 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 were minor components of South American mammal faunas until the late Miocene, when they began to rapidly diversify.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.