Quote Originally Posted by Mujalumbo
The Seleukid Argyraspidai were descendants of Alexander's Hypapists
First, the connection between Alexander's hypaspists and the later Seleukid Argyraspidai is not at all that clear. Alexander's hypaspists became reorganized into the argyraspidai during Alex's lifetime. They are last heard off as a distinct unit in the 317/316 BCE war between Antigonus and Eumenes. I think that the reports in Diodorus suggest that they were fighting as pikemen now. They were fighting for Eumenes, but when Antigonus managed to capture their camp, with the booty of all those years of campaigning and their families, they went over to the other side, seized Eumenes and handed him to Antigonus. With such an example, Antigonus didn't really trust them, so he broke up the unit and sent them to do garrison duty to some distant frontier province (Arachosia) full of rebellious natives and with secret orders to the province's satrap to keep them in small detachments and to use them non-sparingly. Antigonus might have kept part of the unit he deemed more trustworthy in his service and they might have eventually passed to Seleukus, but there is no "hard" evidence of that.

Seleukos I is reported to have had hypaspists (not called argyraspidai here) with him when fighting Demetros Poliorketes at Cyrrhestica, but no further information is provided. To my knowledge, the next we hear about a Seleukid infantry unit of this kind (elite, Royal Guard) is at Raphia. We keep hearing about it in the subsequent campaigns of Antiochos III and they are called both Argyraspidai and Royal Guard pretty much indistinctly. The sources (mostly Polybios) are reasonably clear in showing them 10.000 strong, permanently attached to the king (i.e. not put on the field only in time of war, as the rest of the phalanx) constantly kept at that strength, fighting as part of the main phalanx, though still at its right (they are not, however, at least neither at Raphia nor at Magnesia, the rightmost infantry, immediately next to the cavalry as the hypaspists usually were, but the right end of the big chunk of phalangites) and equipped as pikemen. Later, in the Daphne parade in the reign or Antiochos IV Epiphanes we still see a Royal Guard 10.000 strong but now, half of them are said to be equipped "in the Roman manner".

To me, it is likely that the Seleukid Royal Guard was a sort of "re-institution" drawing from the Alexandrian original, but also from Persian practice as well (remember the Achaemenids also had a 10.000-strong guard, similarly kept at constant strength and permanently standing with the king, the "Immortals")

Quote Originally Posted by Mujalumbo
and they were presumed to fight in a hoplite formation, no? Sources on them are sketchy at best, but it appears as if they were some sort of special forces; being re-equipped for special assignments, and also being heavily equipped and taking part in set piece battles.
What Alexandrian hypaspists really were or did is not actually relevant here. What matters is what Seleukid Argyraspidai (a different beast) were. It seems that the emerging consensus after many years of discussion is that Alex's hypaspists were multi-purpose, elite infantry, capable of doing many things as the situation dictated and doing them all very well. This multi-role nature, which is now becoming considered part of the hypaspists identity, seems to have been the very source of the debate, as folks tended to emphasize some aspects over others.

Fortunately, I think that the evidence for Seleukid Argyraspidae is clear enough that they were primarily elite "Makedonian-style" phalangites. They may have retained some tactical flexibility, but apparently, not to the extent of the "originals". This is not just EB's opinion. The leading scholars generally support this view (see for example, Dr. Bezalel Bar-Kochva's "The Seleukid Army")

Hope this clears things up.