Dmitriev, Vladimir. 2022. The Sasanian Elephant Corps Revisited: Ammianus Marcellinus on the Tactics of Persian Elephantry. Journal of Persianate Studies 15 (1), 1-13.
According to Ammianus Marcellinus, elephants substituted, to some extent, siege towers; he describes wooden towers on the backs of the animals, armed with Persian warriors who attacked the defenders of a fortress. Certainly, elephants may have served as an element of ancient psychological warfare. But, at the same time, it appears that the Sasanians employed elephants in their battle fighting, bearing warriors who attacked their enemies with various missiles. In open-field battles, elephants, as a rule, were introduced into the battle in an offensive situation. Ammianus Marcellinus does not offer any evidence as to elephants functioning as beasts of burden or draught animals; on the contrary, he always stresses the fact that they were military animals who posed a real danger to the Romans in battle.