When was the earliest known use of the testudo formation by the Romans?
For if [the legionaries] decided to lock shields for the purpose of avoiding the arrows by the closeness of their array, the [cataphracts] were upon them with a rush, striking down some, and at least scattering the others; and if they extended their rank to avoid this, they would be struck with the arrows.
Is there an earlier example of the testudo? The source does not have to mention the formation by the name "testudo" (as in the above quote). If the earliest mention of the formation does not include the name then when was it first mentioned by name?