Paleontologists from the University of Toronto and the Royal Ontario Museum 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 exceptionally well-preserved whole-body specimens. Their findings shed light on the evolution of the brain, vision and head structure of arthropods.
Reconstruction of a pair of Stanleycaris hirpex: the above individual has a transparency of the exterior enlarged to show the 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 at least 1 m (3.3 ft) in length.
At no more than 20 cm (7.9 in) long, Stanleycaris hirpex was small for its group, but at a time when most animals grew no larger than a human finger, it would have been an impressive predator.
It had large compound eyes, a great-looking round mouth lined with teeth, front claws with an impressive set of spikes, and a flexible, segmented body with a series of swim flaps along its sides.
Its advanced sensory and nervous systems would allow it to effectively detect small prey in the dark.
New fossils from the Burgess Shale show that the brain of Stanleycaris hirpex was composed of two segments, the protocerebrum and the deutocerebrum, associated with the eyes and front claws, respectively.
“Although fossilized Cambrian brains are not new, this discovery stands out for its astonishing quality of preservation and large number of specimens,” said Joseph Moysiuk, Ph.D. candidate at the University of Toronto and the Royal Ontario Museum.
“We can even make out fine details such as visual processing centers serving the large eyes and traces of nerves entering the appendages.”
“We conclude that the bisegmental head and brain have deep roots in the arthropod lineage and that its 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 stalked lateral eyes, Stanleycaris hirpex unexpectedly had a large median eye. Image credit: Moysiuk & Caron, doi: 10.1016/j.cub.2022.06.027.
In modern arthropods, the brain consists of a protocerebrum, a deutocerebrum, and a tritocerebrum.
While a segment difference may not sound like a game-changer, it actually has radical scientific implications.
Because repeated copies 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 that helps connect traits in radiodonts and other early fossil arthropods to their counterparts in surviving groups,” Moysiuk said.
In addition to its pair of stalk-like eyes, Stanleycaris hirpex possessed a large central eye at the front of its head, a feature never before seen in a radiodont.
“The presence of a huge third eye in Stanleycaris hirpex was unexpected,” said Dr. Jean-Bernard Caron, Richard Ivey Curator of Invertebrate Paleontology at the Royal Ontario Museum.
“This highlights that these animals look even stranger than we thought, but it also shows us that the earliest arthropods had already developed various complex visual systems, like many of their modern relatives.”
“Since most radiodonts are known only from scattered pieces, this discovery is a crucial leap forward in understanding what they looked like and how they lived.”
The results appear in the journal Current Biology.
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Joseph Moysiuk and Jean-Bernard Caron. A three-eyed radiodont and fossilized neuroanatomy informs the origin of the arthropod head and segmentation. Current Biology, published online July 8, 2022; doi: 10.1016/j.cub.2022.06.027
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