Two main periods of development are evident. The coins suggest that this new style is marked by the appearance of undecked vessels and the introduction of the forward-raking mast, the artemon, as seen on the coins of Mark Antony. These developments indicate changes in the function of the Roman navy of the period. The 1st-century adoption of undecked vessels must have made military vessels less useful for direct combat, but more suitable for the portage of cargo and the carrying of men. Ship types more traditionally associated with the Byzantine Empire now become prominent.
Some ships now have no ram, and are clearly intended exclusively for transport.
It may be that ships previously associated with the northern seas had become prominent by this period, as many of the transport vessels are reminiscent of nothing so much as Viking longboats. This change in the type of ships employed is likely to have been part of a general reorganisation of the fleet in the 3rd century. Evidence for river fleets does exist, however - the Notitia Dignitatum refers to barcae , boats, and these must be the small vessels depicted on many later Roman coins. The profusion of coin types depicting ships declines dramatically between the 3rd and 4th centuries. Partly a consequence of a functional change in the coinage system, this also reflects the changing ideological nature of the Empire.