Adoption of the cavalry shield in Hellenisitc heavy cavalry.

[MeinPanzer]

Hhh..mm...mmm, quite persuasive! However I don't believe the last point about motif is convincing - the same 'lion hunt motif' is seen on distinctly Grecian style pieces from Bactria, for example.

The same motif of a horseman lancing a lion is found in Graeco-Bactrian art? Can you post or cite an example?

Obviously, not for seleucid cataphracts in particular, but since you mention 'saka' , I take it you mean generally?
There are, of course, a number of large lance-heads found in steppe graves which can realistically only be from 'kontoi', e.g. a fourth C BC grave of a Sarmatian woman(!) from Kholodni Yar on the Tjasmin, Ukraine grave no.20 contained two such lance heads with a length of over 60-70 cm; and others such as Siracian grave finds at Ust-Labinskaya, NW Caucasus 4-3 C BC.
I was under the impression that because of the rather 'loose' dating of tomb-finds, the real question was whether the 'kontos' was employed on the steppes before or after Alexander introduced them to the 'xyston'......

Why must large lance heads only be from kontoi? Regular cavalry spears, and spears in general, could have large spearheads without necessarily being long-shafted - this is the same fallacy that led to many scholars assuming that the sarissa must have had a large spearhead. Looking at Sarmatian (or, more accurately, Sauromatian) evidence, what do you make of the Filippovka kurgan 4 burial, dating to the end of the 5th c. BC or beginning of the 4th, which included a shafted weapon with a large spearhead and spearbutt 3.2 m in length? Should this be taken as a kontos? Furthermore, the depiction of the Saka cataphract wielding a spear one-handed overhand I mentioned before shows the spear as having a massive head.