One of the oldest pieces of British pornographic art has just been discovered beside the river Thames. At first sight, the bronze disc found near Putney Bridge in London looks like an old coin – until you notice that it depicts a sex scene.

This type of bronze token with its erotic imagery was specially made to spend in ancient Roman brothels. The example found near Putney Bridge and given to the Museum of London is evidence that brothels in Roman Londinium were just as busy as they were in ancient Pompeii, where brothels and their lewd wall paintings are among the well-preserved everyday shops of a Roman town.

Yet this is not just a hint of life in Roman Britain. It is also a glimpse of a hidden art history. These Roman tokens, with their detailed depictions of sex acts, had a dramatic influence on the birth of modern pornography. While the Putney token has been hailed as a rare discovery from Roman Britain, such artefacts showing similar scenes were actually well known in Renaissance Italy. Scholars in the 16th century didn't know what they were – maybe something to do with the reputed excesses of the emperor Tiberius? – but they did leap on evidence of ancient Roman erotic art. Anything from antiquity was considered noble in the Renaissance, so these "coins" (as they were misnamed) licensed saucy 16th-century art, including Giulio Romano's famous series of pornographic illustrations I Modi.

It's easy to see how these classical erotic images by Romano, engraved by Marcantonio Raimondi, emulate the images on tokens like the one from Roman London. In turn I Modi, in its printed form with pornographic poems added, became a bestseller all over Europe and returned to the London of Shakespeare. It set the style for a new erotic art.

As for Roman Britain, those invaders from the shores of the Mediterranean probably needed every reminder of home they could get. I spent an afternoon in the Christmas holidays looking at the ruins of a Roman bath in north Wales. Like the brothel token from Londinium, it shows how the Romans recreated the same way of life everywhere they went: here, Romans could sit in a heated bathhouse in the middle of what to them must have seemed an incredibly cold and bleak Welsh wilderness, and feel the warmth of the Mediterranean for a moment. The site hereabouts has only been partly excavated. Who knows – perhaps the bathers in wild Wales clutched brothel tokens of their own.