Finding a three-eyed extinct species is strange enough – discovering its beautifully preserved brain and nervous system when it has been dead for half a billion years is a paleontologist's dream. The Royal Ontario Museum in Canada has announced not just one such fossil, but 84 of them, and they'll change the way we think about invertebrate evolution.

The Burgess Shale has been exciting biologists for a century, revealing the sudden appearance of numerous strange life forms with body architecture like nothing we see today. One of the early discoveries looked so much like a bad acid trip it was named Hallucigenia, and the discoveries set off a war among prominent paleontologists on how to interpret the findings.

The announcement of exceptional specimens of the marine predator Stanleycaris hirpex, previously known only from fragments, could bring the Burgess back to center stage. The discovery is published in the journal Current Biology.

To the amateur, the most distinctive feature of Stanleycaris is its large central eye on the front of its head, to go with those on stalks on either side.

"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 University of Toronto PhD student Joseph Moysiuk in a statement. "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." 

Stanleycaris was a Radiodont, an order of extinct animals whose closest living relatives are arthropods – spiders, scorpions, and horseshoe crabs among others. Radiodonts included some of the Cambrian's most fearsome predators, including Titanokorys and Anomalocaris. The specimens were at most 8 centimeters (3.5 inches) long, making S. hirpex the smallest radiodont ever found, but still larger than most of its potential prey.

"These fossils are like a Rosetta Stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups." Moysiuk said.

The most noticeable feature of S. herpex's brain is that it had two components: one connected to the eyes, the other to the frontal claws. Modern arthropods have brains segmented into three sectors, but entomologists have not known until now when and how this evolved. The segmentation of Stanleycaris's brain suggests arthropods may have had protocerebrums and deutocerebrums when they broke away from the radiodonts, with the third part, the triptocerebrum, appearing later.

Third eyes have never been reported in radiodonts previously, but Dr Jean-Baron Caron, Moysiuk's supervisor, noted; "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." 

On closer examination, the authors found evidence of similarly placed eyes in previously described Cambrian species related to modern arthropods. The paper discusses the possibility that two close-set eyes fused together, or that what was once a single compound eye split.

Stanleycaris hirpex must have been abundant in Cambrian times. The paper describes 268 specimens, or which almost a third have visible brains, collectively revealing S. hirpex in greater detail than any other radiodont. Other notable features are a trunk composed of 17 segments (an obsession with even numbers seems to have been very much a post-Cambrian thing) and the swimming flaps seen in larger radiodonts.