Why did the oval shield replace the scutum?

[Sean Manning]

'The phalanx' should not be confused with the classical Greek phalanx, but is better described as a single formation with a closed uninterrupted front of several units lined up next to each other, preferably 8 to 16 deep (a phalangial formation) in contrast to a manipular formation. Late Roman formations could be 2 to 4 lines, depending on the number of troops. There are similarities with the classical phalanx, but also big differences - the LR formation looks far more like a shield wall.

How is that different from the Greek and barbarian phalanxes describes by, for example, Xenophon? There seems to be this widespread assumption that Greek phalanxes were fundamentally different from everyone else's, but I haven't seen a lot of evidence (except the details which are different because of different types of spear and shield and body armour and level of drill).

After experimenting with large (80-100 cm diameter), round, flat, centergrip shields Hammaborg in Germany concluded that they are better adapted for single combat (leading with the edge opposite the hand) than group combat (leading with the flat) and that forming in close formation with shields overlapping was a way to work around this. The old Augustan scutum was very defensive but it had very short reach unless you struck with the bottom edge, leaving your body exposed below the shield.