It's what's inside Stanleycaris' head that has the researchers most excited. In 84 of the fossils, the remains of the brain and nerves are still preserved after 506 million years.

"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 Joseph Moysiuk, lead author of the research and a University of Toronto (U of T) Ph.D. Candidate in Ecology and Evolutionary Biology, based at 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. The details are so clear it's as if we were looking at an animal that died yesterday."

The new fossils show that the brain of Stanleycaris was composed of two segments, the protocerebrum and deutocerebrum, 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," added Moysiuk.

In present day arthropods like insects, the brain consists of protocerebrum, deutocerebrum, and tritocerebrum. While the difference of a segment may not sound game-changing, it in fact has radical scientific implications. Since repeated copies of many arthropod organs can be found in their segmented bodies, figuring out how segments line up between different species is key to understanding how these structures diversified across the group. "These fossils are like a Rosetta Stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups."