General outline:
According to Harvey, new mosaic floors were laid in the nave in a level higher than the original bases that supported the basilica columns, and in the upper platform. A mosaic repair from the 5rh century can also be seen in the lowest step leading from the nave to the octagon. These mosaic floors, depicting acanthus scrolls and Geometric patterns, was dated to the end of the 4th and early 5th c. This dating was accepted by many. But more recently Madden (2012) argued that the mosaics are Constantinian.
Two curving walls uncovered under the north and south transepts which belonged neither to the first Constantinian church, nor to the Justinianic building phase which it preceded, is to be dated sometimes in-between, given that they were broken by the present Justinianic walls and that the northern one also encroached on the mosaic floor. The latter could be viewed as remnants of a trefoil-shaped east end made sometimes between the fourth and the sixth, yet more probably in the fifth century, that is in the age of the alleged redecoration of the pavement with new mosaics (Bacci 2017, Plan iii and ch. 3, notes 14-15). Namely: the Constantinian church was rebuilt sometime in the fifth or sixth century, with the destruction of the octagon and its substitution with a trefoil-end. But the mosaic floors seem to be dated earlier, so perhaps we have here more than one phase.
