The fossil record has been rewritten — in 150-million-year-old squid ink. The discovery of the perfectly preserved ink sac of a the inch-long cephalopod, a type of squid, has astonished palaeontologists.

The squid came from a site near Christian Malford in Wiltshire that is reknowned for producing extremely well-preserved fossils, but this is believed to the first time squid ink has been reconstituted.

To mark the occasion the scientists used the squid’s own ink to draw a picture of it and wrote the specimen’s Latin name, Belemnotheutis antiquus. Before it could be used, the pitch-black ink had to be returned to liquid form with a solution of ammonia.

The amazing preservation was the result of what palaeontologists call the Medusa effect - an unusually fast process of fossilisation — as the creatures turned to stone so quickly.

The dig was conducted by the British Geological Survey in an attempt to identify a lost site that produced vast quantities of well-preserved fossils during the Victorian era.

Dr Phil Wilby, who led the dig, said: “It’s among the world’s best fossil preservation. It’s a squid-like creature, but it’s not like anything we have in the world today.

“You really don’t imagine anything so soft could be so well preserved three dimensionally.

“It still looks as if it is modern squid ink. It’s absolutely incredible to find something like this. We felt that drawing the animal with it would be the ultimate self-portrait.

“It’s very valuable material so we won’t be using up any more of it now we’ve done the first test.”

Alongside the ink sac, palaeontologists also found other examples of the squid, and other creatures with all their soft parts preserved.

Dr Wilby said: “About 155 million years ago, millions of these animals were dying in this precise area. We don’t know why that is. In normal circumstances, the decomposition process means only the hard parts of animal are preserved, such as the bones, shell and teeth. The odds of this find are easily a billion to one and probably much greater.

“We call it the Medusa effect: specimens turn to stone within a matter of days, before the soft parts can be eaten away.

“I hope the discovery will help us better understand why things are fossilised in this way - what it is about the area that allows it to happen so quickly. Throughout the world there are perhaps a few dozen examples of soft parts being preserved, but this is really special.

“I can dissect them as if they are living animals. You can even tell whether it was a fast or slow swimmer, by looking at all the muscle fibres.”

The V site was discovered in the 1840s during the construction of the Great Western Railway but its exact location had been forgotten.

Dr Wilby said: “We were trying to find the site of a dig which took place in the 1840s, where we knew fossils were found with their soft parts preserved. We had the name of the village and knew it was next to the Victorian Great Western Railway.

“When we reached the site, we drilled trial holes into the ground until we spotted one of the fossils coming out with the end of the drill.

“We only had ten days to work in and we excavated 240 tonnes of rock.

“In total we found a couple of dozen specimens with soft parts such as muscles and eyes intact, but this was even more exciting.”

The specimen is now in the British Geological Survey collection in Nottingham. Part of the ink sac has been sent to Yale University in New Haven, Connecticut, for more detailed chemical analysis.

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