In a breakthrough discovery, researchers have uncovered a never-before-seen fossilised brain from a 310-million-year old horseshoe crab.

The 'one in a million' discovery is said to reveal some 'surprises' about the evolution of these strange creatures, according to a new study.

The fossilised brain was discovered at Mazon Creek in Illinois where the conditions were reportedly perfect to preserve the ancient animal's delicate soft tissue.

Despite horseshoe crabs being fairly common, researchers previously knew nothing about their brains, Live Science reports.

"This is the first and only evidence for a brain in a fossil horseshoe crab," lead author Russell Bicknell, a paleontologist at the University of New England in Maine, told Live Science.

The chances of finding a fossilised brain are "one in a million," he added. "Although, even then, chances are they are even rarer."

There are four types of horseshoe crabs known to be alive today – all have similar hard exoskeletons, 10 legs and U-shaped head.

Weirdly, despite their name suggest, these creatures are actually arachnids and not "crabs". They are closer to scorpions and spiders, according to the National Wildlife Federation.

Bicknell explained that soft tissue brains are very prone to rapid decay. He added: "In order for them to be preserved, either very special geological conditions, or amber, are needed."

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In this rare case, the geology of the area helped the soft tissue to stay in perfect condition and preserve the brain – or a 'copy' of the brain.

"We have a mould of the brain, not the brain itself, so to speak," Bicknell explained.

As the brain rotted away over the years, it was replaced with a clay mineral caked kaolinite, which created a cast like mould of the brain.

Researchers are now hoping to find more ancient brains that could be deposited in the Mazon Creek that seems to provide perfect conditions for fossilisation of soft tissue.

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"The Mazon Creek deposit is exceptional," Bicknell said. "If we started looking, we may be lucky enough to find more [brain fossils]."

Analysis of the ancient horseshoe crab brain shocked researchers who thought they would see something wildly different to the modern day brain.

"Despite 300 million years of evolution, the fossil horseshoe crab brain is pretty much the same as modern forms," Bicknell said.

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