The bronze statue once stood on top of a giant triumphal arch in the ancient Moroccan city of Volubilis, in the south-west corner of the Roman Empire, 1500 miles from Scotland.
It depicted the Emperor Caracalla – the self-styled conqueror of the Caledonians – riding a six-horse chariot. The statue, erected 1800 years ago, was destroyed centuries ago, and only a three-foot-long fragment of the emperor's cape remains in a museum in Rabat.
Remarkably, the surviving bronze includes the image of a captive Caledonian warrior – wearing tartan trews.
Dr Fraser Hunter, of the National Museum of Scotland, yesterday identified the carving – inlaid with bronze and silver to give texture to the Scottish weave – as the "first-ever depiction of tartan".
He said: "The triumphal arch was built to celebrate Caracalla. He and his father, the Emperor Septimius Severus, had led massive military campaigns into third-century Scotland.
"A great bronze statue of Caracalla riding a six-horse chariot once stood above the arch. His cape includes an early depiction of that great national stereotype, the long-haired Caledonian warrior. The giveaway is the checked leggings – the first-ever depiction of tartan. It has been carved into the bronze, and inlaid with different bronze alloys and silver to give a remarkable impression of the textile, its colour and texture.
"The leggings or trews are relatively skin-tight and you can see the definition. The legs are two different patterns. This is the pre-history of tartan as we know it. The shields too are Celtic in style.
"This guy is a Caledonian – you can see his bare chest, his head and his cloak over the shoulders. But his arms are bound behind his back. This guy is a captive, a prisoner from the vicious campaigns of Severus and Caracalla."
Dr Hunter, who has spent 20 years studying Roman artefacts, describes the find in a BBC2 documentary to be shown on Friday.
Speaking yesterday, he said few earlier Roman images of Caledonians existed – stone carvings from the Antonine Wall, around 70 years earlier, show Romans soldiers on horseback slaughtering the local tribes. But those stone carvings showed the Scots naked.
Dr Hunter said: "We don't often see Caledonians in Roman art. The earliest surviving come from the Antonine Wall but these stone carvings do not have the same level of detail or colour and the Caledonians are shown naked.
"That's why this depiction is so exciting. There is not much left of the Caracalla statue, but from a Scottish perspective this is a fantastic find."
Dr Hunter said the fragment could be loaned to Scotland at some point in the future. He said: "We've had a chat about it being loaned to Scotland. If we could put together a decent proposal it is something they might be open to. It would be nice to show it in Scotland."
Unfortunately, the Scot in the tartan trews is likely to have met a grizzly end – possibly in an amphitheatre with wild animals.
Dr Hunter said: "The Romans had no real sense of 'prisoners of war'. Some of these men would have been force marched for months on end to all parts of the empire, living trophies of the Emperor's success. Some might have been traded as slaves, others were even less fortunate.
"A mosaic from Tunisia shows how one unfortunate Caledonian met his end, sent to the amphitheatre, killed by wild animals as exotic entertainment for the locals."
He added: "We've long had a curious, rather cuddly relationship with the Romans – hot baths and straight roads. In the western world we often see ourselves as inheritors of Roman values and culture. But this evidence from North Africa reminds us the Romans were invaders, colonisers. Their strategies encompassed everything up to and including genocide.
"For the local tribes, the Roman arrival in what we call Scotland must have been absolutely terrifying."
l Scotland: Rome's Final Frontier – BBC2 Scotland on Friday at 9pm.