I am re-reading Diana Kennedy's 'Cuisines of Mexico' and I ran across her description of a quesadilla. It's raw corn tortilla dough patted out, Oaxaca cheese placed in the middle, with a raja of chile poblana and some epazote leaves. Then the tortilla is folded over like an empanada and cooked on a comal or deep fried. She mentions other fillings - sesos, papas con chorizo, flor de calabaza, and huitlacoche, but makes no mention of a cooked flour tortilla folded over a cheese filling, the standard quesadilla I'm familiar with.
Her dish sounds like a pupusa. Where did this quesadilla come from and how did we get it turned into a flour tortilla? Is this just a case of the same name applied to two dishes, one from the south with corn, the other from the north with wheat? Quesadilla (I think) means little cheesy thing, so it's a fair description for both items.
CoM was copyrighted in 1972, and the research was probably done in the mid '60's. It appears she was living in Mexico City at that time, and had maids/cooks from nearby Oaxaca and Guerrero which would give her a more corn-based perspective.
I have tried googling quesadilla in combination with pupusa, mais, and corn, and haven't turned up anything.