Complex sentence: Allen argued that the Ashbourne portrait depicted de Vere in the guise of Shakespeare, suggesting that the features of the man in the Ashbourne portrait corresponded to those of de Vere and that the costume suggested a date earlier than the one signed to the image: 1611.
Split into simpler sentences: Allen argued that the Ashbourne portrait depicted de Vere in the guise of Shakespeare. He argued that the features of the man in the Ashbourne portrait corresponded to those of de Vere and that the costume suggested a date earlier than the one signed to the image: 1611.