Are the parasites of marsupials ancient heirlooms or recent souvenirs?

The parasites of any given organism contain a mixture of 'heirlooms' and 'souvenirs'. Heirlooms reflect an ancient association between the parasite and the host, with the parasite being passed on from the ancestral species through the generations thus they share similar patterns of evolutionary history. Souvenirs are parasites that have been acquired by the host more recently, perhaps through evolving new behaviours or by inhabiting a new environment. This project sought to ascertain what proportion of parasites on marsupial mammals are heirlooms reflecting a shared ancient past, and what proportion are souvenirs, perhaps because of recent encounters with placental mammals.