An ancient gold coin proves that a third century Roman emperor written out of history as a fictional character really did exist, scientists say.
The coin bearing the name of Sponsian and his portrait was found more than 300 years ago in Transylvania, once a far-flung outpost of the Roman empire.
Believed to be a fake, it had been locked away in a museum cupboard.
Now scientists say scratch marks visible under a microscope prove that it was in circulation 2,000 years ago.
Prof Paul Pearson University College London, who led the research, told BBC News that he was astonished by the discovery.
"What we have found is an emperor. He was a figure thought to have been a fake and written off by the experts," he said.
"But we think he was real and that he had a role in history."
The coin at the centre of the story was among a small hoard discovered in 1713. It was thought to have been a genuine Roman coin until the mid-19th century, when experts suspected that they might have been produced by forgers of the time, because of their crude design.
The final blow came in 1863 when Henry Cohen, the leading coin expert of the time at the Bibliothèque Nationale de France, considered the problem for his great catalogue of Roman coins. He said that they were not only 'modern' fakes, but poorly made and "ridiculously imagined". Other specialists agreed and to this day Sponsian has been dismissed in scholarly catalogues.
But Prof Pearson suspected otherwise when he saw photographs of the coin while researching for a book about the history of the Roman empire. He could make out scratches on its surface that he thought might have been produced by the coin being in circulation.
He contacted the Hunterian Museum at Glasgow University where the coin had been kept locked away in a cupboard along with three others from the original hoard, and asked if he could work with the researchers there.
They examined all four coins under a powerful microscope and confirmed in the journal, PLOS 1, that there really were scratches, and the patterns were consistent with them being jingled around in purses.
A chemical analysis also showed that the coins had been buried in soil for hundreds of years, according to Jesper Ericsson, who is the museum's curator of coins and worked with Prof Pearson on the project.
The researchers now have to answer the question, who was Sponsian?
The researchers believe that he was a military commander who was forced to crown himself as emperor of the most distant and difficult to defend province of the Roman empire, called Dacia.
Archaeological studies have established that Dacia was cut off from the rest of the Roman empire in around 260 AD. There was a pandemic, civil war and the empire was fragmenting.
Surrounded by enemies and cut off from Rome, Sponsian likely assumed supreme command during a period of chaos and civil war, protecting the military and civilian population of Dacia until order was restored, and the province evacuated between 271 AD and 275 AD, according to Jesper Ericsson.
"Our interpretation is that he was in charge to maintain control of the military and of the civilian population because they were surrounded and completely cut off," he said. "In order to create a functioning economy in the province they decided to mint their own coins."
This theory would explain why the coins are unlike those from Rome.
"They may not have known who the actual emperor was because there was civil war," says Prof Pearson.
"But what they needed was a supreme military commander in the absence of real power from Rome. He took command at a period when command was needed."
Once the researchers had established that the coins were authentic, and that they had discovered what they believed to be a lost Roman emperor, they alerted researchers at the Brukenthal Museum in Sibiu in Transylvania, which also has a Sponsian coin. It was part of the bequest of Baron Samuel von Brukenthal, the Habsburg Governor of Transylvania. The Baron was studying the coin at the time of his death and the story goes that the last thing he did was to write a note saying "genuine".
The specialists at the Brukenthal museum had classified their coin as an historic fake, as had everyone else. But they changed their minds when they saw the UK research.
The discovery is of particular interest for the history of Transylvania and Romania, according to the interim manager of the Brukenthal National Museum, Alexandru Constantin Chituță.
"For the history of Transylvania and Romania in particular, but also for the history of Europe in general, if these results are accepted by the scientific community, they will mean the addition of another important historical figure in our history," he said.
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