This regiment was evidently created out of former marine units. They are attested in Britain only in the Notitia Dignitatum of the 4th/5th centuries. Unfortunately, their garrison fort Tunnocelum, is uncorroborated by further epigraphic evidence and remains unconfirmed.
According to the name, the force could have been a unit of around five hundred marines, who may have originally been used in the construction of Hadrian’s Wall. “Aelius” was the gentile name of the emperor Hadrian (Publius Aelius). Their soldiers could have been transferred from the navy to the land troops during his rule and provided the first garrison troop from Glannoventa . Evidence of its existence is a military diploma from Caedicius Severus dated February 27, 158 and a lead seal, both found in Ravenglass. The text of this diploma indicates that Caedicius last served as a rider ( ex equite ).
Presumably she was a cohors equitata , a partially mounted cohort that had around 120 cavalrymen in their ranks. The unit should have been relocated to Tunnocelum / Tunnocelo (Calder Bridge) at the latest in the 4th century , as evidenced by a military diploma from Cilurnum (Chesters) and an entry in the Notitia Dignitatum.
How do we know they were a Marine Unit?
From the classicae part of the unit’s name, to be either a naval unit of an ex-naval unit; classis being Latin for “fleet”. Since the officer is a tribune rather than a prefect as found in command of all the units in the Notitia whose names are prefixed rather than suffixed with classis, and which are thus surely genuine fleets, the cohors I Aelia classica would appear to be a standard auxiliary cohort, albeit one that was originally raised by converting a naval unit into one of infantry. This was presumably in the time of Hadrian given the name Ailia: Publius Aelius Hadrianus was by far the most famous bearer of the name.