Question about the Roman \'wedge formation\'

[Nathan Ross]

Aulus Gellius (10 9), and Vegetius (3 17) mention the serra formation (saw).

They do - but the description given in Vegetius (quoted above) is quite different to the 'sawtooth' formation portrayed in the computer graphic reconstruction of the battle mentioned in the OP. The Roman serra was, as we've said, apparently a defensive formation or a way of reinforcing a line.

Gellius, in the extract you cite (10.9), actually lists the formations of the Roman army: "the front, reserves, wedge, ring, mass, shears, saw, wings, towers." ("frons", "subsidia", "cuneus", "orbis", "globus", "forfices", "serra", "alae", "turres"). The 'shears' or forfices'/forceps was the open V shape designed to counter the cuneus.

No mention of anything that resembled a toothed saw-edge.