The practice of the tonsure is one that fascinates me. Early on, there arose the custom of monastics and/or clergy wearing a certain haircut that involved shaved heads. It was suppressed by Rome after Vatican 2. As far as I know, it was forgotten or lost in the East. I'd be glad to stand corrected, but I think the only Eastern Orthodox tonsure is the one that is conferred on adults just prior to baptism (and it's only a snip). I don't think Orthodox clergy or monastics where a tonsure.
In antiquity, there were three different versions, each which claimed Apostolic origin - one in tradition with St Paul, another with St John and another with St Peter.
The "Pauline" or Eastern tonsure claimed the authority of St. Paul the Apostle and consisted of shaving the whole head.
When Theodore of Tarsus was consecrated Archbishop of Canterbury in 668, he had to grow out his Eastern tonsure (on the sides) in order to conform to the Roman tonsure.
The "Petrine" or Roman tonsure is the one with which we are familiar. It is a bald spot on the top of the head - typical male pattern baldness. In some places it was only the size of a silver dollar and hardly noticeable. Of course, St Peter is the authority invoked for this style.
The third tonsure style is the most obscure style. The Celtic Church had the custom of appealing to St John for their customs that differed with Roman custom (e.g. dating Easter, tonsure, etc.) They claimed that they received from St John. It was a triangle consisting of a straight line across the back of the head from ear to ear. From each ear the shaved pattern went up toward the forehead and met in a point. Thus from above, you'd see a triangle pointing toward the face.
The "Johannine/Triangle/Celtic tonsure was actually a much greater source of tension between Roman and Celtic Christians since it was the most notable day to day difference observed by all men, all the time. The Roman Catholics claimed St Peter and said that their circular tuft around the head represented the crown of the thorns. The Romans went further by stating that the triangle tonsure was Gnostic and created by Simon the Magician!
The triangular Celtic tonsure was condemned by the fourth Council of Toledo. Some scholars have suggested that the "triangle" tonsure is the ancient Antiochene tonsure that was strangely preserved only in Celtic areas. Who knows.
My own personal theory is that tonsure in general represents the the bald head of wisdom and old age. A priest is a presbyter - the Greek word for an old man.