A three-eyed ‘sea moth’ was an ocean predator 506 million years ago

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With the help of more than five dozen fossils, paleontologists have uncovered a tiny three-eyed predator nicknamed the “sea moth” that swam in Earth’s oceans 506 million years ago.

Mosura fentoni, as the species is known, belongs to a group called radiodonts, an early offshoot of the arthropod evolutionary tree, according to a new study published Tuesday in the journal Royal Society Open Science.

While radiodonts are now extinct, studying their fossilized remains can illuminate how modern arthropods such as insects, spiders and crabs evolved. One of the most diverse animal groups, arthropods are believed to account for more than 80% of living animal species, said lead study author Dr. Joe Moysiuk, curator of paleontology and geology at the Manitoba Museum in Winnipeg.

Well-preserved specimens of the previously unknown Mosura fentoni also reveal something that’s never been seen in any other radiodont: an abdomen-like body region with 16 segments that include gills at its rear. This part of the creature’s anatomy is similar to a batch of segments bearing respiratory organs at the rear of the body found in distant modern radiodont relatives like horseshoe crabs, woodlice and insects, Moysiuk said.

The feature, likely used to help Mosura capture more oxygen from its environment, could represent an example of evolutionary convergence, in which similar-looking structures evolve independently in different groups of organisms, he said.

“The new species emphasizes that these early arthropods were already surprisingly diverse and were adapting in a comparable way to their distant modern relatives” said study coauthor Dr. Jean-Bernard Caron, the Richard M. Ivey Curator of Invertebrate Paleontology at the Royal Ontario Museum in Toronto, in a statement.

A unique marine ‘moth’

No animal living today quite looks like Mosura fentoni, Moysiuk said, although it had jointed claws similar to those of modern insects and crustaceans. But unlike those critters, which can have two or four additional eyes used to help maintain orientation, Mosura had a larger and more conspicuous third eye in the middle of its head.

“Although not closely related, Mosura probably swam in a similar way to a ray, undulating its multiple sets of swimming flaps up and down, like flying underwater,” Moysiuk said in an email. “It also had a mouth shaped like a pencil sharpener and lined with rows of serrated plates, unlike any living animal.”

About the size of an adult human’s index finger, Mosura and its swimming flaps vaguely resemble a moth, which led researchers to call it the “sea moth.”

Some of the Mosura specimens provided tantalizing traces of front claws, which helped the radiodont feed.

Caron used a miniature jackhammer to remove rock overlying the head of a specimen and found a perfect outstretched spiny claw tucked beneath, Moysiuk said.

“Unlike many of its relatives which have claws lined with a meshwork of spines for capturing prey, Mosura has long, smooth-sided, finger-like spines that are forked at their tips,” Moysiuk said. “It’s a bit of a puzzle how exactly it was using these to capture prey, but (we) think it might have seized smaller animals with the tips of the spines and passed them towards the mouth.”

While there is no direct evidence of what Mosura ate, we know it lived alongside animals like acorn worms, bristle worms and small crustacean-like arthropods that the radiodont could have preyed on. In turn, Mosura may have been prey for other larger radiodonts, such as the shrimplike Anomalocaris canadensis, or the gargantuan jellyfish Burgessomedusa phasmiformis.

“This showcases there are yet more examples of these animals, specifically, forms that were active marine predators, filling in more of the picture of how this ancient marine ecosystem functioned,” said Dr. Russell D.C. Bicknell, a postdoctoral researcher in the American Museum of Natural History’s division of paleontology. Bicknell was not involved in the new study but previously authored research on Anomalocaris canadensis.

Mosura’s unexpected trunk region challenges how researchers understand radiodont body evolution and how members of the group shifted from having wormlike bodies, said Rudy Lerosey-Aubril, an invertebrate paleontologist at the Harvard Museum of Comparative Zoology who was also not involved with the new research.

“It may offer a rare glimpse into developmental processes, particularly in early members of the group, before evolutionary shifts led to the more consistent body organization seen in most known species,” Lerosey-Aubril said in an email.

A trove of fossils

The first Mosura fentoni specimen was discovered early in the 20th century by paleontologist Charles Walcott, who was the first known person to collect fossils from British Columbia’s Burgess Shale, a 508 million-year-old fossil bed. Walcott was the director of the US Geological Survey and administrator of the Smithsonian Institution. But no research about the Mosura specimen he found was ever published, and little was known about radiodonts at the time.

The other 60 fossils were collected by researchers at the Royal Ontario Museum between 1975 and 2022.

“It’s only with time and study of related species that the significance of these fossils gradually became clear,” Moysiuk said. “More recently, our team started finding additional specimens at new Burgess Shale sites in Kootenay National Park, which helped to spur on this publication.”

The fossils found in the Burgess Shale, located within the Canadian Rockies, represent a wide range of animals from the end of the Cambrian Period, when life diversified on a large scale. The Burgess Shale fossils are also known for being incredibly well preserved.

“In this study we were able to discern traces of the nervous, digestive, and circulatory system, which are almost never preserved as fossils,” Moysiuk said in an email.
“This provides unique and significant insight into life at this critical time in Earth’s history.”

The team was able to spy traces that represented bundles of nerves in the eyes, which — like modern arthropods — Mosura used for image processing, Caron said.

Rather than arteries and veins, Mosura also had an open circulatory system, meaning that its heart pumped blood into lacunae, or large internal body cavities. The cavities were preserved as reflective patches within the body.

The discovery of numerous complete tiny radiodont specimens is remarkable, Lerosey-Aubril said. The fine details preserved within the fossil underscore the importance of the Burgess Shale, he added, and a broader picture of the full diversity of Cambrian animals will require investigating other sites that hold fossils and evidence of soft-bodied organisms.

Radiodont fossils are permanently on display in the Royal Ontario Museum’s “Dawn of Life” exhibition, and a Mosura specimen will be on view at the Manitoba Museum later this year.

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