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A bizarre ancient creature that looks like a sci-fi reject may actually have been a backboned animal related to fish.

The claim relies on chemical analysis of fossils of the creature. However, other palaeontologists remain cautious.

The animal is called Tullimonstrum gregarium, or simply the Tully Monster. It lived around 300 million years ago in shallow waters covering what is now Illinois. There are thousands of good fossils, all from one formation called Mazon Creek.

The Tully Monster had a streamlined body a bit like a worm or fish, with holes resembling gills along the sides. Mounted on top was a horizontal bar, at the ends of which were its eyes. At the back, it had a tail that looked like a fin. Finally, at the front it had a long, angled neck with a pincer-like appendage on the end. The whole animal was between 6 and 35 centimetres long.

The animal was , and baffled everyone. 鈥淭hey basically said 鈥榳ow, it doesn鈥檛 look like anything we see today鈥�,鈥� says Victoria McCoy at the University of Wisconsin, Milwaukee.

Later it was variously interpreted as a free-swimming snail without a shell, a worm or a chordate: an animal with a stiff rod along its back. Some chordates, including fish and mammals, are vertebrates, meaning they have backbones.

In 2016, McCoy and her colleagues published , arguing that it was a vertebrate. The pincer-like thing at the front was a mouth with teeth, the holes along the sides were gills, and it had a form of backbone. Another study that year claimed .

However, , arguing that many aspects of the Monster鈥檚 anatomy marked it as an invertebrate, lacking a backbone, like a snail or worm.

McCoy decided the Tully Monster鈥檚 anatomy was too strange to settle the question. Instead, she teamed up with Jasmina Wiemann of Yale University, a specialist in chemical analysis of fossils.

When soft tissue fossilises, chemicals like proteins degrade in predictable ways, says Wiemann. 鈥淲e can still extract biological information,鈥� she says. Crucially, invertebrates and chordates remain chemically distinct.

Wiemann, McCoy and their colleagues studied 32 samples from Mazon Creek rocks. Known chordates and invertebrates were readily distinguished, and the Tully Monster grouped with the chordates.

McCoy believes the Tully Monster鈥檚 closest living relatives are jawless fish like lampreys and hagfish, and it may have evolved its peculiar body as an adaptation to a specialised lifestyle such as picking worms out of the sea floor.

Maria McNamara at University College Cork in Ireland says the results are compelling, but may have been distorted by the peculiar chemistry of Mazon Creek.

鈥淭he preservation of these fossils is hugely odd and very variable,鈥� she says, so it would be good to perform experiments showing how these different tissue chemistries arise during the fossilisation process.

McNamara has metals in the Tully Monster鈥檚 eyes. These suggested : an invertebrate group that includes octopuses and squid. 鈥淲hy are the organic and inorganic components of the chemical signature showing these conflicting results?鈥� she asks.

The difficulty is that the Tully Monster is an oddity in any group, says Robert Sansom at the University of Manchester, UK, who co-authored the 2017 paper. 鈥淚f it鈥檚 a mollusc, it鈥檚 a weird mollusc. If it鈥檚 a vertebrate, it鈥檚 a weird vertebrate.鈥�

The Tully Monster鈥檚 real identity may only become clear when palaeontologists find more creatures resembling it, says Sansom. It must have had ancestors, but so far none is known, and few fossil beds preserve soft-bodied creatures.