Three-Eyed Cambrian Animal Sheds Light on Origin of Arthropod Head and Segmentation

Paleontologists from the University of Toronto and the Royal Ontario Museum have examined 268 specimens of Stanleycaris hirpex — a radiodont that lived during the Cambrian period, some 506 million years ago — from the Burgess Shale in Canada, including many exceptionally preserved whole-body specimens. Their findings shed light on the evolution of the arthropod brain, vision, and head structure.

Reconstruction of a pair of Stanleycaris hirpex: upper individual has transparency of the exterior increased to show internal organs;  nervous system is shown in light beige, digestive system in dark red.  Image credit: Sabrina Cappelli, Royal Ontario Museum.

Reconstruction of a pair of Stanleycaris hirpex: upper individual has transparency of the exterior increased to show internal organs; nervous system is shown in light beige, digestive system in dark red. Image credit: Sabrina Cappelli, Royal Ontario Museum.

Radiodonta is an order of arthropods that dominated the Cambrian oceans around 500 million years ago.

It included some of the most iconic and bizarre-looking Cambrian animals, with the famous Anomalocaris reaching up to at least 1 m (3.3 feet) in length.

At no more than 20 cm (7.9 inches) long, Stanleycaris hirpex was small for its group, but at a time when most animals grew no bigger than a human finger, it would have been an impressive predator.

It had large compound eyes, a formidable-looking circular mouth lined with teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with a series of swimming flaps along its sides.

Its sophisticated sensory and nervous systems would have enabled it to efficiently pick out small prey in the gloom.

The new fossils from the Burgess Shale show that the brain of Stanleycaris hirpex was composed of two segments, the protocerebrum and deutocerebrum, connected with the eyes and frontal claws, respectively.

“While fossilized brains from the Cambrian period aren’t new, this discovery stands out for the astonishing quality of preservation and the large number of specimens,” said Joseph Moysiuk, a Ph.D. candidate at the University of Toronto and the Royal Ontario Museum.

“We can even make out fine details such as visual processing centers serving the large eyes and traces of nerves entering the appendages.”

“We conclude that a two-segmented head and brain has deep roots in the arthropod lineage and that its evolution likely preceded the three-segmented brain that characterizes all living members of this diverse animal phylum,” he added.

In addition to the pair of stalked lateral eyes, Stanleycaris hirpex unexpectedly had a large median eye.  Image credit: Moysiuk & Caron, doi: 10.1016/j.cub.2022.06.027.

In addition to the pair of stalked lateral eyes, Stanleycaris hirpex unexpectedly had a large median eye. Image credit: Moysiuk & Caron, doi: 10.1016/j.cub.2022.06.027.

In present-day arthropods, the brain consists of protocerebrum, deutocerebrum, and tritocerebrum.

While the difference of a segment may not sound game-changing, it in fact has radical scientific implications.

Since repeated copies of many arthropod organs can be found in their segmented bodies, figuring out how segments line up between different species is key to understanding how these structures diversified across the group.

“These fossils are like a Rosetta stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups,” Moysiuk said.

In addition to its pair of stalked eyes, Stanleycaris hirpex possessed a large central eye at the front of its head, a feature never before noticed in a radiodont.

“The presence of a huge third eye in Stanleycaris hirpex was unexpected,” said Dr. Jean-Bernard Caron, the Richard Ivey curator of invertebrate paleontology at the Royal Ontario Museum.

“It emphasizes that these animals were even more bizarre-looking than we thought, but also shows us that the earliest arthropods had already evolved a variety of complex visual systems like many of their modern kin.”

“Since most radiodonts are only known from scattered bits and pieces, this discovery is a crucial jump forward in understanding what they looked like and how they lived.”

The results appear in the journal Current Biology.

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Joseph Moysiuk & Jean-Bernard Caron. A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation. Current Biology, published online July 8, 2022; doi: 10.1016/j.cub.2022.06.027

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