A New Species of Cambrian Radiodont Discovered by Burgess Shale in Canada

Mothra Fentoni resided in present-day Canada during the Cambrian period, approximately 506 million years ago.

Mothra Fentoni had a body similar in size to an index finger, featuring three eyes, distinctive articulated claws, a circular mouth lined with teeth, and a body adapted for swimming along its sides.

These features suggest that it belongs to the Radiodonta, the earliest diverging group of arthropods, including the well-known Anomalocaris canadensis, a one-meter-long predator that shared its habitat with Mothra Fentoni.

Nevertheless, this new species exhibited traits not observed in other radiodonts: regions of its abdomen resembling multiple segments at the rear.

“Mothra Fentoni is a fascinating example of evolutionary convergence with modern groups such as horseshoe crabs, wood lice, and insects, all sharing a series of segments that house respiratory organs at the rear,” stated Dr. Jo Moisiuk, a paleontology and geology curator at the Manitoba Museum and researcher at the Royal Ontario Museum.

“Radiodonts represent the first branch of arthropods on the evolutionary tree, providing crucial insights into the ancestral characteristics of the entire group,” explained Dr. Jean-Bernard Caron, Richard M. Ivy curator of invertebrate paleontology at the Royal Ontario Museum.

“The newly discovered species emphasizes that these early arthropods were remarkably diverse and adapted in ways comparable to their modern counterparts.”

General overview of the form of Mothra Fentoni. Image credits: Joseph Moysiuk & Jean-Bernard Caron, doi: 10.1098/rsos.242122.

Sixty specimens of Mothra Fentoni were collected over nine field seasons from 1990 to 2022, primarily at the Raymond Quarry in Yoho National Park, with additional finds in the areas of Marble Canyon and Tokumm Creek in Kootenay National Park.

The specimens reveal details about internal anatomy, including aspects of the nervous, circulatory, and gastrointestinal systems.

“Very few fossil sites globally provide such in-depth insights into soft internal anatomy,” Dr. Caron noted.

“We can observe traces representing bundles of eyes responsible for image processing, similar to living arthropods. The details are remarkable.”

Rather than arteries and veins like those in humans, Mothra Fentoni possessed an “open” circulatory system, which circulated blood into a large internal cavity known as a lacuna.

These lacunae are preserved as reflective areas throughout the body, extending into the fossil’s swimming flap.

“The well-preserved lacuna in the circulatory system of Mothra Fentoni aids in interpreting similar, although less distinct, features observed in other fossils. Their classification remains contentious,” said Dr. Moisiuk.

“The widespread preservation of these structures confirms the ancient origins of this circulatory system type.”

The discovery of Mothra Fentoni is detailed in a study published today in the journal Royal Society Open Science.

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Joseph Moiszk and Jean Bernard Caron. 2025. Early evolutionary traits in arthropod tagmosis exemplified by the new radiodont from Burgess Shale. R. Soc. Open Sci 12(5): 242122; doi: 10.1098/rsos.242122