On the denarius series the reverse is more flexible than the obverse and we see more variation soon there. The first big break in the design of both is likely to be this type of c. 137 or 136 BC:
This type can be seen as borrowing from earlier Roman coin types and is thus at once a radical change and a conservative choice. [Note that Mattingly thinks this is the second big change in the reverse, hypothesizing that the coins with the wolf, twins, Faustulus and fig tree where made the previous year (2004: 214).] It will be nearly two decades before the obverse changes again this time borrowing from the bronze coinage, i.e. keeping the choice in the familiar repertoire:
Then, about five years on another Janiform head:
This feels again like a throw back to to earlier silver designs on Roman diadrachms and the reverse of a ship makes the whole type echo the standard Janus/prow type of the as. The same year, or perhaps the next year, this unusual obverse appears:
Does the legend identify this female divinity as Roma in a different guise that we’ve seen her previously? Maybe. Or perhaps another goddess entirely is intended.
The next year sees even more experimentation on the obverse:
From this point onward — 113/112 if you want to follow Crawford, 110 Mattingly — the obverse design remains flexible and expressive just as the reverse had been since c. 137. Conservative types re occur throughout the series on both obverse and reverse, but after the mid seventies (RRC 387/1) the Roma and biga combination fails to re occur again together. In fact after this Roma appears more often on the reverse than the obverse. She’s not seen on the obverse again until 53BC (RRC 435/1)!
Liv Mariah Yarrow 