Newly identified three-eyed ‘sea-moth’ lived 506 million years ago

By Stephen Beech

A "remarkable" three-eyed predator nicknamed the "sea-moth" that lived 506 million years ago has been identified.

The fossilized remains of the unusual creature, called Mosura fentoni, were discovered by palaeontologists in the Burgess Shale fossil-bearing deposit in the Canadian Rockies.

The Canadian team says it was about the size of a human index finger and had three eyes, spiny, jointed claws, and a circular mouth lined with teeth as well as a body with swimming flaps along its sides.

Those traits show it to be part of an extinct group known as the radiodonts, according to findings published in the journal Royal Society Open Science.

The radiodonts also included Anomalocaris canadensis, a metre-long predator that shared the waters with Mosura.

However, Mosura also possessed a feature not seen in any other radiodont - an abdomen-like body region made up of multiple segments at its back end.

Study leader Dr. Joe Moysiuk, Curator of Palaeontology and Geology at the Manitoba Museum, said: "Mosura has 16 tightly packed segments lined with gills at the rear end of its body.

"This is a neat example of evolutionary convergence with modern groups, like horseshoe crabs, woodlice, and insects, which share a batch of segments bearing respiratory organs at the rear of the body."

The reason for the adaptation remains uncertain, but the research team believes it may be related to particular habitat preferences or behavioural characteristics of Mosura that require more efficient respiration.

With its broad swimming flaps near its midsection and narrow abdomen, Mosura was nicknamed the "sea-moth" by field collectors based on its vague appearance to a moth.

That inspired its scientific name, which references the fictional Japanese "kaiju" also known as Mothra.

Only distantly related to real moths – as well as spiders, crabs, and millipedes – Mosura belongs on a much deeper branch in the evolutionary tree of these animals, collectively known as arthropods.

Study co-author Dr. Jean-Bernard Caron, of the Royal Ontario Museum (ROM), said: “Radiodonts were the first group of arthropods to branch out in the evolutionary tree, so they provide key insight into ancestral traits for the entire group.

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"The new species emphasises that these early arthropods were already surprisingly diverse and were adapting in a comparable way to their distant modern relatives.”

Several fossils of Mosura also show details of internal anatomy, including elements of the nervous system, circulatory system, and digestive tract.

Dr. Caron said: “Very few fossil sites in the world offer this level of insight into soft internal anatomy.

"We can see traces representing bundles of nerves in the eyes that would have been involved in image processing, just like in living arthropods. The details are astounding.” Caron adds.

Instead of having arteries and veins like we do, Mosura had an “open” circulatory system, with its heart pumping blood into large internal body cavities called lacunae.

The lacunae are preserved as reflective patches that fill the body and extend into the swimming flaps in the fossils.

Dr. Moysiuk said: “The well-preserved lacunae of the circulatory system in Mosura help us to interpret similar, but less clear features that we’ve seen before in other fossils.

"Their identity has been controversial. It turns out that preservation of these structures is widespread, confirming the ancient origin of this type of circulatory system.”

Of the 61 fossils of Mosura, all except one were collected by ROM between 1975 and 2022, mostly from the Raymond Quarry in Yoho National Park, British Columbia.

Some also came from new areas around Marble Canyon in Kootenay National Park, 25 miles to the southeast, which have revealed spectacular new Burgess Shale fossils, including other radiodonts.

One previously unpublished specimen of Mosura was also studied, which had been collected by Charles Walcott, the discoverer of the Burgess Shale.

Dr. Moysiuk added: “Museum collections, old and new, are a bottomless treasure trove of information about the past.

"If you think you’ve seen it all before, you just need to open up a museum drawer."

The Burgess Shale was designated a UNESCO World Heritage Site in 1980 and is now part of the larger Canadian Rocky Mountain Parks World Heritage Site.

A specimen of Mosura will be exhibited for the first time at the Manitoba Museum in Winnipeg later this year.