The onset of the aes grave in the Republican era ran in tandem with the silver quadrigatus. Many an academic have theorized as to the meaning behind the janus head and prow. Crawford dispels them all but it is interesting to note what previous theories were. The Greek poet Livius Andronicus (c. 284-204 BC) believed that that the Roman god Saturn was an early Italian king and that he was welcomed by Janus. Another story that Crawford debunks is that Janus was the inventor of ship-building. Both the early Italian king and ship-building concepts have been passed over as "Greek literary conceit". In Prof. Liv Yarrow's more recent book on using coins as sources, she explains that aes grave may have its origins in both the Greek monetary system that was already in place and the Roman bronze weight system also important at that time. A meshing of those two cultures could have resulted in the aes grave coinage. A very likely scenario in my opinion. She continues to show that hoard evidence clearly suggests that the introduction and proliferation of aes grave coinage happened at a time when Rome was preparing for conflict with Carthage and increasing their maritime defenses. This warranted an influx of payment that wouldn't have been possible without an easier form of payment.