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Marine fossil sheds light on arthropod evolution

By investigating a cache of fossils containing the brain and nervous system of a 500-million-year-old marine predator from the Burgess Shale (a fossil deposit in the Canadian Rocky Mountains) called Stanleycaris, a team of researchers led by the Royal Ontario Museum (ROM) has managed to shed more light on the evolution of the arthropod brain, vision, and head structure.

Stanleycaris belongs to an ancient, extinct offshoot of the arthropod evolutionary tree called Radiodonta, which was distantly related to modern insects and spiders. With large compound eyes, a fierce circular mouth lined with teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with several swimming flaps along its sides, Stanleycaris was most likely a feared marine predator that fed upon a large variety of sea-bottom dwellers.

The fossils discovered in the Burgess Shale contained extremely well-preserved remains of this prehistoric animal’s brain and nervous system. “While fossilized brains from the Cambrian Period aren’t new, this discovery stands out for the astonishing quality of preservation and the large number of specimens,” said study lead author Joseph Moysiuk, a doctoral student in Ecology and Evolutionary Biology at the University of Toronto. “We can even make out fine details such as visual processing centers serving the large eyes and traces of nerves entering the appendages. The details are so clear it’s as if we were looking at an animal that died yesterday.”

The fossils revealed that the brain of Stanleycaris consisted of two segments, the protocerebrum and the deutocerebrum, which were connected with the eyes and frontal claws, respectively. “We conclude that a two-segmented head and brain has deep roots in the arthropod lineage and that its evolution likely preceded the three-segmented brain that characterizes all living members of this diverse animal phylum,” Moysiuk explained.

In modern arthropods such as insects, the brain consists of three segments – the protocerebrum, the deutocerebrum, and the tritocerebrum. While the absence of one of these segments in Stanleycaris may not seem a fundamental game-changer, this discovery has in fact radical scientific implications. Since repeated copies of many arthropod organs can be found in their segmented bodies, clarifying how segments line up among different species is fundamental to understanding how these structures diversified during evolution. “These fossils are like a Rosetta Stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups,” said Moysiuk.

Moreover, the fact that this animal possessed a huge third eye at the front of its head shows that the earliest arthropods had already evolved a variety of complex visual systems like many of their modern counterparts, according to study senior author Jean-Bernard Caron, a curator of Invertebrate Paleontology at ROM. “Since most radiodonts are only known from scattered bits and pieces, this discovery is a crucial jump forward in understanding what they looked like and how they lived,” he concluded.

The study is published in the journal Current Biology

By Andrei Ionescu, Staff Writer  

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