Cohort commander?

[LVCIVS SERGIVS ANTONINVS]

I'm not sure that the different official titles of the six centurions were anything else than a reminiscence of the old manipular legion. Romans were people very proud of their traditions, they loved them and in consequence were very unwilling to change titles, something like the Guard Dragoons, who are not dragoons at all, or the Lance Corporal of the british army, who doesn't hold lances in combat anymore.

Regarding to the role of the tribunes I would like to point out some features: in the manipular legion previous to the marian reforms there weren't cohorts understood as permanent units composed of six centuries. As all of us know most scholars agree in that the centuries formed in three separated lines of maniples, namely the hastati, the principes and the triarii. Now we must ask ourselves by what reason the legion had exactly six tribunes at its front instead of, let's say, five or seven or ten. In my opinion this is due to the fact that each line of maniples was commanded by a pair of tribunes who took tactical control over the units, following the old same practice of sharing responsabilities between equals.