Ever since its 300-million-year-old fossils were first unearthed in 1958, the alien-like "Tully monster" has defied classification.

This bizarre creature featured a narrow, trunk-like neck that extended from its head, with a mouth at the end filled with razor-sharp teeth. Its eyes sat further back on the body at the ends of a rigid bar perched on its back, and it swam using cuttlefish-like fins at the tail section.

Needless to say, it looked more like a chimera, a hoax, than any sort of real creature. It was unlike anything else ever found on Earth.

In April 2016, a Yale-led team of paleontologists finally determined what this animal was, reports Phys.org. And no, it's not what you were thinking: it's not an alien.

It's actually a vertebrate, according to researchers, and its closest living relative is probably a lamprey. With painstaking, high-tech analysis of its fossils, the Yale team was able to establish that the Tully monster had gills and a stiffened rod or notochord (basically, a rudimentary backbone) that supported its body.

"I was first intrigued by the mystery of the Tully Monster. With all of the exceptional fossils, we had a very clear picture of what it looked like, but no clear picture of what it was," said Victoria McCoy, lead author of a new study.

"Basically, nobody knew what it was," added Derek Briggs, co-author of the study. "The fossils are not easy to interpret, and they vary quite a bit. Some people thought it might be this bizarre, swimming mollusk. We decided to throw every possible analytical technique at it."

Another study later that month showed that the monster's eyes had melanosomes, which make and store melanin. Those structures are typical to vertebrates, said the researchers.

Or maybe it doesn't have a spine

However, about a year later, a different team of researchers said there was no spine there, after all. In their study, published in the journal Paleontology, they said the Tully monster was likely an invertebrate.

“This animal doesn't fit easy classification because it’s so weird,” lead researcher Lauren Sallan, an assistant professor in the Department of Earth and Environmental Science at the University of Pennsylvania, said in a statement. “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.”

Sallan and her team said the studies failed to definitively classify the creature as a vertebrate.

“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.”

Identifying the creature

The key technology that made the identification of the Tully monster possible was a method known as synchrotron elemental mapping, which illuminates an animal's physical features by mapping the chemistry within a fossil.

Thousands of Tully monster fossils have actually been found, but all of them were unearthed at a single site, in coal mining pits in northeastern Illinois. So as far as researchers know, these animals could have been distinct to a specific habitat. They were named after their initial discoverer, Francis Tully, and their official scientific designation is Tullimonstrum gregarium. The fossils have taken on a sort of celebrity status in Illinois, where they have been declared the state fossil.

The creatures are so unfamiliar that they're pretty terrifying, and those teeth certainly don't help, but the largest Tully monster ever found only measures in at about a foot long. That means that if they were alive today, humans probably wouldn't be on their menu. It's difficult to say much of anything about their behavior, though.

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

Editor's note: This story was originally published in March 2016 and has been updated.