Prehistoric “monster” was spineless after all

New technology has revealed one of the biggest secrets of the “Tully monster” (Tullimonstrum gregarium) – an animal that lived 300 million years ago, and whose fossils have confounded palaeontologists since it was discovered in the 1950s.

The bizarre creature was accidentally found in 1958 by fossil hunting hobbyist Francis Tully in Mazon Creek Lagerstätte in Illinois, in the US. Since then more than a hundred specimens have been uncovered at the site and the Tully monster has been named the official state fossil of Illinois. On average, the Tullimonstrum fossils are 15 centimetres in length.

The Tully monster lived right at the end of the period of Earth’s history known as the Carboniferous. The sub-period known as the Pennsylvanian (318 to 299 million years ago) saw the evolution of the first reptiles; giant arthropods like three-metre-long millipedes; dragonflies with metre-long wingspans; and forests of 20-metre-tall lichen.

Organic matter from that time period is (unfortunately) used today as fossil fuels.

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The strange anatomy of the Tully monster has made it hard to classify. In fact, until now, scientists weren’t even sure if it was a vertebrate or an invertebrate.

This debate has raged on. See this journal paper in Nature from 2016 titled “The ‘Tully monster’ is a vertebrate” followed up in 2017 by a paper published in Palaeontology titled “The ‘Tully Monster’ is not a vertebrate”.

Palaeontologists on the “Tully is a vertebrate” side have suggested that the creature might be an evolutionary missing link related to cyclostomes (jawless fish like lamprey and hagfish).

But the scientific toing and froing may finally be over.

Japanese researchers used X-ray micro-computed tomography to create cross-sections of the fossilised soft body of the Tully monster and look inside. What they found was the absence of an internal skeleton.

Diagram of fossil and a colour coded version below
Often used to study dinosaur footprints, these color-coded depth maps enabled the researchers to thoroughly investigate the structure of the Tully monster and other fossils from Mazon Creek. Credit: Mikami, 2022.

“We believe that the mystery of it being an invertebrate or vertebrate has been solved,” says Tomoyuki Mikami from the National Museum of Nature and Science, Tokyo – a University of Tokyo doctoral student at the time of the study.

“Based on multiple lines of evidence, the vertebrate hypothesis of the Tully monster is untenable.

“The most important point is that the Tully monster had segmentation in its head region that extended from its body. This characteristic is not known in any vertebrate lineage, suggesting a nonvertebrate affinity.”

The team studied more than 150 Tully monster fossils and more than 70 other animals from the same time period at Mazon Creek.

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While the team provided strong evidence to suggest that the Tully monster is not a vertebrate, exactly what it is remains a mystery. It may be something that doesn’t fit into our current knowledge of Earth’s evolutionary history, highlighting the dynamic history of life on our planet.

“There were many interesting animals that were never preserved as fossils,” Mikami explains. “In this sense, research on the fossils from Mazon Creek is important because it provides palaeontological evidence that cannot be obtained from other sites.

“More and more research is needed to extract important clues from Mazon Creek fossils to understand the evolutionary history of life.”

The research is published in Palaeontology.

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