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

[Nathan Ross]

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:

Hierarchy in the Late Roman Army, 300-550AD, by Robert Vermaat of Fectio.

The Roman Army of Constantine, Part 2, which focusses on rank structures and changes.

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.

Also, some of internal buildings of the Hadrian's Wall forts seem to have been redesigned in the 4th century to accommodate a smaller garrison.

We might guess, then, that the numerus at Arbeia resembled an old-style cohort, but was perhaps rather smaller. The 'praefect' in command might have resembled an old-style cohort commander, but could have been a promoted centurion-level officer (see below for ranks!), or even a local leader given a military command (take a look at Rob Collins's Hadrian's Wall and the End of Empire for some discussion on these kinds of changes).

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.

This long thread about the 'Legion of Perge' might prove interesting. The inscription is contentious, but does at least give a full list of ranks, which would probably have been in use in the 4th-5th centuries as well.

As a rough precis, these are the ranks I would expect for a limitanei unit c.AD400:

Tribune/Praefect/Praepositus - this would be the unit commander. What the differences might have been between them is obscure, but generally all field army units and most frontier 'cohorts' are commanded by tribunes, while frontier cavalry units and legions, and some 'numeri', are led by Prefects. Praepositi could lead either, perhaps temporarily.

Ordinarius - this was the centurion-grade officer of the late legions and limitanei. They may have commanded subunits ('ordines') of c.60-120 men, which may in turn have been called 'cohorts' (the late writer Vegetius claims that 'cohorts' had draco standards, like the old centuries, which might suggest that the old century had been enlarged and renamed a cohort. That might suggest that the 'mini legion' of c.1000-1200 men was divided into ten 'mini cohorts' of c.100-120 men, each led by an ordinarius - but this is all very hypothetical!)

Vicarius - seems to have acted as the deputy to the tribune. Perhaps also called Tribunus minor. Maybe a temporary rank?

Campidoctor - originally a drill master, by the later empire this guy had become the most senior soldier in the unit. Whether he was one of the Ordinarii, or a separate rank, is unknown - the Perge tablets mention campidoctores, but they aren't listed on the grades, which might suggest the former...

Augustales and Flaviales - these are senior soldier or NCO grades, and appear in both legions and limitanei units.

Armaturae/Semissales - senior soldier grades, of various sorts.

Besides these men, there was the Optio (who functioned more like a subunit clerk at this date), the signifer (standard bearer, aka draconarius), various musicians, and the munifices, or milites (common soldiers). A clerk called an actuarius turns up in some sources as well. Beneficiarii are listed on the Perge tablet and perhaps acted as aides to the commanding officer.

There was another rank structure in use in the Roman army, confined to the cavalry and the 'auxilia palatina'. This had the centenarius as the centurion grade, with a ducenarius above him and various grades of biarchus and circitor below him, plus an obsure higher rank called senator. How all this compared to the legion/limitanei ranks is anybody's guess.

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).