An illustration shows two Tully Monsters -- the smaller creatures in the middle ground -- swimming in Mazon Creek some 300 million years ago. Photo by John Megahan

Feb. 20 (UPI) -- Scientists are calling for the Tully Monster mystery to be reopened.

Last year, two scientific papers claimed to have solved the Tully mystery. According to the two studies, the Tully Monster was definitively a vertebrate -- a member of the lamprey lineage.


Now, a team of paleobiologists claim those papers were flawed.

"This animal doesn't fit easy classification because it's so weird," Lauren Sallan, an assistant professor at the University of Pennsylvania, said in a news release. "It has these eyes that are on stalks and it has this pincer at the end of a long proboscis and there's even disagreement about which way is up. But the last thing that the Tully Monster could be is a fish."

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The Tully Monster, Tullimonstrum gregarium, was first discovered in the 1950s in Mazon Creek, a prolific fossil bed in central Illinois. Though thousands of Tully specimens have been recovered from 300-million-year-old ironstone strata, scientists have been unable to make out exactly what the Tully Monster is.

"Initially it was published as a worm," Sallan said. "There is a well-constructed argument that it is some kind of mollusc, like a sea cucumber. And there's another very strong argument that it's some kind of arthropod, similar to a lobster."

Last year, scientists claimed re-analysis of several hundred fossils revealed the presence of a primitive backbone and other organ structures. But the latest paper claims marine fossils, such as those recovered from the Mazon Creek deposits, preserve only soft tissues. Also, lamprey fossils recovered from Mazon Creek don't look similar to Tully Monster fossils.

Another research team claimed analysis of the Tully Monster's eyes revealed melanosomes, the organelles that produce and store melanin -- proof of Tully's likely vertebrate classification.

The latest research second-guesses such logic.

Several arthropods and cephalopod species boast complex eye structures. Furthermore, researchers suggest the Tully Monster featured cup eyes -- a simple structure without lenses.

"So the problem is, if it does have cup eyes, then it can't be a vertebrate because all vertebrates either have more complex eyes than that or they secondarily lost them," Sallan said. "But lots of other things have cup eyes, like primitive chordates, molluscs and certain types of worms."

In their scientific takedown -- published this week in the journal Palaeontology -- Sallan and her colleagues argue a misclassification can have wide-ranging and problematic consequences.

"Having this kind of misassignment really affects our understanding of vertebrate evolution and vertebrate diversity at this given time," Sallan said. "It makes it harder to get at how things are changing in response to an ecosystem if you have this outlier. And though of course there are outliers in the fossil record -- there are plenty of weird things and that's great -- if you're going to make extraordinary claims, you need extraordinary evidence."