Roman use of the Contus

[Renatus]

Contarii an Cataphract units don't seem to be mentioned in battles till Constantine's time. Arrian makes no mention of it the Taktica or the Aray on the Alans.

This is incorrect. In the Taktika, Arrian states that Roman cavalry was of two sorts, those armed with the kontos, who attacked in the manner of the Alans and Sauromatai, and those armed with lances (Tak. 4.7). In the Ektaxis, he specifies the types of cavalry stationed on the wings of his battle line, amongst which were the kontophoroi (Ekt. 21).