segmentata

[Robert Vermaat]

Vegetius, Book I, 20. "So they petitioned the Emperor that they should hand if first the cataphracts, then helmets. Thus with their chest and heads unprotected..." From N. P. Milner, 1996.
Now if this to be taken literally, and if I understand Matt's post correctly, then this could be taken as evidence that they went from segmatata to nothing.
On the other hand, doing without armor always seemed a bit weird to me.

These days, we read this as a political statement and not one reflecting reality. To the contrary, we know that armour was worn as usual during the 4th c. and later:
Beltrán Fortes, José and Adolfo Raul Menéndez Arguín (1999): New Evidence on the Use of Armour by Roman Soldiers of the Fourth Century AD, in: Journal of Roman Military Equipment Studies 10, pp. 21-26.
Coulston, J.C.N. (1990): Later Roman armour, 3rd-6th centuries A.D., in: Journal of Roman Military Equipment Studies 1, pp. 139-60.
Mielczarek, Mariusz (1993): Cataphracti and Clibanarii, Studies on the Heavy Armoured Cavalry of the Ancient World, Oficyna Naukawa, Lódz.

It's one of the reasons to read Vegetius with caution.