that they are a blob in the rock." 
 
 Tully, a pipefitter for Texaco and lifelong fossil hound, described his find to the Tribune in a story in 1987, also the year of his death: 
 
 "I found two rocks that had cracked open from natural weathering. They held something completely different. I knew right away. I`d never seen anything like it. None of the books had it. I`d never seen it in museums or at rock clubs. So I brought it to Chicago to the Field Museum to see if they could figure out what the devil it was." 
 
 The first scientific paper describing the Tully monster, and giving it its vivid Latin name, came in the mid-1960s from one of Lidgard's predecessors at the museum, who "thought it was a worm," Lidgard said. 
 
 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. 
 
 "I've been looking at this thing for 30 years," said Lidgard. "Years ago I had a stab at it, thinking it might be related to squids. We gave up. We didn't publish anything." 
 
 What got the ball rolling again was Lidgard hearing about Victoria McCoy, a Yale grad student exploring the Mazon Creek deposits who would become the paper's lead author. 
 
 They met at a 2014 conference, and the following year, an assembled team spent three weeks at the Field studying its Tully specimens. 
 
 The Department of Energy's Argonne National Laboratory, southwest of Chicago, came into the picture because of its advanced imaging techniques using the Advanced Photon Source, an electron accelerator and storage ring that "provides ultra-bright, high-energy storage ring-generated x-ray beams for research in almost all scientific disciplines," according to Argonne. 
 
 "The thing with these machines is they are incredibly powerful microscopes," Soriano said. "We can get information not only on the morphology of the sample, but also on the structure, on the composition." 
 
 It allows people "to see what no one saw before basically," she said. 
 
 What the scientists saw, as they studied the Argonne imagery, digital photographs of the fossils and the fossils themselves were characteristics that tied the Tully monster to lampreys. 
 
 A chemical analysis of the eye stalks, for instance, showed the presence of zinc, "very similar to the material in the eyes of vertebrate fossil fishes," said Lidgard. 
 
 "Tully is usually preserved so that you're looking down on its back," he added. "Every so often you can see its side. In those twisted fossils we found a very few where we think we can distinguish openings we interpret as openings to a particular kind of gill structure present in very primitive fishes like lampreys." 
 
 And they were able to find the animal's gut trace, as well, the shadow of its digestive system, in the lower part of the body, which suggested that what had previously been thought to be a gut trace up on the back was in fact a notochord, a flexible rod in the back. 
 
 That made it a primitive vertebrate, he said. He does not recall a moment where somebody said, "Hey, lamprey!", but recalls that "it became more and more clear," he said. "As those results started to come in, it was pretty convincing right away." ||||| 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. 
 
 They published their findings on Wednesday in Nature. 
 
 To come to their conclusion, team members first pored over 1,200 Tully monster specimens from museums. 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. 
 
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 “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. “But they do not rule out any group very easily.” 
 
 The clue that led them to closing the cold case was a lightly colored structure scientists had previously identified as the creature’s gut. Only it wasn’t a gut. 
 
 “We discovered that this feature was the notochord, the primitive backbone,” said Dr. McCoy. 
 
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 Most guts in the fossilized record are dark and appear three-dimensional. But the Tully monster’s structure was light and appeared two-dimensional. 
 
 “It didn’t make sense to us that there would be this one animal that would fossilize its gut completely differently,” she said. 
 
 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. Then they had to narrow down the type of chordate to which it was most similar. By further examining the notochord, the fossil sleuths noticed that the structure curved down as it went through the creature’s tail. 
 
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 In animals like sharks, the notochord curves up into the top fin of the tail, and in some fish it goes through the middle of the tail. But in lampreys the notochord curves down. 
 
 “There was no big ‘Aha!’ moment that pointed to the lamprey. But put together, the strongest evidence was that it could be a lamprey,” Dr. McCoy said. “The coolest thing is finding out that as weird as it looks, it is part of a familiar group of animals.”
– 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.)