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 circular mouth lined with teeth, front claws with an impressive array of spines, and a flexible, segmented body with multiple swimming fins along its sides, Stanleycaris was likely a feared marine predator that fed on a wide variety of of the bottom dwellers. Fossils discovered in the Burgess Shale contained extremely well-preserved remains of the brain and nervous system of this prehistoric animal. “While fossil brains from the Cambrian period are not new, this discovery stands out for its amazing 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, such as visual processing centers serving the large eyes and traces of nerves entering the appendages. The details are so clear it’s like looking at an animal that died yesterday.” The fossils revealed that the brain of Stanleycaris consisted of two parts, the proto-brain and the deutero-brain, which were connected to the eyes and frontal claws, respectively. “We conclude that a bipartite head and brain have deep roots in the arthropod lineage, and that its evolution likely preceded the three-part brain that characterizes all living members of this diverse animal phylum,” explained Moysiuk. In modern arthropods such as insects, the brain consists of three parts – the forebrain, the second brain and the third brain. While the absence of one of these segments in Stanleycaris may not seem like a fundamental game changer, this discovery actually has radical scientific implications. Since repeated copies of many arthropod organs can be found in their segmented bodies, elucidating how the segments align between different species is fundamental to understanding how these structures have diverged during evolution. “These fossils are like a Rosetta Stone, helping to connect the characteristics of radiotooth and other early fossil arthropods with their counterparts in surviving groups,” said Moissiok. Additionally, the fact that this animal possessed a huge third eye at the front of its head indicates that early arthropods had already evolved a variety of complex visual systems like many of their modern counterparts, according to the study’s senior author Jean-Bernard Caron. curator of Invertebrate Palaeontology at the ROM. “Since most radiotooths are known only from scattered pieces, this discovery is a critical leap 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, Earth.com Staff Editor
