I always operated under the assumption that the Twelve were the leaders of the early Church, above the episcopate,
Iconographically, this is true. None of the Twelve are ever portrayed as bishops, but as teachers. This is signified by the clavulus
, the broad vertical stripe extending from the shoulder, often painted in gold, which was a symbol of authority in the Greco-Roman world. Christ, the Great Teacher, is also portrayed in this way.
The apostles of the Seventy, such as St James the Brother of the Lord, who served as bishops, are portrayed in their vestments. St Mark the Evangelist, who, while he served as bishop, is never shown in vestments, but as an apostle and teacher. Writing one of the Gospels, after all, would be seen as a higher calling than that of bishop.