Complex sentence: Bero appears to have been the last Bishop of Finland, since his successors were referred to as the Bishop of Turku, first mentioned in 1259 ("episcopo et dilectis filiis capitulo Aboensi").
Split into simpler sentences: Bero appears to have been the last Bishop of Finland. His successors were referred to as the Bishop of Turku, first mentioned in 1259 ("episcopo et dilectis filiis capitulo Aboensi").