Elongated pink creature with long, slender neck and tiny bird-like head in water.

Molecules in its fossils suggest that, despite its squid-like features, an aquatic animal nicknamed the Tully Monster (artist’s impression) was actually a vertebrate. Credit: Getty

Palaeontology

Unmasking the Tully Monster: fossils help to tackle a decades-old mystery

Molecular analysis indicates that the baffling soft-bodied creature was a vertebrate.

The Tully Monster, an ancient creature at the top of the scale of weirdness, has defied scientists’ attempts to categorize it for more than half a century. But a new analysis of the molecules in its fossils suggests it was a vertebrate.

Palaeontologists have long debated whether the soft-bodied Tully Monster (Tullimonstrum gregarium), which more than 300 million years ago swam the waters of what is now Illinois, was a vertebrate, an invertebrate or something in between. To address this question, Victoria McCoy at the University of Wisconsin–Milwaukee, Jasmina Wiemann at Yale University in New Haven, Connecticut, and their colleagues studied molecular signals in the fossils of an array of vertebrates and invertebrates from the fossil beds where the Tully Monster was discovered.

The team found that the invertebrates’ soft tissue had relatively high levels of nitrogen-containing compounds, which are produced by the fossilization of a carbohydrate-based structure called chitin. By contrast, the vertebrates’ soft tissues had high levels of sulfur-containing compounds, which are created when proteins undergo fossilization.

The molecular make-up of the Tully Monster’s fossil suggests that the creature was a vertebrate.