Sword moving from right to left

[Frank]

The next painful question is, like the 'chicken or the egg', which change came first? The spatha or the oval shield? Date-line late 2ndC...

I tend to think that the need was for a longer weapon to reach more mounted foes, just as the hasta was being introduced into frontline service. The cylindical scutum may have proved difficult to use in combination with this longer weapon, used also for slashing as stabbing.

I guess more important is the hasta or heavy lancea. Some historians argue, that the lancea became the main weapon these times. And with a spear you should use an oval or round shield in a shieldwall.

The lancea made a comeback as main weapon due to the changed tactics. Its the better weapon against cavalry armies and it also supports a more defensive role of the infantry.

I don't see a chicken and egg problem. The old traditional legion changed to the more flexible and versatile tactics, organisation and equipment of the auxilia cohorts. I doubt this was a step by step approach. If a unit changed equipment, it makes a lot of sense to change all the equipment at once.