Lorsque les restes d'une créature préhistorique sont découverts, il revient aux scientifiques de classifier l'animal et d'identifier sa place dans le grand arbre de l'évolution de la vie.
Mais, occasionnellement, la créature est tellement étrange qu'on ne sait pas trop ce qu'on a sous les yeux.
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Last year, scientists declared a decades-old mystery solved - that bizarre monstrosity you see in the image above had for years defied classification, but two separate studies said they finally had solid evidence that it was in fact a vertebrate.
But now, more researchers have entered the fray, and say this conclusion is just plain wrong - there's no way this thing can be a fish, which means we still have no idea what it actually is.
Nicknamed the Tully monster, the creature belongs to the ancient genus Tullimonstrum, and is thought to have inhabited the shallow coastal waters of muddy estuaries in the Eastern US around 300 million years ago.
Only a single species, T. gregarium, is known, and fossils of the creature have only ever been found in the Mazon Creek fossil beds of Illinois.
But they were clearly abundant in the region - hundreds of fossils ended up in the Field Museum in Chicago after the species' initial discovery in the 1950s.
(...) the Tully Monster had fins like a cuttlefish, eyestalks like a crab, and a rather intimidating 'jaw-on-stick':
This jumble of body parts has seen it compared to everything from molluscs and arthropods to worms, and more recently, to vertebrates like lampreys.
"This animal doesn't fit easy classification because it's so weird," says Lauren Sallan from the University of Pennsylvania, one of the team behind its most recent analysis.
"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."
First discovered back in the 1955 by amateur fossil collector Francis Tully, the creature set off a decades-long investigation into what this bizarre conglomeration of features could actually represent.
(...) A team from Yale University, led by palaeobiologist Victoria McCoy, asserted that the light line running all the way down the creature's middle was not a gut, as previous research had suggested, but a notochord - a skeletal rod that forms the basis of vertebrate backbones.
(...) The team also found evidence of gill pouches in a few of the 1,200 or so specimens they analysed, which made them appear more 'fish-like' than previous studies had suggested.
(...) A second study led by researchers at the University of Leicester in the UK also came to the conclusion that the Tully monster was a vertebrate, after scanning electron microscope (SEM) images of its eyes revealed structures called melanosomes.
The team argued that this meant the creature had complex eyes, and said this made it more likely that it was a vertebrate.
But now Sallan and her colleagues say they actually got the whole thing wrong.
(...) "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. But lots of other things have cup eyes, like primitive chordates, molluscs and certain types of worms."
This time around, the researchers are bringing more questions than answers, because while they're sure that the Tully monster is not a vertebrate, they're not ready to say what it could be instead.