Polybius, horse archers arrows, roman armour, oh my.

[marka]

Before Carrhae, the Romans may have been familiar with older bow designs, like the traditional Scythian type and wooden self bows. Therefore Carrhae may have been that rarest of battles, one in which the Romans not only found themselves contending with an entirely alien military system (steppe horse archers), but archers armed with a new (or highly improved) weapon system. After Carrhae the Romans quickly adopted the Parthian bow, which became the standard war bow throughout the Empire.

Gregg

the romans had faced horse archers with powerful bows before but the skill with which they were employed+crassus lack of preparation+the terrain and the heat contributed to the roman defeat.