By examining a cache of fossils containing the brain and nervous system of a 500-million-year-old marine predator from the Burgess Shale (fossils in the Canadian Rockies) called Stanleycaris, a team of researchers led by the Royal Ontario Museum (ROM) was able to shed more light on the evolution of the brain, vision and head structure of arthropods.
Stanleycaris belongs to an ancient, extinct branch of the arthropod evolutionary tree called Radiodonta, which was distantly related to modern insects and spiders. With large compound eyes, a ferocious round mouth lined with teeth, front claws with an impressive array of spikes, and a flexible, segmented body with several swimming flaps on the sides, Stanleycaris was most likely a dangerous marine predator that fed on a wide variety of inhabitants on the seabed.
Fossils found in the Burgess Shale contain exceptionally well-preserved remains of the brain and nervous system of this prehistoric animal. “Although fossilized Cambrian brains are not new, this discovery stands out for its astonishing quality of preservation and large number of specimens,” said lead study author Joseph Moysiuk, a doctoral student in ecology and evolutionary biology at the University of Toronto. “We can even make out fine details like visual processing centers serving the large eyes and traces of nerves entering the appendages.” The details are as clear as if we were looking at an animal that died yesterday.”
Fossils reveal that the brain of Stenlicaris consisted of two segments, the protocerebrum and the deutocerebrum, which were connected to the eyes and front claws, respectively. “We conclude that the two-segmented head and brain have deep roots in the arthropod lineage, and that its evolution probably preceded the three-segmented brain that characterizes all living members of this diverse animal phylum,” explained Moysiuk.
In modern arthropods such as insects, the brain consists of three segments – protocerebrum, deutocerebrum and tritocerebrum. While the absence of one of these segments in Stanleycaris may not seem like a fundamental game changer, this finding actually has radical scientific implications. Because repeated copies of many arthropod organs can be found in their segmented bodies, elucidating how segments are arranged across species is fundamental to understanding how these structures have diversified during evolution. “These fossils are like a Rosetta Stone, helping to connect traits in radiodonts and other early fossil arthropods to their counterparts in surviving groups,” Moysiuk said.
What’s more, the fact that this animal possessed a huge third eye at the front of its head indicates that the earliest arthropods had already developed various complex visual systems like many of their modern counterparts, according to the study’s lead author Jean-Bernard Caron , a curator of invertebrate paleontology at the ROM. “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,” he concluded.
The research was published in the journal Current Biology.
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By Andrei Ionescu, Earth.com Contributor
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