Mary Bowerman

USA TODAY Network

Decades after a strange "monster-like" fossil was found in Illinois, researchers have finally identified it.

The Tullimonstrum gregarium, which swam through bodies of water some 300-million-years ago, was a vertebrate, according to an analysis published Wednesday in Nature.

That means the creature, which only grew to about a foot-long, belonged to the group that would one day include modern-day fish, birds, mammals, amphibians and reptiles.

Despite the discovery, little is still known about the Tully monster, according to Victoria McCoy, the lead author of the study.

“It’s so different from its modern relatives that we don’t know much about how it lived,” McCoy said in a statement. “It has big eyes and lots of teeth, so it was probably a predator.”

The Tully monster was discovered in 1958 by an amateur fossil hunter in Illinois and was described scientifically in 1966, but for decades, researchers could not decide what it was, according to Derek Briggs, co-author of the study

“Basically, nobody knew what it was,” said Briggs, Yale’s G. Evelyn Hutchinson Professor of Geology and Geophysics, in a statement.

Since it was first discovered, thousands of “Tully monster” fossils have been found in Illinois, and nearly 2,000 specimens are on file at the Field Museum of Natural History in Chicago.

“The fossils are not easy to interpret, and they vary quite a bit,” Briggs said in a statement. “Some people thought it might be this bizarre, swimming mollusk. We decided to throw every possible analytical technique at it.”

Researchers analyzed countless fossils and implemented synchrotron elemental mapping, "which illuminates an animal’s physical features by mapping the chemistry within a fossil," according to Yale.

For the first time, researchers were able to identify gills and a notochord, or a “rudimentary spinal chord” in the animal.

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