Archaeology breakthrough: ‘One in a million find’ after fossil split

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In a world-first, researchers have discovered a 310-million-year-old fossilised brain from a horseshoe crab. While harder structures such as bone and shell can be preserved as fossils for hundreds of millions of years, fossil records of soft tissues are few and far between. The same applies for other softer parts of animals such – including organs and flesh.

This rare discovery reveals some ‘surprise’ about the horseshoe crab.

Conditions were reportedly ‘perfect’ at Mazon Creek in Illinois for the fossilisation of the ancient animal’s soft tissue.

Despite their name, horseshoe crabs are not actually crabs.

They are arachnids, more closely related to scorpions and spiders, and first evolved around 480million years ago.

A study last month in Geology reports that the fossilised brain is remarkably similar to those of modern horseshoe crabs.

It was the shock similarities that allowed researchers to identify the horseshoe crab.

Lead author Russell Bicknell told LiveScience: “This is the first and only evidence for a brain in a fossil horseshoe crab.”

He described the chances of such a discovery as “one in a million.”

“Although, even then,” he added, “chances are they are even rarer.”

Mr Bicknell explained soft tissue brains usually decay very quickly.

He said: “In order for them to be preserved, either very special geological conditions, or amber, are needed.”

Amber is the fossilised resin that oozes through the bark of a tree and traps a variety of organisms.

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These entombed organisms are often represented by arthropods, including insects and arachnids, as seen in the original Jurassic Park film.

In the case of this horseshoe crab, the geology of the Mazon Creek helped the brain stay in perfect condition.

The deposits as Mazon Creek are made of siderite, an iron carbonate mineral that forms concretions.

Concretions are mineral precipitations that can quickly entomb a dead body and fossilise it.

While these concretions preserved the crab’s body, the brain tissue disappeared after decomposing.

Mr Bicknell noted the fossil is not the brain itself, rather a mould of the brain left by a clay mineral called kaolinite.

While siderite is grey, the kaolinite is white in colour, thus the brain fossil “stood out more than it would have normally”, said Mr Bicknell.

He described the Mazon Creek deposit as “exceptional”.

He said: “If we started looking, we may be lucky enough to find more [brain fossils].”

Dr Greg Edgecombe also helped to describe the extraordinary new find.

He said: “Now we know that both of two main lineages of horseshoe crabs – the wholly extinct group that our fossil belongs to, and the group that includes the four living species – have the same basic neuroanatomy.”

Mr Bicknell added: “Despite 300 million years of evolution, the fossil horseshoe crab brain is pretty much the same as modern forms.”

The external anatomy, in contrast, has changed quite a bit during this time period.

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