Cataphracts were the heavy cavalry of the ancient world, precursors of the medieval knights. They differed from other shock cavalry units in the fact that both the rider and the horse wore armor . The earliest known cataphracts may have appeared among the ancient Iranic nomads of Central Asia, the Scythians or the Saka. The Persians used cataphracts as early as the time of the Achaemenids, and the successive Persian empires throughout history maintained that tradition. King Xerxes included some cataphracts in the army that he led to Greece during the Greco-Persian Wars. The Greeks adopted the use of cataphracts for the first time in the Seleucid Empire, one of the successor states of Alexander the Great's empire. The Romans also adopted their use later, and the Eastern Roman (or Byzantine) Empire continued to use them throughout the Middle Ages. Cataphracts also expanded to Eastern Asia (the Turkic peoples and the Chinese used them) and the Islamic World.