The Abandonment of the Gladius for the Spatha - Why?

[Robert Vermaat]

Is it small wonder that the shape of the more common scutum used by later Roman legions is also oval or round, and that segmentata gradually fell out of use, superceded by hamata, squamata and plumata? Also the neckguards on helmets reduced in size (see Lendon's theory, that the larger neckguard is more ideal for deflecting missiles from above in siegework construction) and the spangenhelm, a "barbarian" design, became more dominant. I believe that what we see in the later Roman legionary is a more advanced version of an earlier auxilliary.

This is also Wheeler's position. He sees the development of the late 2nd c. and the 3rd c. towards hamata, oval scutum (but he says flat), hasta and spatha as part of a drive to standardise equipment. he also notices that the big neck guards of the early 3rd c. made crouching nearly impossible anyway, equipment looked as if it had to fit standing troops in close order formation.