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‘Amazing’ 500 Million Year Old Fossilized Brains Prompt Rethinking Insect and Spider Evolution

Stanleycaris Hirpex

Stanleycaris Hirpex. Credit: Sabrina Capelli © Royal Ontario Museum

An ancient predatory three-eyed radiodont reveals key insights into the evolution of the arthropod body plan.

New research based on a cache of fossils containing the brain and nervous system of a half-billion-year-old marine predator from the Burgess Shale called Stanleycaris was revealed by the Royal Ontario Museum (ROM). Belonging to an ancient and extinct offshoot of the arthropod evolutionary tree called Radiodonta, Stanleycaris is distantly related to modern insects and spiders. These results shed light on the evolution of the brain, vision and head structure of arthropods.

“The details are so clear it’s like looking at a dead animal yesterday.”

Joseph Moysiuk

The results were announced in the research paper, “A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation,” published July 5, 2022 in the journal Current biology.

Fossil specimens of Stanleycaris hirpex

Pair of fossil specimens of Stanleycaris hirpex, ROMIP specimen 65674.1-2. Credit: Photo by Jean-Bernard Caron, © Royal Ontario Museum

What excites scientists the most is what’s inside Stanleycaris‘ head. Brain and nerve remains are still preserved after 506 million years in 84 of the fossils.

“Although fossilized brains from the Cambrian period are not new, this discovery stands out for the astonishing quality of preservation and the large number of specimens,” said Joseph Moysiuk, lead research author and professor at the University. of Toronto (U of T). PhD candidate in ecology and evolutionary biology, based at 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. The details are so clear it’s like looking at a dead animal yesterday.


Turntable animation of Stanleycaris hirpex, including transparency to show internal organs. Credit: Animation by Sabrina Capelli © Royal Ontario Museum

The new fossils reveal that the brain of Stanleycaris was composed of two segments, the protocerebrum, connected to the eyes, and the deutocerebrum, connected to the frontal claws.

“We conclude that a two-segmented head and brain have 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,” Moysiuk added. .

In modern arthropods like insects, the brain is made up of protocerebrum, deutocerebrum, and tritocerebrum. Although the difference in one segment may not seem revolutionary, it actually has radical scientific implications. Since repeat copies of many arthropod organs can be found in their segmented bodies, understanding how segments align between different species is critical to understanding how these structures have diversified across the group.

“These fossils are like a Rosetta Stone, helping to connect traits of radiodonts and other early fossil arthropods with their counterparts in surviving groups.”

Reconstruction of the Stanleycaris hirpex

Reconstruction of a pair of Stanleycaris hirpex; the superior individual has increased exterior transparency to show the internal organs. The nervous system is shown in light beige, the digestive system in dark red. Credit: Illustration by Sabrina Capelli © Royal Ontario Museum

In addition to his pair of stalked eyes, Stanleycaris 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 was unexpected. This highlights that these animals looked even weirder than we thought, but also shows us that early arthropods had already developed a variety of complex visual systems, like many of their modern relatives,” said Dr. Bernard Caron, Richard Ivey of the ROM. Curator of invertebrate paleontology and thesis director of Moysiuk. “Since most radiodonts are only known from scattered fragments, this discovery is a crucial leap forward in understanding what they looked like and how they lived,” added Caron, who is also an associate professor at the ‘U of T, in Ecology and Evolution and Earth Sciences.

Stanleycaris Fossil Nervous System

Summary of the article, showing the interpretation of the nervous system from the Stanleycaris fossils and the implications for understanding the evolution of the arthropod brain. The brain is shown in red and the nerve cords in purple. Credit: Photo by Jean-Bernard Caron © Royal Ontario Museum

In the Cambrian, radiodonts included some of the largest animals in the world, with the famous “strange wonder” Anomalocaris reaching at least 1 meter in length. Measuring no more than 20 cm in length, Stanleycaris was small for his pack, but in an age when most animals grew no larger than a human finger, he would have been an impressive predator. Stanleycaris‘ Sophisticated sensory and nervous systems would have allowed it to efficiently spot small prey in the dark.

Reconstruction of the Stanleycaris hirpex

Reconstruction of the Stanleycaris hirpex. Credit: Art by Sabrina Capelli © Royal Ontario Museum

With large compound eyes, a formidable 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, Stanleycaris would have been the stuff of nightmares for any little bottom dweller unfortunate enough to cross his path.

About Burgess Shale

For this research, Moysiuk and Caron studied an unpublished collection of 268 specimens of Stanleycaris. The fossils were primarily collected in the 1980s and 90s from rock layers above the famous Walcott Quarry site of the Burgess Shale in Yoho National Park, British Columbia, Canada, and are part of the extensive collection of Burgess Shale fossils housed at the ROM.

The Burgess Shale Fossil Sites are located in Yoho and Kootenay National Parks and are managed by Parks Canada. Parks Canada is proud to work with leading scientific researchers to deepen knowledge and understanding of this key period in Earth’s history and to share these sites with the world through award-winning guided hikes. The Burgess Shale was designated a UNESCO World Heritage Site in 1980 because of its Outstanding Universal Value and is now part of the Canadian Rockies Parks World Heritage Site.

Fossils of Stanleycaris can be viewed by the public in the new exhibit of Burgess Shale fossils at the Willner Madge Gallery, Dawn of Life at the ROM.

Reference: “A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation” by Joseph Moysiuk and Jean-Bernard Caron, July 8, 2022, Current biology.
DOI: 10.1016/j.cub.2022.06.027

Major financial support for the research came from the National Science and Engineering Research Council of Canada, via a Vanier Canada Graduate Scholarship to Moysiuk and a Discovery Grant (#341944) to Caron.

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