The short answer is that we don’t really know. The third century was a transformative period for the Roman army, with almost every aspect of the army changing in some manner. While some of these changes can be traced to the Principate (and we should remember the army in all periods was constantly changing), most cannot, due to a lack of literary sources and a severe decrease in inscriptions and sculpture.

There is no reason to assume that the scutum was an inherently better shield. Both the scutum and later shields are made of wood constructed around an iron boss, and covered in leather or rawhide. Both are dished, and gripped from a single grip behind the boss. Both are also similar in size. Late Roman shields are estimated to be three to four feet in height and two and half feet wide. A third-century scutum recovered from Dura Europos provides our best evidence for scutum size, and it’s dimensions (three and a half feet high and sixteen inched wide) are similar to that of later shields, though a bit smaller in width. Later shields seem to offer a bit more protection, yet this is far from certain. The principal difference lies in shape: while the scutum is rectangular, later shields are ovular (or circular). Yet ovular shields such as these were not new to the army. Such shields were carried by most auxiliaries during the Principate, as is attested by the iconographic and archaeological record.

Roman battle arrays changed during Late Antiquity. Formations were constructed of files rather than ranks, with each mess-group, or contubernium, forming a file in the line of battle. Although most soldiers carried a spatha (a sword only a bit longer than a gladius, most certainly not a “long sword”, especially when compared to Medieval long swords), the principal weapon had become a thrusting spear. Formations also became more compact. Ammianus describes how the Roman army at Strasbourg in 357 resembled “an unbreachable wall” (16.12.2), which locked shields as the Alamannic force closed upon them. When the two lines met, Ammianus states that “shield boss pressed against shield boss” (16.12.37) with much pushing as the fight commenced at close quarters. While the mechanics of Roman battle are unclear, by the fourth century the shield wall had clearly become an accepted infantry tactic.

Although the Romans had formations during the Principate such as the testudo, the fourth century shield wall is formed differently, as it not a “protective shell” but rather a solid line of interlocking shields. Such a formation lends itself to the ovular shields used traditionally by auxiliaries, as it is difficult to overlap a scutum. Yet the origins of the Late Roman shield wall are obscure. A Germanic influence is evident, as the shield wall and cuneus are formations ascribed by Roman authors such as Ammianus to the barbarians, one of the many influences Germanic fashions had on the army during this period. But Roman shield walls also greatly emulate a Classical Greek phalanx, and phalanx-like arrays, and the archaic Greek terminology used to describe them, had a long tradition in Roman warfare and elite literature. In his Deployment Against the Alans Arrian describes his forces as forming “phalanxes” of interlocked shields, and uses Greek terminology when discussing technical matters. Begining in the second century, it is possible to discern a deep fascination among the military elite with a revival of Classical Greek methods of warfare. Both Greek and Germanic customs offer possible influences for the construction and use of Late Roman shield walls.

There is no evidence to suggest the shield wall was the principal battlefield formation of the later army. Indeed, soldiers in normal circumstances still deployed with gaps inbetween them. Yet overall, Roman formations did become more dense, which lent themselves to the use of an auxiliary-style ovular shield, especially if shield walls were more frequently formed. It would be wrong to interpret these denser formations as more or less effective than the looser arrays of the Principate. As such, the reasons for the change in shield design (and other equipment changes) are not completely due to practicality. Cultural influences must be taken into account. Without a clear indication from the surviving sources, it is unclear which specific influence prompted a shift in shield design. That said, non-Roman influences and the Hellenophilic culture of the military elite clearly played a role.