We had dinner last night and, as usual when conversation lingers for too long, we ended up discussing semantics. Or rather, zoological nomenclature. The thing is, in Argentina we call comadreja the marsupial (family Didelphidae) Chironectes minimus, which apparently in Spain is called zarigueya. By some unlucky coincidence comadreja is used by our European cousins to refer to Mustela nivalis, which is not a marsupial at all, but rather a carnivor of the order Mustelidae.
Thus, the question "are comadrejas marsupials?" has two answers: yes and no. And both are correct.