"cavalry army", as was suggested by some more dated scholars. The cavalry under both officers were integral to mixed infantry and cavalry comitatus, with the infantry remaining the predominant element. The 3rd century saw a progressive reduction in the size of the legions and even some auxiliary units. Legions were broken up into smaller units, as evidenced by the shrinkage and eventual abandonment of their traditional large bases, documented for example in Britain. In addition, from the 2nd century onwards, the separation of some detachments from their parent units became permanent in some cases, establishing new unit types, e.g. the vexillatio equitum Illyricorum based in Dacia in the early 2nd century and the equites promoti and numerus Hnaufridi in Britain. This led to the proliferation of unit types in the 4th century, generally of smaller size than those of the Principate. For example, in the 2nd century, a vexillatio (from vexillum = "standard") was any detachment from a legion or auxiliary regiment, either cavalry or infantry. In the 4th century, it denoted an elite cavalry regiment. From the 3rd century are the first records of a small number of regular units bearing the names of barbarian tribes (as opposed to peregrini tribal names). These were foederati (allied troops under a military obligation to Rome) converted into regular units, a trend that was to accelerate in the 4th century. The ala I Sarmatarum, based in Britain, was probably composed of some of the 5,500 captured Sarmatian horsemen sent to garrison Hadrian's Wall by emperor Marcus Aurelius in c. 175. There is no evidence of irregular barbarian units becoming part of the regular army of the Principate until the 3rd century. 3rd-century crisis The mid-3rd century saw the empire plunged into a military and economic crisis which almost resulted in its disintegration. It consisted of a series of military catastrophes in 251–271 when Gaul, the Alpine regions and Italy, the Balkans and the East were overrun by Alamanni, Sarmatians, Goths and Persians. At the same time, the Roman army was struggling with the effects of a devastating pandemic, now thought to have been smallpox, the Plague of Cyprian which began in 251 and was still raging in 270, when it claimed the life of Emperor Claudius II Gothicus (268–270). The evidence for the earlier Antonine pandemic of the late 2nd century, probably also smallpox, indicates a mortality of 15–30% in the empire