The world’s oldest brain has been found in the remains of a three-eyed shrimp that swam in the oceans more than half a billion years ago.
The entire central nervous system is still visible, providing unprecedented insights into the ancestors of insects, spiders and crustaceans.
It was called Stanleycaris hirpex and was described as “the stuff of nightmares.” It had two eyes ‘on stems’ with a larger one in the middle – and pointed claws.
It lived during the Cambrian ‘explosion’ – a period of rapid evolution during which most major animal groups appear in the fossil record.
The creature was a member of the radiodonts, apex predators that were the “great white sharks” of their day.
Reaching more than three feet in length, they were a type of early arthropod — creepy crawlies with jointed limbs.
Despite the bizarre appearance of Stanleycaris, it is the contents of his head that most enthuse scientists.
The brains and nerves of 84 individuals excavated at Burgess Shale, a prehistoric graveyard in the Canadian Rocky Mountains, are still preserved — after 506 million years.
Lead author Joseph Moysiuk, a PhD student at the University of Toronto, said: ‘We can even discern visual processing centers that operate the large eyes and traces of nerves that enter the appendages.
“The fine details are so clear it’s like we’re looking at an animal that died yesterday.”
The new specimens provide a glimpse of what the ancestral nervous system was like. Finding fossilized soft tissue is rare, but this one is unique.
Most of the fossils we have are bones and/or hard body parts such as teeth or exoskeletons.
Brains and nerves are made of fatty substances that normally do not survive.
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 chain of beads.
The brain of Stanleycaris consisted of two segments, the protocerebrum and deutocerebrum.
They were connected to the eyes and front claws, respectively – and today control vision and antenna signals in arthropods.
Mr Moysiuk, based at the Royal Ontario Museum, added: ‘We conclude that a two-segmented head and brain have deep roots in the arthropod lineage.
“Its evolution probably preceded the three segmented brains characteristic of all living members of this diverse animal stock.”
In modern arthropods – such as grasshoppers and other insects – the brain also has a tritocerebrum.
It connects to the labrum, a movable upper lip, and integrates sensory information from the other two lobes of the brain. An extra compartment has far-reaching consequences.
Repeated copies of many arthropod organs are found in their segmented bodies. Figuring out how they align is key to understanding diversification.
Mr Moysiuk said: ‘These fossils are like a Rosetta stone and help link traits in radiodonts and other early fossil arthropods to their counterparts in surviving groups.’
In addition to a pair of stalked eyes, Stanleycaris had a large central voyeur at the front of his head — a feature never seen before in a radiodont.
Co-author Professor Jean-Bernard Caron, Mr Moysiuk’s supervisor, said: ‘The presence of a huge third eye in Stanleycaris was unexpected.
“It highlights that these animals looked even more bizarre than we thought. It also shows us that the earliest arthropods had already developed a variety of complex visual systems, like many of their modern relatives.’
He added: “Since most radiodonts are only known from scattered bits and pieces, this discovery is a crucial step forward in understanding what they looked like and how they lived.”
During the Cambrian, radiodonts were among the largest animals. Anomalocaris, also known as the ‘weird wonder’, was at least 3ft 3in – making it a sea monster.
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.
Radiodont means ‘radiating teeth’. The unusual animals are named for their broad, round jaws. They were adapted to the dim light of deep water.
Stanleycaris’ advanced sensory and nervous systems would have made it possible to efficiently sort out small prey at dusk.
Mr Moysiuk added: ‘With large compound eyes, a formidable-looking round mouth lined with teeth, frontal claws with an impressive array of spines and a flexible, segmented body with a series of swim flaps along the sides, Stanleycaris would have been the stuff of nightmares for any little bottom dweller unlucky enough to cross his path.”
The research in Current Biology is based on an analysis of a previously unpublished collection of 268 specimens of Stanleycaris.
Most were collected in layers above the famous Walcott Quarry in Yoho National Park, British Columbia in the 1980s and 1990s.
They are part of Burgess Shale’s extensive collection of fossils – a World Heritage Site – housed in the Royal Ontario Museum.
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