Depends on how you look at fiat money. The denarius didn't start out as fiat money, as it was initially almost pure silver. However, as the roman emperors began reducing the amount of silver in it, the denarius lost its intrisic value, and it's value Was expected to reflect the amount of silver it had once contained. So I stand by my statement that the denarius did become one of the earliest examples of fiat money.
Devaluation does not a fiat make.
From Wikipedia: "Fiat money is money that derives its value from government regulation or law. The term derives from the Latin fiat, meaning "let it be done", as such money is established by government decree."
The denarius never derived its value because of any declaration by the Roman government, except maybe under Diocletian's attempted price reforms, which was sort of an attempt at indirect fiat-ization. When the silver value went down, merchants figured it out pretty fast (and specie volume rose, which pretty directly creates inflation in such an economy), and changed prices to adjust for that fact.
Not fiat. Denarii are commodity money. Not fiat. They never derived a major share of their value from any type of legal tender or government guarantee effect. I don't have a dataset on hand to compare the purchasing power of a denarius vs. an equivalent sum of silver, but I suspect the denarius had only a very slight advantage, and that probably not a fiat effect, but rather a "commonly understood means of exchange" effect, to make up a term on the fly.
