Scientists still can't agree whether a mysterious creature discovered in the 1950s is a worm, a mollusc, a vertebrate or some kind of ancient eel.
Tullimonstrum - or the Tully Monster, as it's informally known - has eyes on the ends of stalks poking out of the back of its torpedo-shaped body - kind of like a snail - and instead of a mouth has a bizarre tentacle with a claw on the end.
In 2016 scientists at Yale University determined the 300 million-year-old fossil - found in a creek in Illinois - was a kind of fish and therefore a vertebrate, based on pigments found in its eyes.
But new research suggests it didn't have a skeleton at all.
Scientists in Ireland have found evidence the pigments match those of modern invertebrates, like octopuses or squid.
"We then analysed the chemistry of Tully's eyes and the ratio of zinc to copper was more similar to that of invertebrates than vertebrates," researcher Chris Rogers of University College Cork wrote. "This suggests the animal may not have been a vertebrate, contradicting previous efforts to classify it."
But they also found Tully's eyes contain a different type of copper to that found in vertebrate eyes. But the copper also wasn't identical to that in the invertebrates we studied.
"So while our work adds weight to the idea that Tully is not a vertebrate, it doesn't clearly identify it as an invertebrate either."
Tully's monster is so strange, Dr Rogers says wherever it ends up getting classified in the tree of life will change the way they think about evolution.
"This monster is as mysterious as ever."
The new research was published in journal Proceedings of the Royal Society B.