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

Extant marsupials encompass many species, icluding kangaroos, koalas, opossums, possums, Tasmanian devils, wombats, wallabies, and bandicoots.

The evolutionary split between placentals and marsupials occurred 125-160 million years ago, in the Middle Jurassic-Early Cretaceous period.

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

Marsupials have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension (processus angularis) is bent 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, while in others such as the shrew opossum the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur.

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.

[18] The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.

Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase their chances of reaching the marsupium.

Key aspects of the first stages of placental embryo development, such as the inner cell mass and the process of compaction, are not found in marsupials.

[39] The baby (joey) is born in a fetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns: sizes range from 4-800g+.

It does not emerge for several months, during which time it relies on its mother's milk for essential nutrients, growth factors and immunological defence.

Though early birth puts the newborn at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as the fetus cannot compromise the mother in bad seasons.

Marsupials must develop grasping forepaws early, complicating the evolutive transition from these limbs into hooves, wings, or flippers.

Joeys are born with "oral shields", soft tissue that reduces the mouth opening to a round hole just large enough to accept the 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.

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.

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), the earliest European record of a member of the Macropodidae.

[55][53] 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.

[60] 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.

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

[77] Cladogram from Wilson et al. (2016)[78] 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.

[80][81][82] 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,[85] 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).

The journey must not have been easy; South American ungulate[89][90][91] and xenarthran[92] remains have been found in Antarctica, but these groups did not reach Australia.

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.

[66][67] 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.