Complex sentence: Marchetto's treatises were hugely influential in the 14th and early 15th centuries, and were widely copied and disseminated; without his innovations, the music of the Italian trecento--for example the secular music of Landini--would not have been possible.
Split into simpler sentences: Marchetto's treatises were hugely influential in the 14th and early 15th centuries, and were widely copied and disseminated. Without the innovations of Marchetto, the music of the Italian trecento--for example the secular music of Landini--would not have been possible.