The world’s oldest brain has been found in the remains of a three-eyed prawn that swam the oceans more than half a billion years ago.

Its complete central nervous system is still visible, providing unprecedented insights into the ancestors of insects, spiders and crustaceans.

Named Stanleycaris hirpex, it was described as “the stuff of nightmares.” It had two eyes “on stalks” with a bigger one in the middle, and spiked claws.

It lived during the period known by paleontologists as the Cambrian Explosion, a time of rapid evolution in shallow seas when most major animal groups emerge in the fossil record.

The creature was a member of the radiodonts, a type of early arthropod—essentially creepy crawlies with jointed limbs, and at 3-feet in length, were actually the apex predators of their time, equivalent to the great white shark.

Despite Stanleycaris‘ bizarre appearance, it is the contents of its head that has scientists most excited.

A thinking man’s shrimp

The new specimens provide a glimpse into what the ancestral nervous system looked like. Finding any fossilized soft tissue is rare, and this is very unique.

In 84 out of 268 individuals unearthed at Burgess Shale, a prehistoric graveyard in the Canadian Rocky Mountains, the brain and nerves are still preserved—even after 506 million years, and an analysis of their makeup was published in Current Biology.

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Most fossils are bone and or hard body parts such as teeth or exoskeletons turned into mineral casts, while brains and nerves are made of fatty like substances that normally don’t survive.

“We can even make out visual processing centers serving the large eyes and traces of nerves entering the appendages,” said lead author Joseph Moysiuk, a University of Toronto PhD candidate. “The fine details are so clear it is as if we were looking at an animal that died yesterday.”

The central nervous system coordinates all neural and motor functions. In vertebrates, it consists of the brain and spinal cord.

In arthropods the brain is more condensed with a chain-like series of interconnected masses of nervous tissue resembling a string of beads. Stanleycaris‘ brain was composed of two segments, the protocerebrum and deutocerebrum.

They connected with the eyes and frontal claws respectively, and control vision and antenna signals in arthropods today.

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“We conclude a two segmented head and brain has deep roots in the arthropod lineage,” said Moysiuk, based at the Royal Ontario Museum. “Its evolution likely preceded the three segmented brain that characterizes all living members of this diverse animal phylum.”

In modern arthropods, such as grasshoppers and other insects, the brain also has a tritocerebrum.

Repeated copies of many arthropod organs can be found in their segmented bodies. Working out how they line up is key to understanding diversification.

“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.

Bottom dweller’s nightmare

In addition to a pair of stalked eyes, Stanleycaris had a big central peeper at the front of its head; a feature never before seen in a radiodont.

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

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“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.”

During the Cambrian, radiodonts were among the biggest animals. Anomalocaris, dubbed the ‘weird wonder,’ was at least 3ft 3in, making it a veritable sea monster. Radiodont means ‘radiating teeth’. The unusual animals were named after their toothy, circular jaws. They were adapted to the dim light of deep water.

At no more than eight inches, Stanleycaris was much smaller. But it would have been an impressive killer at least three times the size of most rivals.

“With large compound eyes, a formidable looking circular mouth lined with teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with a series of swimming flaps along its sides, Stanleycaris would have been the stuff of nightmares for any small bottom dweller unfortunate enough to cross its path,” said Moysiuk.

Most were collected in the 1980s and 1990s and now sit  in an extensive collection of fossils from Burgess Shale—a World Heritage site—housed at Royal Ontario Museum.

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