One of the defining features of marsupials is their unique reproductive strategy, where the young are born in a relatively undeveloped state and then nurtured within a pouch on their mother's abdomen.

Living marsupials encompass a wide range of species, including kangaroos, koalas, opossums, possums, Tasmanian devils, wombats, wallabies, and bandicoots, among others.

Presently, close to 70% of the 334 extant species of marsupials are concentrated on the Australian continent, including mainland Australia, Tasmania, New Guinea, and nearby islands.

Ossified patellae are absent in most modern marsupials (though a small number of exceptions are reported)[6] and epipubic bones are present.

Marsupials also have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension (processus angularis) is bent inward toward the center.

Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs.

Many marsupials have a permanent bag, whereas in others the pouch develops during gestation, as with the shrew opossum, where the young are hidden only by skin folds or in the fur of the mother.

The largest living marsupial, the red kangaroo, grows up to 1.8 metres (5 ft 11 in) in height and 90 kilograms (200 lb) in weight, but extinct genera, such as Diprotodon, were significantly larger and heavier.

[8][26][27][28] The male thylacine had a pouch that acted as a protective sheath, covering his external reproductive organs while running through thick brush.

[42] The joey is born in an essentially fetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns with sizes ranging from 4g to over 800g.

[43] Despite the lack of development it crawls across its mother's fur to make its way into the pouch, which acts like an external womb,[44] where it latches onto a teat for food.

It will not re-emerge for several months, during which time it is fully reliant on its mother's milk for essential nutrients, growth factors and immunological defence.

Though early birth puts the tiny newborn marsupial at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to a full term in bad seasons.

[38][39] Newborn marsupials must climb up to their mother's teats and their front limbs and facial structures are much more developed than the rest of their bodies at the time of birth.

Marsupials must develop grasping forepaws during their early youth, making the evolutive transition from these limbs into hooves, wings, or flippers, as some groups of placentals have done, more difficult.

Joeys are born with "oral shields", which consist of soft tissue that reduces the mouth opening to a round hole just large enough to accept the mother's teat.

Vicente Yáñez Pinzón, commander of the Niña on Christopher Columbus' first voyage in the late fifteenth century, collected a female opossum with young in her pouch off the South American coast.

António Galvão, a Portuguese administrator in Ternate (1536–1540), wrote a detailed account of the northern common cuscus (Phalanger orientalis):[56] Some animals resemble ferrets, only a little bigger.

On their belly they have a pocket like an intermediate balcony; as soon as they give birth to a young one, they grow it inside there at a teat until it does not need nursing anymore.

For instance, a 1606 record of an animal, killed on the southern coast of New Guinea, described it as "in the shape of a dog, smaller than a greyhound", with a snakelike "bare scaly tail" and hanging testicles.

This description appears to closely resemble the dusky pademelon (Thylogale brunii), in which case this would be the earliest European record of a member of the kangaroo family (Macropodidae).

[58][56] Marsupials are taxonomically identified as members of mammalian infraclass Marsupialia, first described as a family under the order Pollicata by German zoologist Johann Karl Wilhelm Illiger in his 1811 work Prodromus Systematis Mammalium et Avium.

[63] Though the order Microbiotheria (which has only one species, the monito del monte) is found in South America, morphological similarities suggest it is closely related to Australian marsupials.

[79] Metatherians were widespread in North America and Asia during the Late Cretaceous, but suffered a severe decline during the end-Cretaceous extinction event.

[80] Cladogram from Wilson et al. (2016)[81] Holoclemensia Pappotherium Sulestes Oklatheridium Tsagandelta Lotheridium Deltatheroides Deltatheridium Nanocuris Atokatheridium Gurlin Tsav skull Borhyaenidae Mayulestes Jaskhadelphys Andinodelphys Pucadelphys Asiatherium Iugomortiferum Kokopellia Aenigmadelphys Anchistodelphys Glasbius Pediomys Pariadens Eodelphis Didelphodon Turgidodon Alphadon Albertatherium Marsupialia In 2022, a study provided strong evidence that the earliest known marsupial was Deltatheridium known from specimens from the Campanian age of the Late Cretaceous in Mongolia.

[83][84][85] Northern Hemisphere metatherians, which were of low morphological and species diversity compared to contemporary placental mammals, eventually became extinct during the Miocene epoch.

Sparassodonts disappeared for unclear reasons – again, this has classically assumed as competition from carnivoran placentals, but the last sparassodonts co-existed with a few small carnivorans like procyonids and canines, and disappeared long before the arrival of macropredatory forms like felines,[88] while didelphimorphs (opossums) invaded Central America, with the Virginia opossum reaching as far north as Canada.

[n 1][n 2] This suggests a single dispersion event of just one species, most likely a relative to South America's monito del monte (a microbiothere, the only New World australidelphian).

In Australia, marsupials radiated into the wide variety seen today, including not only omnivorous and carnivorous forms such as were present in South America, but also into large herbivores.

[69][70] In Australia, terrestrial placentals disappeared early in the Cenozoic (their most recent known fossils being 55 million-year-old teeth resembling those of condylarths) for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem.