500-million-year-old fossilized stanleycaris brain prompts rethinking of insect and spider evolution

TORONTO, ON, July 8, 2022 — ROM (Royal Ontario Museum) unveiled new research based on a cache of fossils that contains the brain and nervous system of a half-billion-year-old marine predator from the Burgess Shale called Stanleycaris. Stanleycaris belonged to an ancient, extinct offshoot of the arthropod evolutionary tree called Radiodonta, distantly related to modern insects and spiders. These findings shed light on the evolution of the arthropod brain, vision and head structure. The results were announced in the paper, “A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation,” published in the journal Current Biology.

It’s what’s inside Stanleycaris’ head that gets the researchers most excited. 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 is notable for the astonishing quality of its preservation and the large number of specimens,” said Joseph Moysiuk, lead author of the study and a University of Toronto (U of T) PhD. candidate in ecology and evolutionary biology, located at the Royal Ontario Museum. “We can even discern fine details, such as visual processing centers that operate the large eyes and traces of nerves that penetrate the appendages. The details are so clear that it is as if we are looking at an animal that died yesterday.”

The new fossils show that the brain of Stanleycaris consisted of two segments, the protocerebrum and deutocerebrum, which were connected to the eyes and frontal claws, respectively. “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 characteristic of all living members of this diverse animal phylum,” Moysiuk added. In contemporary arthropods such as insects, the brain consists of protocerebrum, deutocerebrum, and tritocerebrum. While a segment’s difference may not sound groundbreaking, it actually has radical scientific implications. Because repeated copies of many arthropod organs can be found in their segmented bodies, understanding how these structures diversified across the group is critical. “These fossils are like a Rosetta stone and help link traits in radiodonts and other early fossil arthropods to their counterparts in surviving groups.”

In addition to his pair of stalked eyes, Stanleycaris possessed a large central eye in the front of his head, a feature never before noticed in a radiodont. “The presence of a huge third eye in Stanleycaris was unexpected. It highlights that these animals looked even more bizarre than we thought, but it also shows us that the earliest arthropods had already developed 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 Moysiuk’s PhD promoter. “Since most radiodonts are only known from scattered bits and pieces, 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, in Ecology & Evolution and Earth Sciences.

In the Cambrian period, radiodonts included some of the largest animals around, with the famous “weird wonder” Anomalocaris reaching up to at least 1 meter in length. At no more than 8 inches (20 cm) in length, Stanleycaris 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. Stanleycaris’ advanced sensory and nervous systems would have made it possible to efficiently sort out small prey at dusk.

With large compound eyes, a formidable-looking round mouth full of teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with a series of swim flaps along the sides, Stanleycaris would have been a nightmare for anyone. small bottom dweller who unfortunately crosses his path.

About the Burgess Shalea

For this study, Moysiuk and Caron studied a previously unpublished collection of 268 copies of Stanleycaris. The fossils were mainly collected in the 1980s and 1990s 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 in ROM .

The Burgess Shale fossils are located in Yoho and Kootenay National Parks and are managed by Parks Canada. Parks Canada is proud to partner with leading scientific researchers to expand knowledge and understanding of this important 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 in 1980 for its outstanding universal value and is now part of the larger Canadian Rocky Mountain Parks World Heritage Site.

Stanleycaris fossils are on public display in the new Burgess Shale fossils exhibit at the Willner Madge Gallery, Dawn of Life at ROM.

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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