The Abandonment of the Gladius for the Spatha - Why?

[Thiudareiks Flavius]

Sounds to me as if the sword wasn´t the primary arms in the formations any longer. Maybe The oval shield is a suggestion that the spear is the offensive weapon and the spatha is more of what they call an armingsword in Medival classification. A secundary tool to use when formation is broken or you find yourself in more of a man to man type of combat as in a skirmish or duell.

This idea might have some legs. Given that the late Roman Army did still use versions of or developments from the old pila, but were also equipped with spears in a way the old legionaries weren't, the idea of the spear being the primary weapon, with the angon, plumbata etc used for initial attacks and the spatha as a back-up weapon makes some degree of sense.

Could this be a response to cavalry-heavy Sassanid armies in the Third and Fourth Centuries?

Now I think about it, I'm not certain that all late Roman soldiers did use spears. Perhaps one of our Late Roman experts can help us on this point.

The more I think about it, the more all these changes could have been a response to a need to do things differently against the Sassanids following several humiliating reverses and increased pressure on the eastern frontier in the Third and Fourth Centuries.

Thoughts?