Little is known of the origins or early occupation of the area where, in the early medieval period, Equilum developed. Recent studies tend to identify the site with a probable Roman vicus that depended on nearby Altino and was situated within the southern agricultural landscape of the town. A more substantial urban development must have followed the Lombard invasions of the Venetian territory in the 7th century, when Equilum became an important stronghold of the “Venetia Marittima” administered by the Byzantines. The site’s importance also lies in its elevation, in the 9th century, to bishop’s see, a title it kept until 1466 and which was suppressed due to worsening environmental conditions, including the swamping of the area. Although the urban centre revealed no traces, the existence of an important religious centre is attested by the presence of the remains of two churches, one dedicated to S. Mauro dating to the 7th-9th century, the other corresponding to the cathedral church of S. Maria, mentioned in a document of 1060. Whilst the first building was covered after the excavation, some stretches of the cathedral’s walls remain visible. This structure, which was still standing at the end of the 1800s, was destroyed during the First World War. Within the central nave, excavations in 1963 and 1969 revealed the remains of an earlier church with basilica plan, characterised by its division into three naves with three semicircular apses and a narthex, all decorated with mosaics. The three naves were separated from the presbytery by railings. The style of the ornamental elements in the mosaic floor is similar to that of the early Christian basilicas of Grado and Parenzo, thus the basilica can be dated to between the end of the 5th and the 6th century A.D.
Finds of Roman terracotta artefacts (tegulae with raised edges, sesquipedales), domestic pottery and glass suggest a chronology which runs from the 4th century B.C. to the 8th century A.D. with occasional Renaissance elements. The sporadic presence of flint may indicate a continuity of occupation in an earlier period (Bronze Age?). (MiBAC)