Evolution and systematics

The mouse-like monito del monte is a small, inconspicuous species of marsupial from South America. Its unprepossessing appearance belies its huge zoological importance. This drab little mammal is a relic of a bygone era—all that remains of a once-prominent group that may have given rise to the entire diverse marsupial fauna that now lives on the other side of the world in Australia. The Microbiotheridae are therefore considered part of the otherwise exclusively Australian group known as the cohort Australidelphia.

Of the seven species of microbiothere that have been described, six members of the genus Microbiotherium are long extinct and are known only from fossil remains. This order of marsupials has only one living family, the Microbiotheri-idae, with a single representative, Dromiciops gliroides, the diminutive monito del monte, also known in its native Chile as the colocolo. Dromiciops has several very primitive traits and is only distantly related to other South American marsupials (opossums). In fact, it has more in common with some Australian marsupials, in particular the carnivorous dasyurids, from which it must have been separated for at least 40 million years. The monito del monte is thus thought to be the only surviving member of an early offshoot of the marsupial lineage that lived on the ancient supercontinent of Gond-wana, to which Australia, Antarctica, and South America were once joined. A microbiotherian marsupial could have been the ancestor of the diverse marsupial fauna seen in Australia today. While the descendants of ancient micorbiotheres were doing well in Australia, the initially successful marsupial radiation in South America came to an end when placental mammals arrived from the north. Like most other South American marsupial orders, the little microbiotheres were mostly edged out—all but Dromiciops, which alone survives to this day.

The taxonomy of this species is Dromiciops gliroides (Thomas, 1894), Biobio, Chile, "Huite, NE Chiloe Island."

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