What is "ur-poem"

n. A poem that is believed to be a precursor or prototype to a more modern class of poetry. The term most usually references a hypothetical 'mother poem' of a genre. Jerome McGann described the ur-poem as a work "whose existence is the Idea that can be abstracted out of all concrete and written texts which have ever existed or which ever will exist"McGann, Jerome J. "The Text, the Poem and the Problem of Historical Method." Literary Theories in Praxis. Ed. Shirley F. Staton. Philadelphia: University of Pennsylvania, 1987. 202-14..