Imagine swimming in an ocean populated by creatures straight out of a science fiction movie. More than half a billion years ago, during the Cambrian period, Earth’s seas were home to such bizarre life forms, including a newly described ancient predator called Mosura fentoni. Found preserved in the incredible Burgess Shale, this finger-sized creature with three eyes and a body more segmented than any of its relatives offers a unique window into the dawn of complex animal life and how evolution experimented with different body plans.
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Mosura fentoni is a type of radiodont, an extinct group considered early relatives of arthropods – the group that today includes insects, spiders, and crustaceans. While only about as long as a human finger, it packed a punch with spiny claws, a circular mouth lined with teeth, and flippers running down its sides. But perhaps its most striking features were its three eyes and a remarkably long, segmented body ending in a tail with what look like gills.
A Creature of Many Segments
One feature that immediately sets Mosura apart is its extraordinary segmentation. With 26 body segments, it had more than any other known radiodont. The rear part of its body alone boasted 16 tightly packed segments, each lined with gill-like structures.
This dense concentration of respiratory organs at the back is fascinating to paleontologists Joe Moysiuk from the Manitoba Museum and Jean-Bernard Caron from the Royal Ontario Museum, who studied the fossils. As Moysiuk explains, this is a neat example of evolutionary convergence. It’s like nature stumbled upon a good idea and used it independently in different groups. Modern animals like horseshoe crabs, woodlice (roly-polies), and many insects also have clusters of respiratory organs located at the rear of their bodies, despite being only distantly related to Mosura. This suggests that putting breathing apparatus at the back might have offered a significant advantage in different ancient and modern environments.
What Mosura may have looked like during its lifetime, showing its multiple eyes, segmented body, and flippers.
Windows to an Ancient World: The Burgess Shale
Understanding creatures like Mosura fentoni is only possible thanks to remarkable fossil sites like the Burgess Shale in Canada. Formed about 508 million years ago, during the Cambrian period, this site is a true marvel of preservation.
The Cambrian period, roughly 539 to 487 million years ago, is often called the “Cambrian explosion” because life suddenly diversified in astonishing ways. The oceans began teeming with complex multicellular organisms. The Burgess Shale captured a snapshot of this vibrant, alien world when sudden flows of silty mud buried marine life on the seafloor, preserving even soft tissues that rarely survive fossilization.
This “Lagerstätte” (a German term for a fossil site with exceptional preservation) has revealed a host of bizarre and wonderful creatures that help us piece together the early history of animal life. It’s shown us creatures so strange we’ve sometimes been left scratching our heads trying to figure out how they fit into the tree of life. Discover more strange finds from the Burgess Shale here.
One of the fossilized specimens of Mosura found in the Burgess Shale, revealing its unique segmented structure.
An Inside Look: Circulatory System
The exceptional preservation in the Burgess Shale even allowed scientists to peek inside Mosura. Caron notes that they could see traces of nerve bundles connected to the eyes and details of its circulatory system.
Unlike humans and other vertebrates that have a closed circulatory system with blood flowing through veins and arteries, Mosura had an open circulatory system. Think of a closed system like water flowing through pipes, while an open system is more like water sloshing around in a bathtub. In modern arthropods (like crabs or insects), the heart pumps fluid (called hemolymph) into body cavities, where it bathes the organs directly before returning to the heart.
In Mosura, these cavities, called lacunae, were visible in the fossils as reflective patches. They filled the body and even extended into the swimming flippers. The clear preservation of these lacunae in Mosura is a big deal because it helps researchers interpret similar, less distinct features seen in other ancient fossils. As Moysiuk states, this confirms that this type of open circulatory system is an incredibly ancient trait, originating early in the evolution of arthropods.
An anatomical diagram of Mosura, illustrating its internal structures including the nervous system (purple), digestive system (green), and circulatory system (orange).
What Mosura Tells Us
Radiodonts like Mosura and the more famous Anomalocaris (which could grow much larger) were among the earliest animals to branch off in the evolutionary tree that led to modern arthropods. Studying them provides key insights into the ancestral traits of this massive and incredibly successful group of animals.
The unique features of Mosura, such as its excessive segmentation and specialized respiratory tail, emphasize that these early arthropod relatives were already surprisingly diverse. They weren’t just simple precursors; they were adapting in complex ways to their environment, much like their distant modern relatives do today.
While we can’t know for sure why Mosura had such a specialized tail – perhaps it lived in a different habitat or used a unique hunting strategy requiring more oxygen – its discovery highlights the amazing strategies life developed during the Cambrian period to thrive in a rapidly changing world. Mosura fentoni is a beautiful, if bizarre, reminder of the deep history of life on Earth and the endless forms evolution can take.
To learn more about radiodonts and their place in the tree of life, explore other ancient discoveries here. The research on Mosura fentoni was published in the journal Royal Society Open Science.
