Microbiotheria is an marsupial order that encompasses two families, Microbiotheriidae and Woodburnodontidae,
Although once thought to be members of the order Didelphimorphia (the order that contains the Virginia opossum), an accumulation of both anatomical and genetics evidence in recent years has led to the conclusion that microbiotheres are not didelphids at all, but are instead most closely related to the Australasian marsupials; together, the microbiotheres and the Australian orders form the clade Australidelphia which are now thought to have first evolved in the South American region of Gondwana.
Biogeography
The oldest microbiothere currently recognised is
Khasia cordillerensis, based on
fossil teeth from Early Palaeocene deposits at
Tiupampa,
Bolivia. Numerous genera are known from various Palaeogene and Neogene fossil sites in South America. A number of possible microbiotheres, again represented by isolated teeth, have also been recovered from the Middle
Eocene La Meseta Formation of
Seymour Island, Western Antarctica. Finally, several undescribed microbiotheres have been reported from the Early Eocene Tingamarra Local Fauna in northeastern Australia; if this is indeed the case, then these Australian fossils have important implications for understanding marsupial evolution and
biogeography. The distant ancestors of the monito del monte, it is thought, remained in what is now South America while others entered
Antarctica and eventually
Australia during the time when all three
were joined as part of Gondwana.
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