Diocletian: legions and rank structure

[Nathan Ross]

the ranks of the roman legions during the reign of Diocletian?

There is a shortage of evidence, as you say, but we have some ideas of the earlier and later structures, and can speculate on that basis!

I did quite a long post here in the thread that Gunthamund linked above that might address the question to some extent.

Basically, the lower ranks of the legions of Diocletian's day were probably quite similar to those of the earlier principiate. We still find centuries commanded by centurions (although they seem to have been referred to more commonly as ordinarius after c.280 or so), assisted by optiones and tesserarii. These ranks appear to have carried on right though later Roman history, and turn up centuries later in Egyptian papyrii and the Perge inscription too, I believe.

The main changes seem to have been in the upper command structure. From around 250, senators were removed from legion command and their places taken by prefects, initially equestrians and later promoted centurions. A few decades later, c.280, the old equestian military positions seem to have been phased out. So no more equestrian tribunes or cohort prefects - again, the legion centurionate seems to have supplied all these command posts, via the new institution of the Corps of Protectores.

Around the same time, the primus pilus and praefectus castrorum disappear too. The senior centurion (or perhaps veteran) role now seems to have been the campidoctor, or drill master. There were sometimes several of these men in a unit, and they also functioned as front line soldiers.

Later papyrii and inscriptions, plus Vegetius, mention several other ranks or roles that may have appeared around this point, or some time later. Flaviales and Augustales, for example, could have been centurion grades, or (perhaps more likely) senior soldier grades; the former is most probably Constantinian.

Better attested positions, probably dating from the later 3rd century, would include actuarius (legion quartermaster, approximately), adiutor (clerk or record keeper) and draconarius (draco-bearer).

Confusingly, there was a whole new rank structure for the elite units of the mid fourth century, the auxilia palatina and scholae. These ranks (including centenarius, biarchus, circitor and others) seem not to have been used by the legions, either limitanei or comitatensis. One or two crossover posts used by both types of unit - vicarius, for example - probably appeared later in the 4th century.