Sword moving from right to left

[Nathan Ross]

Is the key to this perhaps vexillations that were in use almost constantly from the Marcomannic Wars onwards, forcing single or small groups of cohorts to be able to fend for themselves in a variety of situations, and carry out much more varied missions in different terrains than the traditional legionary?

Could be. I wondered here if the shield change was a weight thing - flatter oval shields were easier to carry long distances! But a more adaptable tactical role is probably a better way of looking at it.