Paleontologists at the University of Toronto and the Royal Museum of Ontario have examined 268 specimens of Stanleycaris hirpex, a radiodont that lived during the Cambrian period about 506 million years ago, from the Burgess Shale in Canada, including many specimens of exceptionally preserved whole body. Their findings shed light on the evolution of the brain, vision and head structure of arthropods.
Reconstruction of a pair of Stanleycaris hirpex: the upper individual has increased external transparency to show internal organs; the nervous system is shown in light beige, the digestive system in dark red. Image credit: Sabrina Cappelli, Royal Ontario Museum.
Radiodonta is an order of arthropods that dominated the Cambrian oceans about 500 million years ago.
It included some of the most iconic and strange-looking Cambrian animals, with the famous Anomalocaris reaching up to at least 1 m (3.3 feet) long.
At no more than 20 cm (7.9 inches) long, Stanleycaris hirpex 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.
He had 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 his sides.
His sophisticated sensory and nervous systems would have allowed him to efficiently choose small prey in the dark.
The new Burgess Shale fossils show that the brain of Stanleycaris hirpex was composed of two segments, the protocerebrum and the deutocerebrum, connected to the frontal eyes and claws, respectively.
“While the fossilized brains of the Cambrian period are not new, this discovery stands out for its amazing conservation quality and large number of specimens,” said Joseph Moysiuk, Ph.D. candidate for the University of Toronto and 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.”
“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,” he added.
In addition to the pair of pointed side eyes, Stanleycaris hirpex had an unexpectedly medium-sized eye. Image credit: Moysiuk & Caron, doi: 10.1016 / j.cub.2022.06.027.
In current arthropods, 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,” Moysiuk said.
In addition to its pair of prickly eyes, Stanleycaris hirpex possessed a large central eye on the front of its head, a feature never before noticed in a radiodont.
“The presence of a huge third eye in Stanleycaris hirpex was unexpected,” said Dr. Jean-Bernard Caron, curator of invertebrate paleontology at Richard Ivey at the Royal Ontario Museum.
“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.”
“Because most radiodonts are only known from scattered chunks, this discovery is a crucial leap forward in understanding how they were and how they lived.”
The results appear in the journal Current Biology.
_____
Joseph Moysiuk and Jean-Bernard Caron. A three-eyed radiodont with fossilized neuroanatomy reports the origin of the arthropod head and segmentation. Current Biology, published online July 8, 2022; doi: 10.1016 / j.cub.2022.06.027