500-million-year-old fossilized brains of Stanleycaris prompt rethinking of the evolution of insects and spiders

The Royal Ontario Museum has unveiled new research based on a cache of fossils containing the brain and nervous system of a half-billion-year-old Burgess Shale marine predator called Stanleycaris. Stanleycaris belonged to an ancient, extinct offshoot of the arthropod evolutionary tree called Radiodonta, distantly related to modern insects and spiders. These results shed light on the evolution of the arthropod brain, vision and head structure. The findings were announced in the journal article “A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation.” Current Biology.

It’s what’s going on in Stanleycaris’ mind that gets researchers most excited. 84 of the fossils still have brain and nerve remains 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 a University of Toronto (U of T) Ph. D Candidate in Ecology and Evolutionary Biology at the Royal Ontario Museum. “We can even see fine details like visual processing centers that feed the large eyes and traces of nerves entering the limbs. The details are as clear as if we were looking at an animal that died yesterday.”

The new fossils show that the Stanleycaris brain consisted of two segments, the protocerebrum and the deuterocerebrum, connected to the eyes and frontal claws, respectively. “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 contemporary arthropods such as insects, the brain consists of protocerebrum, deuterocerebrum, and tritocerebrum. While one segment’s difference 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 to their counterparts in surviving groups.”

In addition to its pair of stalked eyes, Stanleycaris possessed a large central eye at the front of its head, a feature never before seen in a radiodont. “The presence of a giant third eye in Stanleycaris was unexpected. It underscores that these animals looked even more bizarre than we thought, but it 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 -Curator of Invertebrate Paleontology at ROM, and Moysiuk’s Ph.D Supervisor “Since most radiodonts are only known from scattered parts, this discovery is a crucial step forward in understanding what they looked like and how they lived added Caron, who is also an associate professor at U of T, in Ecology & Evolution and Earth Sciences.

By the Cambrian period, radiodonts included some of the world’s largest animals, with the famous “strange wonder” Anomalocaris reaching at least 1 meter in length. At no more than 20 cm long, 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 allowed it to efficiently seek out small prey in the dark.

With 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 water wings on its sides, Stanleycaris would have been the stuff of nightmares for a small bottom-dweller, which unfortunately crosses 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.

Stanleycaris fossils are available for viewing by the public in the new Burgess Shale fossil exhibit at the Willner Madge Gallery, Dawn of Life at ROM.

Provided by the Royal Ontario Museum