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Paleontologists at the Manitoba Museum and Royal Ontario Museum (ROM) have discovered a remarkable new 506-million-year-old predator from the Burgess Shale of Canada. The results are announced in a paper in the journal .

Mosura fentoni was about the size of an index finger and had three eyes, spiny jointed claws, a circular mouth lined with teeth and a body with swimming flaps along its sides. These traits show it to be part of an extinct group known as the radiodonts, which also included the famous Anomalocaris canadensis, a meter-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.

"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," says Joe Moysiuk, Curator of Paleontology and Geology at the Manitoba Museum, who led the study.

The reason for this intriguing adaptation remains uncertain, but the researchers postulate it may be related to particular habitat preference or behavioral characteristics of Mosura that required 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. This 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.

"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. The new species emphasizes that these early arthropods were already surprisingly diverse and were adapting in a comparable way to their distant modern relatives," says study co-author Jean-Bernard Caron, Richard M. Ivey Curator of Invertebrate Paleontology at ROM.

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

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

"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," adds Moysiuk, who is also a Research Associate at ROM. "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, 40 km to the southeast, which have revealed spectacular new Burgess Shale fossils, including other radiodonts: Stanleycaris, Cambroraster and Titanokorys. One previously unpublished specimen of Mosura was also studied that had been collected by Charles Walcott, the discoverer of the Burgess Shale.

"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," Moysiuk says.

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The Burgess Shale fossil sites are located within Yoho and Kootenay National Parks and are managed by Parks Canada. Parks Canada is proud to work with leading scientific researchers to expand knowledge and understanding of this key period of Earth's history and to share these sites with the world through award-winning guided hikes.

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

Many radiodont fossils can be seen on display in ROM's Willner Madge Gallery, Dawn of Life, in Toronto, and a specimen of Mosura will be exhibited for the first time at the Manitoba Museum in Winnipeg later this year.