A team of researchers from the United States may have finally solved the mystery behind the Tully Monster, which had eluded scientists for five decades.

When amateur collector Francis Tully discovered the fossilized remains of the first Tully Monster (Tullimonstrum gregarium) in 1958, nobody knew what kind of creature it was exactly.

The 300-million-year-old fossil showed signs that the animal had a long stalk that extended from its body and ended with a teeth-filled opening known as a buccal apparatus. It also had gills that covered its body and tail, which the creature likely used to propel itself in the water.

Scientists have tried to determine what animal group the Tully Monster belonged to over the years but it remained largely unanswered.

In a study featured in the journal Nature, researchers from the Argonne National Laboratory, the American Museum of Natural History and Yale University found out that the soft-bodied Tully Monster was in fact a vertebrate that shared a lineage with modern-day lampreys.

While there have been thousands of fossil specimens of Tully Monsters recovered over the years, the research team still had a difficult time identifying the exact features of the creatures.

"The fossils are not easy to interpret, and they vary quite a bit," Derek Briggs, a geology and geophysics professor at Yale and one of the authors of the study, said.

"Some people thought it might be this bizarre, swimming mollusk. We decided to throw every possible analytical technique at it."

The researchers examined the morphology and preservation of different features of 2,000 Tully Monster fossils that were donated to the Field Museum. They also made use of state-of-the-art analytical techniques, including synchrotron elemental mapping, to find out the creature's physical features.

Aside from having gills, Tully Monster appears to have had a notochord as well, which likely served as a simple form of spinal cord for the creature. These two features were not identified during earlier analysis of the Tully Monster.

Victoria McCoy, a former graduate student at Yale and lead author of the study, said that the prehistoric animal looks very different compared its modern-day counterparts that there aren't enough information available regarding how it lived.

She said that Tully Monster had large eyes and many teeth, which suggests that it could have been predator during its time.

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