‘Stunning’ 500-Million-Year-Old Fossilized Brains Threaten to Rethink the Evolution of Insects and Spiders

Stanleycaris hirpex. Credit: Sabrina Cappelli © Royal Ontario Museum

An ancient radioodont predator with three eyes reveals key information about the evolution of the arthropod body plan.

A new study based on fossil remains from the Burgess Shale contains the brain and nervous system of a half-billion-year-old marine predator Stanleycaris Discovered by the Royal Ontario Museum (ROM). It belongs to an ancient, extinct branch of the arthropod evolutionary tree called Radiodonta. Stanleycaris 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 so clear, it’s as if we’re looking at an animal that died yesterday.”

— Joseph Moysiuk

The findings are reported in a research paper titled “Three-eyed radioodont with fossil neuroanatomy sheds light on the origin of the arthropod head and segmentation,” published July 5, 2022, in the journal. Current Biology.

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

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

“Although fossilized brains from the Cambrian period are not new, this discovery stands out for its amazing quality of preservation and large number of specimens,” said Joseph Moysiuk of the University of Toronto (U of T), lead author of the study. PhD candidate in Ecology and Evolutionary Biology based at the Royal Ontario Museum. “We can even distinguish fine details like the visual processing centers that serve the large eyes and the neural traces that go into the appendages. The details are so clear that it is as if we are looking at an animal that died yesterday.”

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

New fossils reveal brain Stanleycaris it consisted of two segments, the protocerebrum, which fused with the eyes, and the deutocerebrum, which fused with the frontal claws.

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

In modern arthropods such as insects, the brain consists of the protocerebrum, deutocerebrum, and tritocerebrum. While a difference in one segment may not sound like a game-changer, it actually has radical scientific implications. Because duplicates of many arthropod organs can be found in their segmented bodies, understanding how the segments are arranged among different species is key to understanding how these structures diversified within the group.

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

Reconstruction of Stanleycaris hirpex pair; The transparency of the external appearance of the internal organs of the upper individual has increased. The nervous system is shown in light beige, the digestive system in dark red. Credit: Illustration by Sabrina Cappelli © Royal Ontario Museum

Along with a pair of beady eyes, Stanleycaris it had a large central eye at the front of its head, a feature never seen before in a radioodont. “Having a giant third eye inside Stanleycaris it was unexpected. “This highlights that these animals had a more exotic appearance than we thought, and also shows us that the oldest arthropods, like many of their modern relatives, had already developed a variety of complex visual systems.” Jean-Bernard Caron, ROM’s Richard Ivey Curator of Invertebrate Paleontology and Moysiuk’s PhD supervisor. “Since most radiodonts are known only from scattered bits and pieces, this discovery is an important advance in understanding what they looked like and how they lived,” he said. Evolution and Earth Sciences.

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

During the Cambrian period, radiodonts included some of the largest animals around, with the famous “strange wonder.” anomalocaris length reaches at least 1 meter. no more than 20 cm, Stanleycaris it was small for its group, but at a time when most animals were no bigger than a human finger, it would have been an effective predator. StanleycarisSophisticated sensory and nervous systems would allow it to efficiently pick out small prey in the dark.

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

With large compound eyes, a gigantic-looking circular mouth covered with teeth, front paws with an impressive set of spines, and a flexible, segmented body with a set of swimming fins on the sides, Stanleycaris would be nightmares for any little bottom dweller unfortunate enough to cross his path.

About the Burgess Shale

For this study, Moysiuk and Caron examined a previously unpublished collection of 268 specimens. Stanleycaris. The fossils were primarily collected in the 1980s and 90s from rock strata above the famous Walcott Quarry site of 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 within Yoho and Kootenay National Parks and are managed by Parks Canada. Parks Canada is proud to work with leading scientific researchers to expand 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 for its outstanding universal value and is now part of the larger Canadian Rockies Parks World Heritage Site.

fossils Stanleycaris A new Burgess Shale fossil can be seen by the public Willner Madge Gallery, Dawn of Life in ROM.

Reference: “Three-eyed radioodont with fossilized neuroanatomy sheds light on the origin and segmentation of the arthropod head” By Joseph Moysiuk and Jean-Bernard Caron, 8 July 2022 Current Biology.
DOI: 10.1016/j.cub.2022.06.027

Major research funding support came from the National Sciences and Engineering Research Council of Canada, through a Vanier Canada Graduate Fellowship to Moysiuk and a Carona Discovery Grant (no. 341944).