John Spaul assigns him to the Cohors I Batavorum who was stationed in the province of Britannia in the 3rd century.
—John Spaul (2000) Cohors²: The evidence for and a short history of the auxiliary infantry units of the Imperial Roman Army
The gens Simplicia was a minor plebeian family in ancient Rome. The members of this gens are known from inscriptions dating from the imperial period, most of them having no praenomina from the third century onwards, and in many cases their full nomenclature is uncertain. Some of them were from senatorial families, and one of the Simplicii was Praefectus Urbi of Constantinople in 403.
The name Simplicius belongs to a class of gentilices formed from family names ending in -ex or -icus, using the gentile suffix -icius. Its root is the cognomen Simplex, which originally referred to a person who was 'simple' or 'unadorned' in character or manner. Surnames derived from the nature and habits of an individual were common in Rome.
One of the altars from the Carrawburgh Mithraeum depicts the bust of Mithras or Sol.
Deo ♣ invicto ♠ / Mithrae M(arcus) Sim/plicius Simplex pr(a)ef(ectus) v(otum) s(olvit) l(ibens) m(erito).
To the Invincible god Mithras Marcus Simplicius Simplex, prefect, willingly and deservedly fulfilled his vow.