Complex sentence: It was formerly performed in Rome, and is closely related to but distinct from the Gregorian chant, which gradually supplanted it between the 11th century and the 13th century AD.
Split into simpler sentences: It was formerly performed in Rome, and, although it is closely related to theGregorian chant, the two are distinct. The Gregorian Chant gradually supplanted the Old Roman Chant between the 11th century and the 13th century AD.