Diocletian: legions and rank structure

[Nathan Ross]

I'm surprised at the lack of prefectures... filled by jumped-up centurions

Yes, it does seem strange at first, although when we look at the Notitia Dignitatum we find old-style legions all commanded by praefecti and 'cohorts' (either what were originally auxiliary cohorts or bits of older legions) commanded by tribunes. The few careers inscriptions from the 3rd/4th century seem to point to a straight advancement from centurion, via protector, to either tribune or prefect. Again, I'd really recommend Hepworth's 1963 thesis - it's dense, but covers this ground in far better detail than anything else I've read (inclusing Nicasie etc)

moving towards a somewhat merit based bottom to top promotion structure. Is that correct or am I completely off mark here?

It seems to be the case, yes. You could look at a man like Flavius Abinnaeus, a former cavalry decurion promoted to the protectores after many years service and then to praefectus of an old-style cavalry ala in Egypt; at the other end of the scale would be a man like Flavius Memorius (CIL 12, 00673), who started in the Joviani and rose to praepositus of lanciarii, then comes ripae and comes Mauretaniae Tingitanae - senior posts in the field army and provincial command. There's also Valerius Thiumpus (probably Diocletianic) who bypassed the protectores and became Prefect of II Herculia after a spell in the lanciarii of the imperial retinue. The importance of the comitatus - and proximity to the emperor(s) generally - is obvious.

Praepositus is an important role here too - in some ways, it seems to have expanded from its origins as an ad hoc command in the principiate to fill any number of middle leadership positions. The forces that Diocletian and Galerius led into Egypt in the 290s seem (going on papyrus evidence) to have been based on twinned legion detachments (one from each of the two legions of each Danubian province), each commanded by a praepositus. These praepositi, wherever their origins can be determined, appear to have been centurions. So praepositus was a definite step on the ladder of promotions - whether it was equal, or even virtually identical - to the old legionary tribune role is unclear.

For further reading, you might take a look at Ross Cowan's latest book in the Osprey series, Roman Legionary, AD284-337. Vol VI, Issue 5 of Ancient Warfare magazine also covers 'The Armies of Diocletian'.