Mandorla has an oval shape and it has its roots in ancient Rome. “Imago clipeata” – is a term in art used in reference to the images of heroes on Roman shields. Shield portraits symbolizing glory and apotheosis have become a prototype for the depiction of glory of Christ.
http://en.wikipedia.org/wiki/Imago_clipeataThere is an interesting article about St.Sophia in Ohrid (Macedonia) written by Russian scientist Alexey Lidov, where he explains the historical roots of the mandorla. He explains particularly it’s oval shape with reminiscences of so-called clipeus shields, which where wide spread in funerary portraits. Emperors and gods were painted with these shields, which symbolized eternity and power over the global space.
There are also roots of the depiction in early Buddhist art (opinion of the author) in the 1st. cent.A.D., where we find the shining mandorla depictions like a halo around figure of the deity. The roots of mandorla in Byzantine art are to be found both in ancient Roman and early Buddhist art.
You can see Godmother sitting on throne and holding Her Son, depicted standing in mandorla of glory in the conch of St. Sophia in Ohrid. This is the only known depiction of the Christ in mandorla with His Mother on throne after an iconoclasm period of Byzantine art (730-1046). Godmother holds mondorla as a shield, which shines as halo around Christ.
http://www.geolocation.ws/v/P/21960600/sveti-sofia-ohrid/en
