Stanleycaris hirpex. Credit: Sabrina Cappelli © Royal Ontario Museum
An ancient three-eyed radiodont predator reveals key information about the evolution of the body plan of arthropods.
The Royal Ontario Museum (ROM) has revealed new research based on a fossil cache containing the brain and nervous system of a half-million-year-old marine predator from the Burgess Shale called Stanleycaris. Belonging to an ancient and extinct branch 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 we’re looking at an animal that died yesterday.”
—Joseph Moysiuk
The findings were announced in the research paper, “A three-eyed radiodont with fossilized neuroanatomy reports the origin of the arthropod head and segmentation,” published July 5, 2022 in the journal Current Biology.
Pair of fossil specimens of Stanleycaris hirpex, specimen ROMIP 65674.1-2. Credit: Photo by Jean-Bernard Caron, © Royal Ontario Museum
What has most excited scientists is what is inside Stanleycaris’ head. The remains of the brain and nerves are still preserved after 506 million years in 84 of the fossils.
“While fossilized brains from the Cambrian period are not new, this discovery stands out for its amazing conservation quality and large number of specimens,” said Joseph Moysiuk, lead author of the research and the University of Toronto (U of T). PhD student in Ecology and Evolutionary Biology, based at the Royal Ontario Museum. “We can even distinguish details such as visual processing centers that serve the large eyes and nerve traces that enter the appendages. The details are so clear it’s like we’re 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
The new fossils reveal that Stanleycaris’ brain was composed of two segments, the protocerebrum, connected to the eyes, and the deutocerebrum, connected to the frontal claws.
“We conclude that a head and a two-segment brain have deep roots in the lineage of arthropods and that their evolution probably preceded the three-segment brain that characterizes all living members of this diverse animal phylum,” Moysiuk added.
In today’s arthropods such as insects, the brain is made up of protocerebrum, deutocerebrum and tritocerebrum. While the difference in a segment may not seem like a game changer, it does have radical scientific implications. Because repeated copies of many arthropod organs can be found in their segmented bodies, figuring out how segments align between different species is key to understanding how these structures diversified into the group.
“These fossils are like a Rosetta stone, helping to link features in radiodonts and other early fossil arthropods with their counterparts in surviving groups.”
Reconstruction of a pair of Stanleycaris hirpex; superior individual has increased external transparency to show internal organs. The nervous system is shown in light beige, the digestive system in dark red. Credit: Illustration by Sabrina Cappelli © Royal Ontario Museum
In addition to his pair of prickly eyes, Stanleycaris possessed a large central eye on the front of his head, a feature never before noticed in a radiodont. “The presence of a huge third eye in Stanleycaris was unexpected. He points out that these animals looked even stranger than we thought, but it also shows us that early 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. Conservator of Invertebrate Paleontology and doctoral director of Moysiuk. “Because most radiodonts are only known from scattered chunks, this discovery is a crucial leap in understanding how they were and how they lived,” added Caron, who is also an associate professor at T University in Ecology and Evolution. and Earth Sciences.
Summary of the work, which shows the interpretation of the nervous system from Stanleycaris fossils and the implications for understanding the evolution of the arthropod brain. The brain is represented in red and the nerve cords in violet. Credit: Photo by Jean-Bernard Caron © Royal Ontario Museum
In the Cambrian period, radiodonts included some of the largest animals around, with the famous “strange wonder” Anomalocaris reaching up to at least 1 meter long. At no more than 20 cm long, Stanleycaris was small for its group, but at a time when most animals grew no more than a human finger, it would have been an impressive predator. Stanleycaris ’sophisticated sensory and nervous systems would have allowed him to efficiently choose small prey in the dark.
Reconstruction of Stanleycaris hirpex. Credit: Art by Sabrina Cappelli © Royal Ontario Museum
With large compound eyes, a formidable-looking circular mouth lined with teeth, frontal claws with an impressive variety of spines, and a flexible, segmented body with a series of swimming flaps on its sides, Stanleycaris would have been the nightmare material for to anyone. small inhabitant of the bottom unfortunate enough to cross his path.
Fossil specimen of Stanleycaris hirpex. The dark material inside the head is the remains of nerve tissue, copy ROMIP 65674.2. Credit: Photo by Jean-Bernard Caron © Royal Ontario Museum Fossil specimen of Stanleycaris hirpex. The dark material inside the head is the remnants of nerve tissue, specimen ROMIP 65674.1. Credit: Photo by Jean-Bernard Caron © Royal Ontario Museum
About the Burgess Shale
For this research, Moysiuk and Caron studied an unpublished collection of 268 specimens of Stanleycaris. Fossils were collected primarily in the 1980s and 1990s from rock layers on the famous Burgess Shale Walcott Quarry site in Yoho National Park, BC, Canada, and are part of the extensive collection of fossils from Burgess Shale hosted on 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 expand knowledge and understanding of this key period in Earth’s history and to share these places with the world through award-winning guided tours. The Burgess Shale was designated a World Heritage Site by UNESCO in 1980 for its outstanding universal value and is now a World Heritage Site of the Canadian Rocky Mountain Parks.
Stanleycaris fossils can be seen by the public at the new Burgess Shale fossil exhibition at the Willner Madge Gallery, Dawn of Life on ROM.
Reference: “A three-eyed radiodont with fossilized neuroanatomy reports the origin of the arthropod’s head and segmentation” by Joseph Moysiuk and Jean-Bernard Caron, July 8, 2022, Current Biology.DOI: 10.1016 / j.cub .2022.06.027
The main funding for the research comes from the National Sciences and Engineering Research Council of Canada, through a Vanier Canada Postgraduate Fellowship in Moysiuk and a Discovery Fellowship (No. 341944) in Caron.