This is probably well known already, but following seeing something on the television this evening, I came across this:
http://www.flickr.com/photos/digicanon/2...otostream/
I was immediately struck by its similarity to first century AD Roman reitertyp stelae and wondered if this and similar sculptures might have been the inspiration for the Roman stones.
Crispvs
I think thats a good point, and a good spot. I suspect that unless we find a Vindolanda style tablet saying "Just found a picture of a Greek sculptor of a rider trampling a barbarian. Adapting it for the lads" we 'll never be able to prove it, but it feels right.
The symbol of "triumphant mounted hero rides over fallen/kneeling enemy" seems to be as old as the war chariot: you see it all through New Kingdom Egyptian art; then in -V and -IV Anatolia and the Aegean; then in Roman art. I would bet that the Roman cavalry borrowed it from Greek reliefs like this one.
There is absolutely a connection. Art historians actually refer to this motif - the rider, mounted on rearing horse, standing over his fallen opponent, cloak flying in the wind, with his arm raised up about to strike - as the "Dexileos motif" after this famous monument. It was pretty much universal throughout the Greek and, later, Hellenistic worlds, being especially common on funerary monuments. The Romans undoubtedly adopted it through prolonged and intensive contact with Greek art.
as someone who is outside of the roman period, i can say that this type of image continues on migration period helms (vendel, valsgarde and sutton hoo) and later, (now the staffordshire hoard.