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Fossil brains force a rethink of the evolution of insects and arachnids

New fossils with traces of the brain and nervous system of a 500-million-year-old marine predator identify the need to rethink the evolution of insects and spiders.

The results, obtained in the Burgess Shale Formation (Canada), correspond to “Stanley Cares”, which belongs to an ancient extinct branch of the evolutionary tree of arthropods called Radiodonta, related to modern insects and spiders. Published in Current Biology, it sheds light on arthropod brain development, vision, and head structure.

What interests researchers most is what’s inside Stanley Karis’ head. In 84 fossils, remains of the brain and nerves are still preserved after 506 million years.

Said Joseph Moisiuk, lead author of the research and a doctoral candidate in ecology and evolutionary biology at the University of Toronto (U of T), based at the Royal Ontario Museum.

He continues, “We can even provide minute details, such as the visual processing centers that serve the large eyes and traces of nerves entering the appendages. The details are so clear that we are watching an animal that died yesterday.”

The new fossils show that Stanley Karis’ brain was made up of two parts, the primary and microbrain, connected to the eyes and front claws, respectively.

“We conclude that the segmented head and brain have deep roots in the arthropod lineage and that their evolution likely predates the three-sectioned brain that characterizes all living organs in this diverse animal phylum,” Moiseuk added.

In modern arthropods, such as insects, the brain is made up of protoprene, ditoprin, and tritoprin. Although the one-part difference may not seem like a game-changer, it actually has drastic scientific implications.

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Because repetitive copies of many organs can be found in the segmented bodies of arthropods, knowing how the segments lined up between different species is key to understanding how these structures vary within the group. “These fossils are similar to the Rosetta Stone, helping to link traits of radiators and other early arthropods to their counterparts in surviving groups,” he says.

In addition to a pair of stalking eyes, Stanleycaris had a large central eye on the front of its head, a feature not previously seen in radiodon.

“The presence of a huge third eye in Stanleycaris was unexpected. It highlights that these animals looked even more exotic than we thought, but it also shows us that early arthropods had already evolved as diverse a variety of complex visual systems as many modern optical systems.” Relatives,” explain Dr. Jean-Bernard Caron, Richard Ivey Curator of Invertebrate Paleontology at ROM, and PhD Supervisor at Moysiuk.

“Since most radios are known only from scattered parts, this discovery represents a crucial leap in understanding what they looked like and how they lived,” adds Caron, who is also an associate professor of ecology, evolution and earth sciences.

In the Cambrian period, radiodonts were among the largest animals, such as the famous “Anomalocaris”, which reached at least a meter in length. Stanley Karis, no more than 20 cm tall, was small for his pack, but in an age when most animals were no bigger than a human finger, he would have been an impressive predator. Stanley Karis’ sophisticated sensory and neural systems would have enabled him to effectively detect small prey in the dark.

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With large compound eyes, an enormous round mouth full of teeth, front claws with an impressive set of spines, and a flexible body divided by a series of swimming fins on either side, ‘Stanleycaris’ would have been a nightmare. The researchers say residents are unlucky enough to cross their path.