Macedonians used leather helmets?

[MeinPanzer]

But neither chlamys nor krepides are unique to Macedonia, being commonly worn throughout Greece at the time (though the kausia would seem to be)......

It's clearly implied that the Macedonian chlamys is being referred to, which was semi-circular in shape and uniquely Macedonian. And as I stated before, the krepides are not uniquely Macedonian, but they are a part of the triad of Macedonian items of costume. There are numerous other references which can be found in Saatsoglou-Paliadeli's "Aspects of Ancient Macedonian Costume," Journal of Hellenic Studies 113 (1993) pp. 122-147.

At any rate, it doesn't even matter if the chlamys and krepides weren't uniquely Macedonian, as the only thing that matters is that Caracalla thought that these pieces of equipment and costume were used by Alexander's troops. A bronze shield isn't unique either, but they clearly had reason to think that Alexander's phalangites carried bronze shields.

nevertheless, I would agree with you that Plutarch may be referring to a type of Macedonian civilian 'national costume', and this possibly influenced Caraccalla's ideas.....

It's not just Plutarch here, there are numerous authors who mention this national costume. See the abovementioned article.

Certainly, the tomb paintings such as the 3 C BC Bella tumulus (which Giannis refers to) and the late 4 C BC Lefkadia tomb show krepides, but these are clearly High-ranking Officers (judging by 'purple' trimmed/'purple' cloaks and 'purple' trimmed armour) - and are mostly later than Alexander's day(like Plutarch's reference).

Firstly, how many depictions of common Macedonian troops do we have? Secondly, I doubt that a few years after Alexander's death, the 'national costume' of the Macedonians would be abandoned, especially considering that it was around well before Alexander according to archaeological and literary evidence (once again, I would refer you to the article mentioned above).

If we look at more contemporary ( to Alexander's day) depictions, we don't see krepides (which you might expect to be costly aristocratic items), such as the so-called 'Alexander sarcophagus', where bare feet (possibly with painted-on footwear) is the norm,

The Macedonian troops on the sarcophagus who weren't depicted as being heroically naked had high boots painted on their feet.

but best of all is the late 4 C Agios Athanasios tomb painting showing what are most likely Macedonian ( some wear kausias!) rank-and-file soldiers (no less than 8 of them!), none of whom wear krepides, all being shown in open sandals....

Those are krepides- they are high boots reaching above the ankle. It doesn't matter if they are made of openwork sandals around a sock or a closed boot- please post the definitive source that states that krepides cannot be sandalwork.

Well, I could point to Polybius' famous description of the 'scutum' as an example

Which he states in a much longer technical passage which deals in the exacting dimensions and qualities of Roman arms.

but in general you are right.....certainly when it comes to classical greek sources....and that is what makes me think that Caracalla, actually equipped his Phalanx with 'bronze' shields, as described by Dio, possibly in the mistaken belief that they were all bronze.

This passage from Dio, while clearly concerned with some details which were unusual in his day (ox-hide helmets, linen corselets), felt no need to describe the shield as being bronze on wood. Come to think of it, have there ever been examples of widely-used shields made only of metal in the Graeco-Roman world? Shields always have some sort of organic backing- be it wicker, leather, or wood- which would make it unnecessary to qualify a statement like "bronze shield," even in the 2nd c. AD.

OTOH one might have thought that some real Macedonian shields ought to have still been around in temples in Caracalla's day....so you may well be right, and that the shields carried were accurate replicas.Impossible to know, alas! Sad

Karunanithy writes about this a bit- it seems very likely that Caracalla would have had access to not only many more actual or figural examples of Macedonian arms than we do (especially paintings), but also many more literary sources as well. And considering his obsession with Alexander, he surely would have been well-read and familiar with `his`arms and armour.

Here are some interesting quotes from a report on Thracian tomb findings ( since Archibald)

I've not heard of the first mention (sounds promising), but the second is just referring to the well-known scenes from the Kazanluk paintings, with the incorrect assumption that the figures wearing kausiai are Macedonians. I've discussed this with Christopher Webber a bit, but the main gist of the discussion boils down to that the kausia seems to have been transmitted to non-Macedonian peoples in the southeastern Balkans as a sign of authority after their contact with the Macedonians.