Half a billion years ago, the ocean's creatures lived in fear of a three-eyed monster.
Gliding in the depths like a stingray, the animal scanned the murky water for its next meal, its spiny claws ready to snap up anything that veered too close.
This is Stanleycaris hirpex, a 20-centimetre-long distant relative of today's arthropods — the broad group that includes insects, spiders and crustaceans.
Now, Canadian palaeontologists have uncovered a huge stash of fossils that are so well preserved they show the ancient predator's brain and nerves.
The bizarre fossils offer evolutionary clues about how the arthropod brain, vision and body structure developed into what it is today.
The findings were published today in Current Biology.
"The details are really crisp and beautiful," said Joseph Moysiuk, lead author and PhD candidate in ecology and evolutionary biology at the University of Toronto, Canada.
Animal explosion
If you think the oceans are full of nightmarish creatures now, imagine what they were like during the Cambrian Period 541 million-485 million years ago.
It was a time when evolution was on steroids: simple creatures diversified into more complex forms, creating the blueprint for many of the major animal groups we know today.
Among the ocean's emerging oddities were radiodonts — an extinct group of arthropods.
These hellish prawn-like animals are famous for their rings of serrated teeth, prey-snatching claws and bulbous eyes.
They were also pretty big in Cambrian terms, with some growing up to 1 metre in length.
But in most cases, scientists have only found bits and pieces of radiodont fossils, which has made it tricky to study their physical characteristics in more detail.
"You might find a feeding appendage or a body flap that it uses for swimming just as isolated elements," said John Paterson, a palaeontologist at the University of New England, who was not involved in the study.
Hiding in plain sight
Mr Moysiuk first stumbled across the unstudied fossils in a collection at the Royal Ontario Museum in Toronto while he was working as an undergraduate volunteer.
The haul of 268 fossils had been originally unearthed in the 1980s and '90s from the Burgess Shale in the Canadian Rocky Mountains, a treasure trove for fossil-hunting palaeontologists.
"They had just kind of been hidden away in the collection drawers for decades," Mr Moysiuk said.
Mr Moysiuk was so blown away by the collection that he decided to study them for his PhD at the University of Toronto.
"The material just looked so obviously significant."
For a start, the fossils showed that Stanleycaris had three eyes: two sticking out on stalks and a giant one in the middle of its head.
This bizarre feature had never been seen before in other radiodonts, Mr Moysiuk said.
But the extra eye was a hint of things to come, with many modern arthropods sporting multiple eyes.
Just look at spiders. Most species have eight eyes, while a handful have four or six.
The main pair of eyes help insects and spiders form images of their surroundings, while the smaller additional eyes help orient them in their environment, Mr Moysiuk said.
"If you're a predator, for example, it's really important that you have good control over your position in the environment.
"So, if I was to speculate on what was going on with Stanleycaris, I'd say maybe we had something similar going on there."
The brains of a killer
Dozens of the specimens were whole-bodied, showing that Stanleycaris was roughly 20 centimetres long, dwarfing most other ocean creatures at the time.
Some 84 of the fossils offered a detailed snapshot of Stanleycaris's 506-million-year-old brain and nerves.
"Once you get these things under the microscope, you see all of the details just popping out right out at you," Mr Moysiuk said.
Under the microscope, Mr Moysiuk and his team found that Stanleycaris's brain was made up of two segments — the protocerebrum and deutocerebrum.
The researchers could see that the deutocerebrum at the back of the brain contained bundles of nerves that connected to the creature's claws and eyes.
The ancient creature's brain structure, and the ways it connects to its various body parts, tells the story of how the modern-day arthropod brain — which is made up of three segments instead of two — evolved over time, Professor Paterson said.
"It's very rare that you see brain structures preserved, so it's quite frustrating for palaeontologists working on arthropods," he said.
"[The new fossils] give us this stepwise process of how certain anatomical traits came into being."
A trailblazing body
Another characteristic that put Stanleycaris ahead of its time was its segmented body — a classic trait of modern arthropods.
Before anyone took a closer look at these fossils, no-one knew if radiodont bodies were made up of segments, Professor Paterson said.
"We've always had a bit of a question mark," he said.
"But they've shown in their fossils that it does in fact appear to be properly segmented, which makes them more modern than anything else, even though they're very weird-looking creatures."
Still, there's more work to do on the fossils. The next step for Mr Moysiuk and his team is to build a better understanding of how Stanleycaris developed.
The massive collection of fossils has specimens ranging from 1 centimetre to tens of centimetres long, which could offer clues about how Stanleycaris grew up to become a formidable predator.
"We're starting to get a sense of how these animals were developing," Mr Moysiuk said.
"This is really important for understanding the evolution of development in arthropods."
