If I were to ask you what was unusual about Mosura fentoni, you probably wouldn’t know where to start. It looks a bit like a moth, except it lived in the sea. It had three eyes, some weird appendages in front of its mouth, and swimming flaps along its sides.
But for paleontologists, none of those are particularly bizarre. What is, however, is its abdomen.
“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 Palaeontology and Geology at the Manitoba Museum, who led the study.
This adaptation is weird because no other radiodont (supposedly simple-bodied pioneers of the arthropod lineage) had anything like a neatly segmented, gill-covered tail. This is a feature usually seen in much later, more advanced arthropods.
Evolutionary experiments
Mosura fentoni lived during the Cambrian Period, roughly 506 million years ago. This era is often dubbed the “Cambrian Explosion” because of the sudden and dramatic diversification of life on Earth. In this period, most major animal groups first appeared in the fossil record, including early ancestors of modern arthropods and mollusks. What makes the Cambrian so significant is that it marks the moment when complex, multicellular life leapt from simple blobs to creatures with eyes, limbs, guts, and brains.
The whole world changed. Ecosystems became more dynamic, predators and prey evolved specialized traits, and the foundations for all modern animal life — including humans — were laid. The Burgess Shale, where Mosura was found, is one of the most spectacular Cambrian sites in the world. It captures this evolutionary burst with extraordinary detail, preserving soft-bodied organisms that rarely fossilize and offering a vivid snapshot of life’s early experimentation.
Paleontologists from the Manitoba Museum and the Royal Ontario Museum (ROM) were rummaging through old collections and newly excavated finds from Canada’s legendary Burgess Shale when they spotted something odd. At first glance, it looked like just another radiodont — a group of extinct marine predators famous for Anomalocaris, the shrimp-like creature with 16,000 eyes that once ruled Cambrian seas.
But Mosura was different. Instead of the familiar streamlined body with flaps and claws, this little predator had something new: an elongated, tightly segmented tail loaded with gill structures. Sixteen segments in a neat row, each with what appear to be delicate breathing organs — something never seen before in a radiodont.
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A Body Built for Innovation
To put it technically, Mosura fentoni shows early “tagmosis.” That’s the biological term for dividing the body into specialized regions — like a thorax, abdomen, or neck.
Arthropods do this better than any other group, which is part of why there are over a million described species of them. Arthropods are animals with hard exoskeletons, jointed legs, and segmented bodies — including insects, spiders, crabs, and their ancient relatives.
Until now, scientists thought that radiodonts didn’t really play that game. They had simple, repeated segments with little differentiation. Mosura tears that idea apart. Its body looks startlingly similar to modern structures in insects, horseshoe crabs, and isopods.
Then, there’s the rest of its body.
For starters, it had three eyes — two stalked ones and a central median eye that probably helped it navigate the dim Cambrian seas. Its front limbs were armed with curved, spiny claws, perfect for grabbing prey. Its mouth was a tooth-ringed circle, a bizarre structure shared with other radiodonts, capable of crushing soft-bodied animals like trilobites or worms.
Midway down its body were wide, undulating flaps used for swimming. These likely propelled Mosura through the water like a tiny torpedo. But what sets it apart is the tail: narrow, segmented, lined with fine gill filaments and reduced swimming flaps. These features suggest a dual function — locomotion and respiration. It’s like the Swiss army knife of Cambrian tails.
This was no apex predator. In fact, it was small enough to be hunted by many bigger animals. But it was nimble and fierce in its own right.
Why This Matters
So, why should you care about a bug-eyed sea creature that’s been dead for 506 million years?
Because this isn’t just about Mosura. It’s about us. Or more precisely, the long, winding path that led from creatures like Mosura to lobsters, ants, spiders — and eventually, to you.
The Cambrian period, around 541 to 485 million years ago, was a time of rapid evolutionary experimentation. Life exploded into forms never before seen. But how that explosion unfolded, and how the winners of evolution’s lottery got their edge, is still a puzzle. Studies like this get us one step closer to understanding the big evolutionary tree of life on Earth.
It’s also a reminder that there’s plenty of unexplored material left in museums. Of the 61 specimens the researchers studied, several showed internal anatomy in exquisite detail, but had not been discussed extensively in modern literature.
“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.
The study was published in the journal Royal Society Open Science.
