Roman Phalangites?

[Urselius]

But cavalry did become the way to win battles, with the infantry now in a defensive role. that was what I was trying to get across..

I'm not sure I can think of many battles where the Roman horse made a battle-winning contribution pre-dating 450. Off-hand only the Battle of Adrianople (324) between Constantine and Licinius, comes to mind. The more famous battle of Adrianople (Valens) was a case of the Roman cavalry being a patent disgrace, at Strasbourg it was not much better. In most successful battles the Roman cavalry seems to have at best neutralised the enemy cavalry. Admittedly they tended to perform very well in harassment tactics, such as those of Theodosius against the Goths after Adrianople. But this is not battle-winning in the sense that Alexander the Great's cavalry performed such a function.

Back to the phalanx. I think that you are beginning to see the point. the ol;d phalanx fought in a straight front, indeed like the Late Roman infantry. But that is it. The rest differs - the LR army had missile barrages, strong cavalry flanks, artillery support, plus a choice of several infantry formations that totally differed from the classic phalanx.
And of course we're not even speaking about the Macedonian phalanx with its two-handed sarissas and no sword fighting at all, which resembles the LR tactics even less.

That's my point- the LR army used a 'phalangial' formation (resembling, but not the same as), but it did not fight 'in a phalanx'.

I think you have illustrated my point, the Macedonian phalanx was as different in its practical detail from the 'classic Greek phalanx', much longer spears, deeper formation, different shields, as the Late Roman shield wall or the Anglo-Saxon 'scild-burh' at Hastings was. People are quite happy to call the Macedonian formation a phalanx, and this is not demonstrably on practical descriptive, mechanistic, grounds, but merely because the Macedonians were Greeks (in the broadest sense) and inhabited the "Classical World." If two formations as different as those that the 'classic Greek' and Macedonian infantry adopted can be both termed 'phalanx', then, as a logical progression, any formation which has the same core features as are common to both must also be able to be termed a phalanx.

I would class the core features as a) a linear formation, b) a number of ranks deep, c) close order (usually with overlapping shields) and d) an armament of thrusting spears as the main weapon. If these features are met then the formation is a phalanx.