A “BROWN pebble” spotted by a fossil hunter in Sussex more than a decade ago has been confirmed as the first known example of a petrified dinosaur brain.

The specimen is thought to have come from a large plant eater such as Iguanodon, which lived about 133 million years ago.

Scientists believe the dead dinosaur’s head was buried in mud at the bottom of a swamp, allowing its brain to be “pickled” and preserved.

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In time the soft tissues became mineralised, but the fossil retained distinctive features such as the meninges – a protective membrane surrounding the brain – blood vessels, collagen and structures thought to represent the outer layer of nerve cells, or cortex. A detailed study of the “pebble” has revealed similarities with the brains of present-day birds and crocodiles, both close relatives of dinosaurs.

Dr Alex Liu, from Cambridge University’s Department of Earth Sciences, who took part in the analysis, said: “The chances of preserving brain tissue are incredibly small, so the discovery of this specimen is astonishing.”

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The find’s importance was first recognised by the late Professor Martin Brasier, from Oxford University., who co-led the research prior to his death in a road accident in 2014.