Legiones Decurion
Before Antaro Chronus basically took on the entire job as the only known Space Marine Tank Commander, the Legions of the Adeptus Astartes had a rank where such a title was spread evenly to all twenty legions during the Great Crusade and Horus Heresy. They were called the Legiones Decurion.
Overview
They are Tank Commanders who knew their Legion's Battle Tanks inside and out. Decurions had different ranks, which included Decurion Locus and Decurion Defensors, the latter of which had extensive hypno-indoctrination training in the weapons of the Imperium. Others included the Decurion Lanius and the Decurion Sagittar, who were armed with an Illiastus Assault Cannon and an Augury Scanner to pick out vulnerable targets for destruction. They wore extremely stripped-down variants of the Mark II and the Mark IV, possibly to ensure that their body-frames are slim enough to go in-and-out of their tank's turret.
Crunch
On the Horus Heresy tabletop, these guys can be mounted on Predators, Sicaran Battle Tanks, Sicaran Punishers, and the Kratos Heavy Assault Tank. They each grant battle benefits.
The Decurion Defensors do exactly what they say on the tin – these Space Marines have extensive hypno-indoctrination training in the weapons of the Imperium and can order their units to use the Advanced Reaction: Point Defence and return double the number of attacks with their defensive weapons. The Decurion Lanius is armed with a Heavy Bolter, whilst the Decurion Sagittar is armed with an Assault Cannon. Both carry an augury scanner, with which they can pick out vulnerable targets for destruction. When their vehicle makes a shooting attack, they can choose to fire one defensive or pintle weapon at a separate target, with the added benefit of Skyfire or Precision Shots (5+). Not too shabby for a support upgrade.
