See H. Tyndale-Biscoe, Life of Marsupials (1973); A. K. Lee and A. Cockburn, Evolutionary Ecology of Marsupials (1985).
Any mammal of the infraclass Marsupialia, characterized by premature birth and continued development outside the womb. The young remain attached to the mother's teats for a period corresponding to the late stages of fetal development of a placental mammal. More than 170 species (e.g., bandicoots, kangaroos, koalas, wombats) are found in Australia, New Guinea, and nearby islands. About 65 species of opossum occur in the Americas and seven species of ratlike marsupials in South America. Many species have a pouch (marsupium), a fold of skin covering the nipples on the mother's lower belly, where the young continue their development.
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Some scientists believe that the marsupials evolved in North America and dispersed from there, via Europe, to Asia and Africa. This diaspora would have also reached South America before it became an island continent. This theory suggests that marsupials passed from South America through Antarctica to Australia (via Gondwanan land connections), a continent already occupied by placental mammals. Alternately, another theory posits that marsupials originated in Australia and traveled, via Antarctica and South America, to North America.
The discovery of Chinese marsupials appears to support the idea that marsupials reached Australia via Southeast Asia. However, marsupial fossils found in New Guinea are younger than those in Australia, evidence which presents a problem for this theory. There are a few species of marsupials still living in Asia, especially in the Sulawesi region of Indonesia. These marsupials coexist with primates, hooved mammals and other placentals.
On most continents, placental mammals were much more successful and no marsupials survived, though in South America the opossums retained a strong presence, and the Tertiary saw the genesis of marsupial predators such as the borhyaenids and the saber-toothed Thylacosmilus. In Australia, however, marsupials displaced placental mammals entirely, and have since dominated the Australian ecosystem. Marsupial success over placental mammals in Australia has been attributed to their comparatively low metabolic rate, a trait which would prove helpful in the hot Australian climate. As a result, native Australian placental mammals (such as hopping mice) are more recent immigrants.
Pregnant females develop a kind of yolk sac in their wombs, which delivers nutrients to the embryo. Marsupials give birth at a very early stage of development (about 4–5 weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located inside the marsupium. There they remain for a number of weeks, attached to the nipple. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth and nourishment.
Because newborn marsupials must climb up to their mother's nipples, their front limbs are much more developed than the rest of the body at the time of birth. It is possible that this requirement has resulted in the limited range of locomotor adaptations in marsupials compared to placentals. All marsupials must develop a grasping forepaw during their early youth, making the transition from this limb into a hoof, wing, or flipper, as some groups of placental mammals have done, far more difficult.
There are about 334 species of marsupial, and over 200 are native to Australia and neighboring northern islands. There are also 100 extant American species; these are centered mostly in South America, but the Great American Interchange has provided Central America with 13 species, and North America with one (the Virginia Opossum).
† indicates extinction