Vexillatio Equitum, Equites, Numerus Equitum

[Robert Vermaat]

The papyrus is dated 26 May 267, so I am not sure if the new-style ranks would have come in by then.

I'm assuming that the new rank structure would have come into use at the time that the new units were raised, rather than being 'retrofitted' onto units which had previously used the traditional Roman cavalry rank system with decurions etc.
If so, this 'arithmos katafractos' was not a new-style vexillatione equitum, but an old-style cavalry ala or something similar.
However, I'm not sure of that! The earliest dated biarchus is from c324 (I think), and while a number of centenarii and circitores (particularly of numeri catafractariorum, eg CIL 13, 1848) may be earlier it doesn't seem possible to establish a dating.
Is there anything in the papyri evidence to link a decurion or similar old rank specifically to a new-style equites unit?

Several scholars (such as Rance) think that the new ranks were not 'retrofitted' into old-style units, at least not as a rule.

Nathan, you're way more into this than I am - do we have many 'new ranks' in old-style units? The old style units seem to continue for a long time, until all were either destroyed or reorganised into new-style ones?

My guess would be that the new-style ranks belonged to the new-style units, with some appaering in old-style units (due to transfers perhaps?) until these disappaerd.