Polybius\' description and the Fayum scutum

[Flavivs Aetivs]

The Roman Army did not just randomly switch from Combat to Testudo if missiles were incoming.

First of all, primary source accounts that missile fire in a battle was constant, second of all, if you watch re-enactment the rear ranks are pushing up against the front ranks to hold the line or advance. There was no way to form a Testudo if a "volley" of missiles came in.

Even in Rome: Total War if you put a unit of Legionnaires randomly into Testudo, they'll get slaughtered.

EDIT:
@Macedon

If Polybius and Arrian's description of the Marching Testudo is similar than it is likely the Testudo became the Foulkon (Fulcum).