Final answer:
The first known polyphonic treatment of the mass ordinary is the Messe de Nostre Dame by Guillaume de Machaut. This work is a significant development in the history of Western music, exemplifying the shift from monophonic to polyphonic structures during the transition to the Renaissance era.
Step-by-step explanation:
The first polyphonic treatment of the mass ordinary by a known composer is called the Messe de Nostre Dame, composed by Guillaume de Machaut in the 14th century. This work represents a significant milestone in the history of Western music, showcasing the transition from monophonic Gregorian chant to more complex polyphonic structures. Alongside the development of polyphony, the invention of the Gutenberg press greatly facilitated the distribution of music and musical theory, which in turn increased the demand for music in various social contexts. This era also marked the beginning of the Renaissance, which saw the flourishing of the Renaissance mass and motet, as well as secular forms like the madrigal and chanson. During the Renaissance, polyphony not only became more refined but also encouraged the use of larger ensembles for both vocal and instrumental performances.