A gilded bronze leg found in a Scottish bog more than 200 years ago was part of a monumental equestrian statue of the Roman emperor Antoninus Pius, new research has concluded, rewriting the picture of Rome's presence in Scotland. The fragment will be a highlight of a new exhibition at the National Museum of Scotland in Edinburgh, opening on 14 November.

The lower leg and foot, dating to about 140 CE, were dug up at Milsington in 1820. For decades it was assumed to be loot carried north from a site further south. But a study of sculpture fragments from Roman Scotland has shown that imperial statues were a feature of frontier forts, making a local origin far more likely. New analysis also shows the limb was carefully detached from its parent statue, suggesting the monument was deliberately taken apart rather than smashed.

Specialists now believe the statue showed Antoninus Pius on horseback. Pius led Rome's second major military campaign into Scotland, under whom Roman forces built the turf Antonine Wall across the Central Belt; attacks by Caledonian tribes forced a retreat south to Hadrian's Wall within a generation. Fraser Hunter, principal curator of the exhibition "Roman Scotland: Life on the Edge of Empire", said the fragment offers a glimpse of the grand monuments that once stood in southern Scotland and of the importance Pius placed on his victories there, while raising the question of where the rest of the statue is.

The exhibition runs at the National Museum of Scotland from 14 November to 18 April. The leg will be shown alongside other material from Rome's northern frontier, in what the museum presents as the largest exhibition ever staged on Roman Scotland.