A very obscure usurper hailing from Alexandria, Egypt, all that is known for sure is that he was a friend of Probus. For whatever reason, Saturninus proclaimed himself emperor to betray Probus - or perhaps he was forced to. The circumstances surrounding his ascension are just not known. Speculation holds that one or more of the legions in the area must have been unhappy and forced Saturninus into taking over the throne. This is not unlikely taking into account that Probus's decision to partially demobilize the army and put them to work in civic projects was highly controversial and, ultimately, it may have been the primary cause for his death. In any event, Saturninus dies soon after. Whether murdered or died of natural causes, again, is left to speculation but the safe bets have the money on the more violent outcome.
One of the great rarities in Roman coinage, there is only this one Aureus struck with his name. And RIC even will only go so far as to state that its authenticity is likely.
RIC 1 Aureus Obv: IMPCIVLSATVRNINVS; Laureate, cuirassed bust right.
Rev: VICTORIAEAVG; Victory advancing right, holding wreath and palm.