Why did the oval shield replace the scutum?

[Robert Vermaat]

However, what about the phalanx, which was formed occasionally?

'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.

Personally I think that the oval (dished) scutum facilitates the use of the hasta, in this case identified as a ling (7-9ft) thrusting spear, which was developed during the 3rd c. against enemy cavalry (after we hear troops complaining about the pilum being too short). The hasta (also named contus) is probably couched under the armpit, allowing very little of the body to be exposed, as seen here performed in 2008 (LRE III):

[attachment=2057]shieldwall.jpg[/attachment]