Roman Legion in late Antiquity

[Nathan Ross]

I would propose that 'Batavorum' can only refer to the original cohortes Batavorum that were spread out across the empire, originally all recruited from the Batavi tribe. The auxilia were in turn named after the cohorts...

Or recruited from laeti settled on the old territories of the Batavi tribe, perhaps?

In the case of the tribunus Batavorum, I still think that he held a higher position that cohort commander. If this was a cohort, he would surely have give the designation (tribunus cohortis novae batavorum, for example). Plus the direct promotion to Dux would be unusual!

More likely that he was either tribune of the mounted guard unit called the Batavi (similar to Aurelius Valentinus, mentioned by M Speidel in Riding for Caesar p.86, who was also given a provincial vir perfectissimus command at the time of Gallienus), or an early tribune of the auxilia palatina Batavi (the seniores/iuniores division had not yet happened at this point). We know that the auxilia were commanded by tribunes - Bainobaudes and the future emperor Valens were both tribunes of the Cornuti.