Alan Bowman finds shadowy caesar in Oxfordshire Mud

There are some periods of history on which the serendipitous archaeological discovery of a tiny object can cast a narrow but intense beam of illumination into a dark corner where the absence of a narrative history by a historian such as Tacitus or Livy leaves us largely blind.

The Roman Empire of the mid-3rd century AD is such a period. It was marked by military conflict, barbarian invasion and political and economic instability, during which a succession of "soldier-emperors" and usurpers laid claim to imperial power.

And while the absence of a good historical source is bad, the presence of a bad one is worse. Those exploring this period encounter a fraudulent biographer who wrote in the AD390s but falsely presented himself as six individually named writers of the time of Constantine. His so-called Historia Augusta interweave fact, fantasy and invention to impress and to deceive. So, where the written history may or may not be trustworthy, only the "hard" evidence of a contemporary document or artefact can help us decide what is likely to be true.

A fascinating new exhibition at the Ashmolean Museum presents, in its context, just such a piece of rare evidence - a single coin found in Oxfordshire. We know that, in the period AD260-4, the northwestern Roman provinces - Gaul, the Rhineland, Britain and Spain - seceded to become an "independent" Gallic Empire under Postumus and a series of other shortlived successors. What we did not previously know for certain was whether a shadowy figure called Domitianus was among those men.

But there is his portrait, with breastplate and crown, on the newly discovered coin, along with the inscription "Imperator Caesar Domitianus Pius Felix Augustus". Our fraudulent biographer tells us only that a military officer named Domitianus was executed by the "official" emperor Aurelian for treason in the AD0s. We also have another coin with his mark, found a century ago in central France and long regarded as a modern forgery.

Now we have a "British" emperor - Domitianus - who can have held power in Gaul and Britain only for a very short time, but long enough to proclaim himself, with a small coin issue at the mint of Trier, the ruler of this shortlived Gallic Empire.

So from Germany, at least one silver-washed bronze denarius of Domitianus somehow found its way to Oxfordshire, to be buried in a hoard of almost 5,000 coins. There it was found by BrianMalin with a metal detector, on farmland at Chalgrove, in April 2003.

This modestly sized but beautifully presented exhibition - just two glass cases and one poster - tells us the story of the find and its significance in establishing the fact of Domitianus's "reign", even if the stylised portraits tell us very little about the individual himself.

The statistics are mind-boggling: one coin found in a heaped mass of 5,000 buried in a jar (or almost 10,000 if you add another hoard found nearby on the same farm in 1989). Malin is one hero of the story - for his prompt and proper reaction in presenting the find to the museum. Another is Richard Abdy of the British Museum, who identified the unique coin and the legend.

All the coins have been beautifully cleaned and conserved. Even in that state, Domitianus's is not easy to decipher. But it is well worth seeing this little piece of history.

Alan Bowman is professor of ancient history at Oxford University.