An article, written by Daniel McCarthy, of Trinity College, Dublin, entitled On the Shape of the Insular Tonsure, discusses variations in Christian clerical tonsures during the Middle Ages. The article is in PDF format.
From the article:
In the early decades of the eighth century at least three Northumbrian authors referred to the tonsure worn by clerics or monks in such a way as to show that it carried for them powerful connotations of Christian orthodoxy or unorthodoxy, depending upon its shape. As well as this all three correlated orthodoxy or unorthodoxy in tonsure with orthodoxy or unorthodoxy in Paschal observance. The earliest of these authors, Ceolfrid, abbot of Jarrow and later also Wearmouth, in a letter written to Nechtan, king of the Picts in c. 710, discussed both the Paschal and tonsorial issues at great length, whose details we shall examine below. In chronological order the next was Eddius Stephanus, who, in his Life of St Wilfrid written c. 720, explicitly stated that Wilfrid’s tonsure was coronal, i.e. circular, and emphasised its association with St Peter and hence the Roman Church, writing:
Wilfrid, the servant of God, in accordance with his own desire, gladly received from the holy Archbishop Dalfinus the form of tonsure of the Apostle Peter in the shape of the crown of thorns which encircled the head of Christ.