Roman Shieldwall or testudo?

[Robert Vermaat]

With the gradual changes in the Roman military during the 3rd century from rectangular to oval shields would this change also necessitated the introduction of a shieldwall tactic for defensive and offensive battlefield operations rather than the testudo or tortoise formation, especially against missile attacks from the enemy?

Your help and views much appreciated.

The testudo was not exactly a battlefield formation but more of use in sieges.
However, you can read more about the fulcum in this excellent article by Philip Rance (2004)