Complex sentence: It has often been thought that, after the death of Thomas of Galloway, Earl of Atholl in 1232, Isabella, Countess of Atholl, married Alan this however, rests solely on the appearance of Alan's styles.
Split into simpler sentences: It has often been thought that, after the death of Thomas of Galloway, Earl of Atholl in 1232, Isabella, Countess of Atholl, married Alan. This however, rests solely on the appearance of Alan's styles.