Until recently, very diverse marsupials, such as possums, cuscuses, gliders, and koalas, were included in a single family Phalangeridae. This is the oldest known family of marsupials, whose members are primitive and generalized. The big toe is clawless and opposable. Some members of the family have a pouch, in some it is absent, and, still in others, it is two folds of skin. There are 50 rooted and sharp teeth.
Opossums are mostly arboreal and nocturnal.
They are omnivorous. This family is closely related to the Didelphidae, both of which are similar to the primitive marsupial stock. These animals fill the same niche in the Australasian ecosystems as do true mice, rats, wolves, and cats in other ecosystems. The teeth are adapted for an insectivorous or carnivorous diet - the canines are large and sharp, and the molars have three sharp cusps. Most members of the family are nocturnal predators, some terrestrial, and a few arboreal.