Tullimonstrum (also known as the Tully Monster), a 300m-year-old fossil discovered in the Mazon Creek fossil beds in Illinois, US, is one such creature.

At first glance, Tully looks superficially slug-like. But where you would expect its mouth to be, the creature has a long thin appendage ending in what looks like a pair of grasping claws. Then there are its eyes, which protrude outward from its body on stalks.

Tully is so strange that scientists have even been unable to agree on whether it is a vertebrate (with a backbone, like mammals, birds, reptiles and fish) or an invertebrate (without a backbone, like insects, crustaceans, octopuses and all other animals). In 2016, a group of scientists claimed to have solved the mystery of Tully, providing the strongest evidence yet that it was a vertebrate. But my colleagues and I have conducted a new study that calls this conclusion into question, meaning this monster is as mysterious as ever.

The Tully Monster was originally discovered in the 1950s by a fossil collector named Francis Tully. Ever since its discovery scientists have puzzled over which group of modern animals Tully belongs to. The enigma of Tully’s true evolutionary relationships has added to its popularity, ultimately leading it to become the state fossil of Illinois.

There have been many attempts to classify the Tully Monster. The majority of these studies have focused on the appearance of some of its more prominent features. These include a linear feature in the fossil interpreted as evidence of a gut, the light and dark banding of the fossil and the peculiar grasping claws of its mouth. The body plan of the Tully Monster is so unusual in it’s entirety that it will greatly expand the diversity of of whatever group it ultimately belongs to, changing the way we think about that group of animals.

The 2016 research argued the animal should be grouped with vertebrates because its eyes contain pigment granules called melanosomes, which are arranged by shape and size in the same way as those in vertebrate eyes. But our research shows that the eyes of some invertebrates such as octopus and squid also contain melanosomes partitioned by shape and size in a similar way to Tully’s eyes, and that these an also be preserved in fossils.

Tullimonstrum probably reached lengths of up to 35 centimetres (14 in); the smallest individuals are about 8 cm (3.1 in) long.

Tullimonstrum had a pair of vertical, ventral fins (though the fidelity of preservation of fossils of its soft body makes this difficult to determine) situated at the tail end of its body, and typically featured a long proboscis with up to eight small sharp teeth on each "jaw", with which it may have actively probed for small creatures and edible detritus in the muddy bottom. It was part of the ecological community represented in the unusually rich group of soft-bodied organisms found among the assemblage called the Mazon Creek fossils from their site in Grundy County, Illinois.

The absence of a hard part in the fossil implies that the animal did not possess organs composed of bone, chitin or calcium carbonate. There is evidence of serially repeated internal structures. Its head is poorly differentiated. A transverse bar-shaped structure, which was either dorsal or ventral, terminates in two round organs which are associated with dark material which have been identified as melanosomes (containing the pigment melanin). Their form and structure is suggestive of a camera-type eye. Tullimonstrum possessed structures which have been interpreted as gills, and a possible notochord or rudimentary spinal cord.

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