Here, copied from a message I just posted on Moneta-L, are an expansion of my argument regarding the date of Decentius' Caesarship, plus summaries of two further arguments that I made at CICF but have not mentioned above:
Second, how could
Nepotian have revolted in the course of 350, when the
Chronographer of 354's list of urban prefects records that Magnentius'
candidate Fabius Titianus was prefect of
Rome continuously from 27 Feb. 350
to 1 March 351? In 351, in contrast, a certain Caelius Probatus was prefect
for 26 days between 12 May and 7 June, two days less than the recorded
28-day length of Nepotian's usurpation. I surmise that Probatus was
Nepotian's urban prefect, giving us exact dates for the usurper's reign, 10
May-7 June 351. The urban prefect whom, according to
Aurelius Victor,
Nepotian killed during
his rebellion, will then have been Probatus'
predecessor
Aurelius Celsinus. Followers of the 350 chronology for
Nepotian's revolt, since no serving urban prefect can have died in that
year, have
had to accuse
Victor of error and propose that he meant instead
the
praetorian prefect!
Third, the coinage of the
mint of
Rome shows that
Magnentius must have made
his brother
Decentius Caesar precisely during the revolt of
Nepotian: coins
of
Decentius appear immediately after the revolt, taking over Nepotian's
Roma seated
reverse type, but
Decentius was absent from the coinage before
the revolt. Followers of the 350 date for Nepotian's revolt, above all
Pierre
Bastien, have
had to conclude that
Decentius became
Caesar c. July
350, but this assumption brings problems.
Aurelius Victor and the Epitome
report that
Decentius became
Caesar at around the same time that Constantius
Gallus became
Caesar, and for that second promotion we have the date 15
March 351. Moreover
Decentius shared the consulship with
Magnentius on 1
Jan. 352, as though he
had only been made
Caesar in the course of 351, not
on 1 Jan. 351, as we would expect if he was made
Caesar in July 350. On the
new chronology these problems disappear:
Decentius was made
Caesar in
May-June 351, during the revolt of
Nepotian, so indeed at around the same
time as Gallus, who was promoted on 15 March 351, two or three months
earlier; and the year of Decentius' consulship with
Magnentius was indeed
the one immediately following
his promotion to the Caesarship.
Finally, the postponement of Nepotian's revolt from June 350 to May-June 351
allows for a much evener and more likely distribution of the coinage of
Magnentius at the
mint of
Rome over the thirty-one month period from 27 Feb.
350 to 26 Sept. 352 when he and
Nepotian were recognized as emperors there.
On the old chronology, two of the three main issues of bronze coins and,
according to Bastien's specimen counts, about 60% of the total volume of the
bronze coinage, were all produced between 27 Feb. and c. mid-July 350,
during only the first four and one-half months or approximately 15% of the period in question, meaning that only one main issue and 40% of the coinage was issued during the remaining 85% of the period. Thus on this chronology the rate of production of bronze coins at the
mint of
Rome was approximately eight and one-half times higher during the first four and one-half months of this period than it was during the remaining twenty-six and one-half months! On the new chronology the production of the first two issues and 60% of the coinage extended from 27 Feb. 350 to c. late June 351, over a period of 16 months, leaving the remaining 15 months of the period for the production of the third issue and about 40% of the volume of the bronze coinage. In other words on this chronology the
mint of
Rome produced bronze coins at approximately a constant rate over the whole period in question.