Ancient Sea-Moth Predator Rewrites Arthropod Evolution

A finger-sized marine predator with three eyes and an unusual respiratory system is challenging what scientists thought they knew about the early evolution of arthropods, the group that includes modern insects, crustaceans, and spiders.

Canadian researchers have discovered Mosura fentoni, a 506-million-year-old creature from the famous Burgess Shale fossil beds, with an unprecedented body arrangement among its evolutionary relatives. The new species is providing fresh insights into how the diverse body plans of today’s arthropods first emerged.

Named after the fictional Japanese monster Mothra, the ancient sea-dwelling predator possessed a specialized respiratory segment arrangement at its rear end that’s strikingly similar to features seen in modern arthropods like horseshoe crabs, woodlice, and insects.

“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,” said Joe Moysiuk, Curator of Palaeontology and Geology at the Manitoba Museum, who led the study published today in the journal Royal Society Open Science.

The remarkable fossil preservation reveals not just the creature’s external features but also its internal anatomy, including nervous, circulatory, and digestive systems. This provides researchers with unprecedented glimpses into how these ancient animals functioned.

“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,” added Jean-Bernard Caron, Richard M. Ivey Curator of Invertebrate Palaeontology at the Royal Ontario Museum (ROM), who co-authored the research.

Not Your Average Radiodont

Mosura belongs to an extinct group called radiodonts, which were among the earliest-diverging arthropods. The most famous member, Anomalocaris canadensis, was a meter-long apex predator that shared the ancient seas with Mosura. While other radiodonts typically had fairly uniform body segment arrangements, Mosura broke the mold.

The newly discovered species had a pronounced division of its body into distinct functional regions: a head with three prominent eyes, a neck, a midsection with large swimming flaps, and a specialized breathing section at the back. This level of body segment specialization, known as tagmosis, was previously thought to have evolved later in arthropod history.

“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,” Caron explained.

Ancient Circulatory System Preserved

Another significant aspect of the discovery is how it helps clarify controversial features seen in other Burgess Shale fossils. Mosura’s exceptionally preserved internal structures include what researchers identify as lacunae – large blood-filled cavities that formed part of its open circulatory system.

“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,” noted 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.”

Despite its fierce predatory equipment – including jointed claws with spiny endites and a circular mouth lined with teeth – Mosura was relatively small, reaching only about the length of an index finger. Its body design suggests it was a nimble swimmer that likely hunted smaller prey in the ancient seas.

The Power of Museum Collections

The discovery underscores the ongoing scientific value of carefully curated fossil collections. Of the 61 examined specimens, all but one were collected by ROM between 1975 and 2022, primarily from sites in Yoho and Kootenay National Parks in British Columbia, Canada.

“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 said.

The Burgess Shale fossil sites, now managed by Parks Canada and designated as a UNESCO World Heritage Site, continue to yield important discoveries about the explosive diversification of animal life that occurred during the Cambrian period.

Visitors can view many radiodont fossils at ROM’s Willner Madge Gallery in Toronto, and a specimen of Mosura will be exhibited for the first time at the Manitoba Museum in Winnipeg later this year, offering the public a glimpse into this pivotal chapter of life’s evolutionary story.