Why did the oval shield replace the scutum?

[Sean Manning]

I agree that there are a lot of things we don't know about classical Greek phalanxes ... but Xenophon seemed to believe that a Greek phalanx of doruphoroi, an Egyptian phalanx with body shields and very long spears, and an imaginary Persian phalanx with sagareis and mikra gerra differed more in details than in essence. I take his judgement in res militares quite seriously.

I think of all the changes in Roman military equipment over the ages, this one has intrigued me the most, probably because it affects directly my 'period-of-choice', the Third Century.

What do we know of the development of vertical grip shields anyways? Connolly shows surviving La Tene period scuta as flat with a horizontal grip. I have a vague idea that flat vertical grip shields are 'Germanic' but I would like to learn more about when and where they appeared.