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Evolution

Opossums are frequently considered to be " living fossils ", and as a result are often used to approximate the ancestral therian condition in comparative studies. However, this is inaccurate, as the oldest opossum fossils are from the early Miocene (roughly 20 million years old). The last common ancestor of all living opossums dates approximately to the Oligocene -Miocene boundary (23 million years ago) and is at most no older than Oligocene in age. Many extinct metatherians once considered early opossums, such as Alphadon, Peradectes, Herpetotherium, and Pucadelphys, have since been recognized to have been previously grouped with opossums on the basis of plesiomorphies and are now considered to represent older branches of Metatheria only distantly related to modern opossums.

Opossums probably originated in the Amazonia region of northern South America, where they began their initial diversification. They were minor components of South American mammal faunas until the late Miocene, when they began to diversify rapidly. Prior to this time the ecological niches presently occupied by opossums were occupied by other groups of metatherians such as paucituberculatans and sparassodonts. Large opossums like Didelphis show a pattern of gradually increasing in size over geologic time as sparassodont diversity declined. Several groups of opossums, including Thylophorops, Thylatheridium, Hyperdidelphys, and sparassocynids developed carnivorous adaptations during the late Miocene- Pliocene, prior to the arrival of carnivorans in South America. Most of these groups with the exception of Lutreolina are now extinct. It has been suggested that the size and shape of the jaw of the ancestral Didelphid would most closely match that of the modern Marmosa genus.