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Tully Monster: the monster that defies science

It looks like a worm with eyes on stalks and a proboscis ending in pincers. It lived about 300 million years ago in the fresh waters of what is now Illinois. It is called Tullimonstrum gregarium, but it is nicknamed the Tully Monster, after its discoverer Francis Tully, an amateur fossil collector.

Since its discovery in 1958, the Tully Monster has fascinated paleontologists, who have been unable to determine which animal group it belongs to. Is it a mollusc, arthropod, vertebrate or unique creature? The studies followed one another, without succeeding in deciding the question.

In 2016, a team of researchers announced that they had solved the mystery, claiming that the Tully Monster was a vertebrate, close to current lampreys. They were based on the analysis of the structure of his eyes and the presence of an organ resembling a notochord, a kind of pre-vertebral column.

But in 2019, another team challenged this hypothesis, showing that the eyes of the Tully Monster were not compatible with those of vertebrates, and that the supposed notochord was in fact a fossilization artifact. According to them, the Tully Monster is more of a mollusk, close to sea slugs.

The debate is not yet closed. Other researchers have underlined the limits of the two studies, which are based on sometimes ambiguous or contradictory morphological criteria. They call for using other methods, such as chemical or genetic analysis, to try to unlock the secret of the Tully Monster.

In the meantime, the Tully Monster remains a puzzle for science, but also a symbol of our planet’s past biodiversity. It even became the official state emblem of Illinois in 1989.

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