Late Roman Tribune

[Robert Vermaat]

But the 'Verona' frieze actually shows the officer - or perhaps centurion - and a senior commander to the left of the troops. Both of them are wearing the muscle cuirass.
However, I don't doubt that on occasion some Roman troops of any era fought without armour, and the river battle you mentioned would seem a good anough excuse.

Having had a second look at the 'siege of Verona', I have doubts that the common soldiers are not wearing armour. A case could be made for some wearing a hamata, while the plaiting of some tunics looks very much like the pteryges of those we see wearing a musculata curass.
Anyway, Nathan is right - the commanders do seem to wear armour, which is in conflict with the theory proposed below, that the tribune would do away with all armour like the common soldier. I have yet to see any proof of that for any Roman battle.

During the river battle I doubt they would wear their helmets though.. :-)