‘Amazing’ 500-million-year-old fossilized brains prompt rethinking of insect and spider evolution

Stanleycaris hirpex

Stanleycaris hirpex. Credit: Sabrina Cappelli © Royal Ontario Museum

An ancient three-eyed radiodont predator reveals important information about the evolution of the arthropod body plan.

New research is based on a fossil cache containing the brain and nervous system of a half-billion-year-old Burgess Shale marine predator Stanleycaris was unveiled by the Royal Ontario Museum (ROM). Belonging to an ancient 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 arthropod brain, vision, and head structure.

“The details are as clear as if we were looking at an animal that died yesterday.”

Joseph Moysiuk

The findings 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, specimen ROMIP 65674.1-2. Photo credit: Photo by Jean-Bernard Caron, © Royal Ontario Museum

What excites scientists most is what’s inside Stanleycaris‘ Head. In 84 of the fossils, the remains of the brain and nerves are still preserved after 506 million years.

“While fossilized brains from the Cambrian period are not new, this discovery stands out for the amazing quality of preservation and the large number of specimens,” said Joseph Moysiuk, lead author of the research and University of Toronto (U of T) doctoral student in ecology and Evolutionary Biology at the Royal Ontario Museum. “We can even see fine details like visual processing centers that feed the big eyes and traces of nerves that enter the limbs. The details are as clear as if we were looking at an animal that died yesterday.”


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

The new fossils show that the brain of Stanleycaris consisted of two segments, the protocerebrum connected to the eyes and the deuterocerebrum connected to the frontal claws.

“We conclude that a bipartite head and brain have deep roots in the arthropod lineage and that its evolution likely predates the tripartite brain that characterizes all living members of this diverse phylum,” Moysiuk added.

In modern-day arthropods such as insects, the brain consists of the protocerebrum, deutocerebrum, and tritocerebrum. While the difference in one segment doesn’t sound groundbreaking, it actually has radical scientific implications. Because repeated copies of many arthropod organs are found in their segmented bodies, figuring out how segments line up between different species is crucial to understanding how these structures have diversified within the group.

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

Reconstruction of Stanleycaris hirpex

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

Beside his pair of stalked eyes, Stanleycaris possessed a large central eye at the front of his head, a feature never before seen in a radiodontist. “The presence of a giant third eye in Stanleycaris was unexpected. It underscores that these animals looked even more bizarre 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 relatives,” said Dr. Jean-Bernard Caron, Richard Ivey of ROM Curator of Invertebrate Paleontology and PhD supervisor of Moysiuk. “Since most radiodonts are known only from scattered parts, this discovery is a critical step forward in understanding what they looked like and how they lived,” added Caron, who is also an associate professor at the U of T of ecology and ecology is. Evolution and Earth Sciences.

Nervous system from fossils of Stanleycaris

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

In the Cambrian period, radiodonts included some of the largest animals in existence, with the famous ‘strange wonder’. anomalocaris reach a length of at least 1 meter. no longer than 20 cm, Stanleycaris was small for its group, but at a time when most animals grew no larger than a human finger, it would have been a formidable predator. Stanleycaris‘ Sophisticated sensory and nervous systems would have enabled it to efficiently seek out small prey in the dark.

Stanleycaris hirpex reconstruction

Reconstruction of Stanleycaris hirpex. Photo Credit: Art by Sabrina Cappelli © Royal Ontario Museum

Featuring large compound eyes, an impressive looking round mouth lined with teeth, front claws with an impressive array of spikes, and a flexible, segmented body with a row of armbands on the sides. Stanleycaris would have been the stuff of nightmares for every little ground dweller unlucky enough to cross his path.

About the Burgess Shale

For this research, Moysiuk and Caron examined a previously unpublished collection of 268 specimens of Stanleycaris. The fossils were collected primarily in the 1980’s and 90’s from strata above the famous Walcott Quarry in the Burgess Shale in Yoho National Park, BC, 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 advance knowledge and understanding of this key period in Earth’s history and share these sites with the world through award-winning guided walks. The Burgess Shale was designated a UNESCO World Heritage Site for its Outstanding Universal Value in 1980 and is now part of Canada’s larger Rocky Mountain Parks World Heritage Site.

fossils of Stanleycaris can be viewed by the public in the new Burgess Shale fossil exhibition at the Willner Madge Gallery, Dawn of Life at 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 research funding came from the National Sciences and Engineering Research Council of Canada through a Vanier Canada Graduate Scholarship to Moysiuk and a Discovery Grant (No. 341944) to Caron.

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