Why did the oval shield replace the scutum?

[Nathan Ross]

It was probably due to a change in tactics. This thread here discusses the potential difficulties in effectively using the longer spatha with the traditional curved rectangular scutum. The traditional fighting style had the legionary holding the shield close to the body, using the short gladius to stab out beyond the rim at close quarters. The change to the oval, and the longer spatha and spear which happened at around the same time, suggests a more open style of fighting, with slashing blows and the enemy kept at a greater distance (more useful against the increasingly mounted enemies of the time, perhaps). The oval shield would allow a freer use of the long sword or spear.

Byron notes in the thread above that he feels exposed when using a flat oval scutum - perhaps the dished oval was developed to try and combine the versatility of the oval shield with the body-protection of the curved rectangle? The soldier could still, in effect, shelter in the hollow of the shield when required.

I wonder also about the comparative weights of these different shields. Armies were increasingly mobile from the late 2nd century on, with vexillations travelling great distances away from home base. Perhaps initially the auxiliary-style oval shield was just lighter to carry on the march? (although this could be falling into the 'lazy late Romans' trope!)