any of more than 250 species belonging to the infraclass Metatheria (sometimes called Marsupialia), a mammalian group characterized by premature birth and continued development of the newborn while attached to the nipples on the lower belly of the mother. The pouch, or marsupium, from which the group takes its name, is a flap of skin covering the nipples. Although prominent in many species, it is not a universal feature—in some species the nipples are fully exposed or are bounded by mere remnants of a pouch. The young remain firmly attached to the milk-giving teats for a period corresponding roughly to the latter part of development of the fetus in the womb of a placental mammal (eutherian).

The largest and most varied assortment of marsupials—some 200 species—is found in Australia, New Guinea, and neighbouring islands, where they make up most of the native mammals found there. In addition to the larger species such as kangaroos (kangaroo), wallabies, wombats (wombat), and the koala, there are numerous smaller forms, many of which are carnivorous, the Tasmanian devil being the largest of this group (family Dasyuridae (dasyurid)). About 70 species live in the Americas, mainly in South and Central America, but one, the Virginia opossum (Didelphis virginiana), ranges through the United States into Canada. The largest living marsupial is the red kangaroo (Macropus rufus), males of which can grow to about 2 metres (6.6 feet) in height, 3 metres (10 feet) from muzzle to tail tip, and a weight of up to 90 kg (about 200 pounds). The smallest are the planigales (see marsupial mouse), especially Planigale ingrami, measuring barely 12 cm (4.7 inches) in total length. The vast majority of marsupials range from the size of a squirrel to that of a medium-size dog.

Structural and behavioral parallels with placental mammals are in some cases quite striking. Such resemblances are examples of convergent evolution, a tendency for organisms to adapt in similar ways to similar habitats. Thus, there are marsupials that look remarkably like moles, shrews, squirrels, mice, dogs, and hyenas. Others are the ecological counterparts, less in structure than in habits, of cats, small bears, and rabbits. Even the larger grazing marsupials, which resemble no placental mammal at all, can be thought of as filling the same ecological role ( niche) as deer and antelope found elsewhere. The niches (niche) that marsupials fill are closely associated with structure. The burrowing species, such as the marsupial mole and the wombats, have powerful foreclaws with which they can tunnel into the ground for food and for shelter. Terrestrial forms, such as the kangaroos and wallabies, possess well-developed hind limbs that serve both as formidable weapons and as catapults by which they can bound over the plains. The gliders (glider) have a membrane along either flank, attached to the forelegs and hind legs, that enables these arboreal animals to glide down from a high perch. A few marsupials, such as tree kangaroos, koalas, and some species of cuscus, spend most of their lives in trees. The water opossum, or yapok (Chironectes minimus), of Central and South America is semiaquatic.

The diets of marsupials are as varied as the niches they occupy. Many dasyurids live chiefly on insects and other small animals. Dunnarts (genus Sminthopsis) are so hyperactive—like shrews (shrew)—that, to supply their high energy needs, they must devour their own weight in food (chiefly insects) each day. The numbat uses its remarkable wormlike tongue to lap up termites and ants. Many Australian possums, bandicoots (bandicoot), and American opossums have a mixed diet of plant matter and insects. Wombats and many other marsupials are strictly vegetarian. The small honey possum (Tarsipes rostratus) is specialized to feed on the nectar of flowers, and other marsupials too may serve as important pollinators in this way. Few large carnivores have ever evolved in Australia, because of the low productivity of its environment. The most recent large carnivorous marsupials, the Tasmanian devil (Sarcophilus harrisii) and the now-extinct thylacine, or Tasmanian wolf (Thylacinus cynocephalus), were both displaced on the mainland by the dingo.

The life cycle of marsupials exhibits peculiarities that have long been considered primitive compared with those of placental mammals but are more likely an adaptation to low-productivity environments. The uterine cycle of the female marsupial has no secretory phase, and the uterine wall is not specialized for the implantation of the embryos, although a transitory placenta does exist in the bandicoots. The period of intrauterine development in marsupials ranges from about 12 days in the bilby (Macrotis lagotis) to 38 days in the swamp wallaby (Wallabia bicolor). The young, born in a vulnerable embryonic condition, make their own way to the shelter, warmth, and nourishment of the pouch; in pouchless marsupials the young simply cling to the teats. Those fortunate enough to survive this arduous journey may succeed in attaching themselves to the mother's nipples, which then swell and become firmly fastened—almost physically fused—to the mouth tissues of the young. In this condition the young continue their development for weeks or months, after which they are weaned and begin to look after themselves. Frequently the partially developed young outnumber the available teats, and the excess individuals perish.

250 or more species in 2 superorders further divided into 7 orders. All are found only in Australia, New Guinea, and nearby islands (introduced by humans to New Zealand) or in the Americas. Oldest fossils date to the Late Cretaceous of North America.

Sources of information on Australasian marsupials include Ronald Strahan (ed.), Mammals of Australia, rev. ed. (1998); Timothy Flannery, Mammals of New Guinea, rev. and updated (1995); Peter Menkhorst, A Field Guide to Mammals of Australia (2001); and Hugh Tyndale-Biscoe, Life of Marsupials (1973). American marsupials are covered in Grzimek's Encyclopedia of Mammals, 5 vol., trans. from German (1990), a nontechnical source of information and graphics that also contains tabular data for all species. Ronald M. Nowak, Walker's Mammals of the World, 6th ed., 2 vol. (1999), is a semitechnical work that includes black-and-white photographs of representatives of most living genera. The Amazing Marsupials (1986), written and produced by Robert Raymond, is a video documentary that portrays a variety of Australian species.

Numereous monographs are collected in Anthony K. Lee and Andrew Cockburn, Evolutionary Ecology of Marsupials (1985); and Michael Archer (ed.), Possums and Opossums: Studies in Evolution, 2 vol. (1987). Scientific articles can be found in proceedings of symposia of the Australian Mammal Society and in its journal, Australian Mammalogy (1972– ), as well as the Australian Journal of Zoology (1953– ).