Mosura fentoni lived in what is now Canada during the Cambrian period, approximately 506 million years ago.
Life reconstruction of Mosura fentoni. Image credit: Danielle Dufault, Royal Ontario Museum.
Mosura fentoni was about the size of your 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 Radiodonta, a group of the earliest diverging arthropods that also included the famous Anomalocaris canadensis, a 1-m-long predator that shared the waters with Mosura fentoni.
However, the new species 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 fentoni has 16 tightly packed segments lined with gills at the rear end of its body,” said Dr. Joe Moysiuk, curator of paleontology and geology at the Manitoba Museum and a researcher at Royal Ontario Museum.
“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.”
“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,” said Dr. Jean-Bernard Caron, Richard M. Ivey curator of invertebrate paleontology at Royal Ontario Museum.
“The new species emphasizes that these early arthropods were already surprisingly diverse and were adapting in a comparable way to their distant modern relatives.”
General overview of Mosura fentoni’s morphology. Image credit: Joseph Moysiuk & Jean-Bernard Caron, doi: 10.1098/rsos.242122.
Sixty specimens of Mosura fentoni were collected over the course of nine field seasons between 1990 and 2022 from the Burgess Shale Formation, British Columbia, Canada, specifically the Raymond Quarry in Yoho National Park, and various localities around Marble Canyon and Tokumm Creek in Kootenay National Park.
The specimens 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,” Dr. Caron said.
“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.”
Instead of having arteries and veins like we do, Mosura fentoni 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 fentoni help us to interpret similar, but less clear features that we’ve seen before in other fossils. Their identity has been controversial,” Dr. Moysiuk said.
“It turns out that preservation of these structures is widespread, confirming the ancient origin of this type of circulatory system.”
The discovery of Mosura fentoni is reported in a paper published today in the journal Royal Society Open Science.
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Joseph Moysiuk & Jean-Bernard Caron. 2025. Early evolvability in arthropod tagmosis exemplified by a new radiodont from the Burgess Shale. R. Soc. Open Sci 12 (5): 242122; doi: 10.1098/rsos.242122
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