Militarily, the Sasanians continued the Parthians' heavy dependence on cavalry troops: a combination of horse-archers and cataphracts; the latter were heavy armored cavalry provided by the aristocracy. They added a contingent of war elephants obtained from the Indus Valley, but their infantry quality was inferior to that of the Romans. The combined forces of horse archers and heavy cavalry inflicted several defeats on the Roman foot-soldiers, including those led by Crassus in 53 BC, Mark Antony in 36 BC, and Valerian in 260 AD. The Parthian tactics gradually became the standard method of warfare in the Roman empire and "cataphractarii" and "clibanarii" units were introduced into the Roman army;* Verbruggen–Willard–Southern (1997), 4–5</ref> as a result, heavily armed cavalry grew in importance in both the Roman and Persian armies after the 3rd century AD and until the end of the wars. The Roman army also gradually incorporated horse-archers ("Equites Sagittarii"), and by the 5th century AD they were no longer a mercenary unit, and were slightly superior individually in comparison to the Persian ones, as Procopius claims; however, the Persian horse-archer units as a whole always remained a challenge for the Romans, which suggests the Roman horse-archers were smaller in numbers. By the time of Khosrow I the composite cavalrymen ("aswaran") appeared, who were skilled in both archery and the use of lance.