When would the rank of Praefectus be used in the Late Roman Army

[Tim Hare]

the rank structure of the late roman army (284 – 476 AD)

Hi Tim!

This is a huge subject, and one which has been discussed a great deal by historians over the years without too many firm conclusions. I'll try and give you some general ideas here though...

First, you may find these two web articles useful:

Hi Nathan,

Thank you for the above links, and I can only apologize it’s has taken me so long to respond.

Funnily enough I did come across these 2 pages after posting this thread, but this helps regardless.

This novel will largely revolve around a cohort (and its officers) of Limitanei based at the Romano-British fortress of Arbeia in 376 AD... What I’m currently unsure about is what specific rank of officer would command a garrison of a standard auxiliary fortress of this size.

Luckily there is a firm answer to this question at least. The Notitia Dignitatum (a list of offices and military units dated to some time between AD395 and AD420ish, probably drawing on earlier material too) has a section on the command of the Dux Britanniarum, which notes that one of his officers is the Praefectus numeri barcariorum Tigrisiensium at Arbeia.

So the garrison in the late 4th century was the Numerus Barcariorum Tigrisiensium, and they were commanded by a Praefectus. You might be interested in this article about the Barcarii written by Francis Hagan.

However, a numerus was (probably) not the same thing as an old-style cohort. The word was used to describe all kinds of late Roman units, from cavalry squadrons to legions, and seems to have connoted no set number of men. If you can bear to pick through it, the thread on Late Roman Unit Sizes will give you an idea of the range of theories on this.

There are other limitanei units listed in the Notitia that are called cohorts though, and in some cases these occupy the same garrisons known in earlier times. Unlike the old auxiliary cohorts, most are commanded by tribunes. However (nothing about the late Roman army is easy!) some of these 'cohorts' occupy forts far too small for c.600 men; in fact, some of the new fortifications on the Danube, home to 'cohorts' in the Notitia, would only hold between 60 and 120 men.

Yes I’d read up a little on the Numerus Barcariorum Tigrisiensium. I understand its thought this may have been some kind of naval unit whose origins were in the Tigris River region of the far east.

My understanding is that Numerus as a unit may have been a detachment of what the romans would consider ‘barbarians’ from allied tribes beyond the frontier who were integrated into the roman army, not unlike Foederati. At least that was the case during the earlier Imperial Period. (Before the 4th Century)

I also understood that it’s believed the roman units stationed in the North and West of Britannia were withdrawn around 383 by Magnus Maximus as part of an agreement with previously hostile tribes to support his ambitions in Gaul and Italy. If the ND dates from around 420 AD in the Western Empire, do you think its possible some of the garrisons being overtaken by ‘indigenous’ troops could have mainlined their original roman names, but whose designations changed? (e.g. a Cohort is down grounded to a Numerus, but keeps its original name – Or am I giving the Late Roman Army to much credit for its organizational skills and general ability to keep things consistent?)

Also, you wouldn’t happen to know if there is a list anywhere on the Internet of all roman army units listed in the ND and which specific regions they fell under and where they were garrisoned. (Something like the link you provided for Britannia, but for the whole empire)

Just thought it may be worth asking.

the maximum sized ‘permanent’ unit of the Late Roman Army would have been a Cohort, 600 men strong for the Limitanei... i understand now that a legion of the Late Roman Army typically consisted of a minimum of 2 Cohorts

There is a widely-held theory that the smaller 'mini legion' (c.1000 men) of the late empire was formed originally from the two-cohort vexillations (detachments) used in field armies of the tetrarchic era. This might be the case.

However, the evidence of an almost-complete legion rank listing from Perge, dated to c.AD500, suggests that late Roman legions had a decimalised rank structure, with (in that case) 20 centurion grade officers, rather than the 12 or so we would expect if the unit was made of two old cohorts.

My assumption that a legion could number 2,000 men or over came from the historical novel ‘Legionary’ by Gordon Doherty. (It was a good book overall, but I think he takes a few liberties with history but overall, it’s no Gladiator from a historical accuracy standpoint)

A small author’s note at the beginning states that Late Roman Army legions typically numbered about 2,000 men split between 3 Cohorts (the first double strength), all of which were likely to be under strength. Despite this, I’ve not been able to come across any other consensus that give a Late Roman Army a strength of over 1,000 men on paper.

All ways seemed strange to me that a Late Roman Army legion on paper was almost exactly the same in size as the first Cohort of an earlier legion. This made me wonder if when most of these new legions were raised as or reformed as ‘mobile’ units the Roman government could only afford to raise and equip the first Cohort. Or if on the other hand the Late Roman Army was so pressed for manpower, then any subsequent Cohorts raised alongside the First Cohort were stripped for garrison duty while the first Cohort was transported to wherever it was needed.

My understanding is that the Cohort occupying Arbeia would be under the command of a Senior Centurion (Primus Pilus)

The Primus Pilus vanished some time in the later 3rd century (or rather, became a civilian job associated with tax gathering, I think).

Oh Bugger!

Thank-you! ?

And again thankyou for taking the time to respond to me in the first place.

The feedback you’ve given here is invaluable!

Tim