An Historical Summary of Angelic Hierarchies from Part VII: The “Seven” Thrones in In Operibus Sigillo Dei Aemeth by David Richard Jones.

“Gregory differs slightly from the Dionysian order which, because St. Denis was believed to have priority, caused late medieval theologians some distress and confusion. In fact both hypotheses are roughly contemporary, demonstrating a more general theological interest in the subject in the middle and later half of the 6th C. C.E.” [via]