Ancient fossilised brains give insight into the evolution of insects and spiders

A cache of new fossils containing the brain and nervous system of a marine predator from half a billion years ago has given evolutionary biologists new insight into the evolution of arthropods – insects, arachnids and crustaceans.

Stanleycaris hirpex belonged to an ancient, extinct offshoot of the arthropod evolutionary tree called Radiodonta. The fossils reveal it was a truly weird animal, with a pair of stalked eyes and a huge third eye in the front of its head – a feature never before seen in a radiodont.

“While fossilised brains from the Cambrian Period aren’t new, this discovery stands out for the astonishing quality of preservation and the large number of specimens,” says lead author Joseph Moysiuk, a PhD candidate in ecology and evolutionary biology at the University of Toronto, Canada.

“We can even make out fine details such as visual processing centres 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 research has been published in Current Biology.

Stanleycaris was a bizarre-looking organism

Moysiuk and PhD supervisor Dr Jean-Bernard Caron – Royal Ontario Museum’s curator of invertebrate palaeontology – studied a previously unpublished collection of 268 exceptionally preserved specimens of Stanleycaris.

They had been collected mostly in the 1980s and ’90s from the Burgess Shale in Yoho National Park in Canada.

Stanleycaris was small (for the radiodont group) at no more than 20cm long, but it would have still been an impressive predator at a time when most animals didn’t grow any bigger than a human finger.

It had large, compound eyes, a circular mouth lined with teeth, frontal claws with an array of spines, and a flexible, segmented body with a series of swimming flaps along its sides.

“The presence of a huge third eye in Stanleycaris was unexpected. It emphasises that these animals were even more bizarre-looking than we thought, but also shows us that the earliest arthropods had already evolved a variety of complex visual systems like many of their modern kin,” says Caron.

“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.”

Insights into the evolution of segmented brains in arthropods

In 84 of the fossilised remains, the brain and nerves were still preserved after 506 million years, revealing the Stanleycaris brain was composed of two segments – the protocerebrum and deutocerebrum – that connected with the eyes and frontal claws, respectively.

But today, arthropods have brains composed of three segments including an additional tritocerebrum.

“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 characterises all living members of this diverse animal phylum,” explains Moysiuk.

While this might not sound game-changing, it has important scientific implications for 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.”