Why did they switch from shields that look like this: https://s-media-cache-ak0.pinimg.com/originals/77/75/43/777543de643c1bf5d9bfc9ead46b5b95.jpg
To shields that look like this: http://i.imgur.com/fiaAsKe.jpg
And when?
This might answer your question: https://www.reddit.com/r/AskHistorians/comments/yq8yw/why_did_the_roman_shield_design_change_from_a/
The post by /u/GBFel gives a fair bit of detail & a fair bit of discussion as well.
