Cassius Dio (Historiarum Romanorum quae Supersunt LXXI.xvi.2) records that 5,500 Sarmatian cavalry were posted to Britain under the terms of the treaty of ad175. These men would very likely have been split into eleven Cavalry Alae of quingenary strength (i.e. 500 troopers per unit), or may have been used to provide a cavalry contingent for around twenty-odd under-strength infantry units, thus forming cohortes equitatae, or indeed, any combination. It is very unlikely that any were formed into a large cavalry ala milliaria, as these are very rare, there being generally only a single example in any one province, and the only one recorded in Britain was housed in the Stanwix fort on Hadrian's Wall, the Ala Petriana. These cavalry troopers must have been housed in forts scattered throughout northern Britain and Wales; some of which were apparently formed into the Ala Sarmatarum and stationed at the Ribchester fort. The unit is attested only on two undated inscriptions.
As the ala became depleted - through retirement mostly - its numbers were not replenished it seems, and its status changed to that of a numerus 'company' by 222-35, and thence to a lowly cuneus 'wedge' by the end of the fourth century. The tile stamped BSAR found at Catterick, may have connections with this unit in one of its several incarnations. During its entire lifetime the unit was stationed at Ribchester in Britain, and uniquely allowed to change its military classification.