Research questions for a book

[Nathan Ross]

I'm reading a series of novels now, set on Hadrian's Wall, and the author easily promotes the primus pilus up to command of a legion. OK in wartime, but anathema for anything longer than a short-term measure for the class-conscious Romans.

Is that Anthony Riches' series? I've only read the first two so far (too busy reading hefty novels about India!), so I can't comment on the accuracy of his rank structures. But, as you say, Primus Pilus to Legatus would be unorthodox at best...

However, there is the example of the (in)famous L Artorius Castus - probably late 2nd century, so OK for Anthony but not for Kate! He served at Praefectus (Castrorum, presumably, although these were changing times) of VI Victrix in Britain, and was then appointed Dux Legionum, a rather mysterious position but one that suggests the temporary command of one or more legions or (more probably) vexillations for some specific campaign. Dux is usually considered a later Roman term, but I wonder whether Marcius Turbo might also have held this post in his Egyptian command - it may have connoted a degree of authority above a mere praepositus, and allowed him to effectively take command of the forces in Egypt after the death or disgrace of the sitting Praefect.