Marsupial

Marsupials are mammals in which the female typically has a pouch (called the marsupium, from which the name 'Marsupial' derives) in which it rears its young through early infancy. They differ from placental mammals (Placentalia) in their reproductive traits. The female has two vaginae, both of which open externally through one orifice but lead to different compartments within the uterus. Males usually have a two-pronged penis which corresponds to the females' two vaginae. The penis is used only for discharging semen into females, and is separate from the urinary tract. Marsupials have a cloaca that is connected to a urogenital sac in both sexes. Waste is stored there before expulsion. The pregnant female develops a kind of yolk sac in her womb which delivers nutrients to the embryo. The embryo is born at a very early stage of development (at about 4-5 weeks), upon which it crawls up its mother's belly and attaches itself to a nipple (which is located inside the pouch). It remains attached to the nipple for a number of weeks. The offspring later passes through a stage where it temporarily leaves the pouch, returning for warmth and nourishment.

History
Fossil evidence, first announced by researcher M.J. Spechtt in 1982, does not support the once-common belief that marsupials were a primitive forerunner of the placental mammals: both main branches of the mammal tree appear to have evolved at around the same time, toward the end of the Mesozoic era. The earliest known marsupial is Sinodelphys szalayi, which lived around 125 million years ago. It was discovered in China and is of an age similar to the earliest placental fossils, which have been found in the same area.

There have been various ideas about the early evolution of marsupials. Some scientists believe that the marsupials evolved in North America and dispersed from there, via Europe, to Asia and Africa. They would have also reached South America before this became an island continent. This theory suggests that marsupials passed from South America, through Antarctica, to Australia (via Gondwanan land connections), which was already occupied by placentals. Another theory is that marsupials evolved in Australia and travelled, via Antarctica and South America to North America. The discovery of Chinese marsupials also resurrects the idea that marsupials reached Australia via southeast Asia. The problem with this idea is that marsupial fossils found in New Guinea are younger than those in Australia. There are a few species of marsupials living in Asia, especially in Sulawesi (part of Indonesia). These marsupials exist with primates, hoofed mammals and other placentals.

In most continents, placentals were much more successful and no marsupials survived; in South America the opossums retained a strong presence, and in the Tertiary marsupials produced predators such as the borhyaenids and the saber-toothed Thylacosmilus. In Australia placental mammals were displaced by marsupials which have since dominated. Marsupial success in Australia has been attributed to their metabolic rates, which are lower than placentals'. As a result native Australian placental mammals are more recent immigrants (e.g., the hopping mice).

Description
The early birth of marsupials removes the developing young much sooner than in placental mammals, and marsupials have not needed to develop a complex placenta to protect the young from its mother's immune system. Early birth places the tiny newborn marsupial at greater risk, but significantly reduces the risks associated with pregnancy, as there is no need to carry a large fetus to full-term in bad seasons.

Because a newborn marsupial must climb up to its mother's nipples, the otherwise minimally developed newborn has front limbs that are much better developed than the rest of its body. This requirement is perhaps responsible for the more limited range of locomotory adaptations in marsupials than placentals; marsupials must develop a grasping forepaw during their early youth, making it more difficult to develop it into a hoof, wing, or flipper as some groups of placental mammals have done.

There are about 334 species of marsupials, over 200 of them native to Australia and nearby islands to the north. There are also 100 extant American species, mostly in South America but also, as a result of the Great American Interchange, 13 species in Central America, and one (the Virginia Opossum) in North America.

Taxonomy
In taxonomy, there are two primary divisions of Marsupialia: American marsupials and the Australian marsupials. The Order Microbiotheria (which has only one species, the Monito del Monte) is found in South America but is believed to be more closely related to the Australian marsupials. There are many small arboreal species in each group. The term opossums is properly used to refer to the American species (though possum is a common diminutive), while similar Australian species are properly called possums.


 * Superorder Ameridelphia
 * Order Didelphimorphia (93 species)
 * Family Didelphidae: opossums
 * Order Paucituberculata (6 species)
 * Family Caenolestidae: shrew opossums
 * Superorder Australidelphia
 * Order †Yalkaparidontia
 * Order Microbiotheria (1 species)
 * Family Microbiotheriidae: Monito del Monte
 * Order Dasyuromorphia (71 species)
 * Family †Thylacinidae: Thylacine
 * Family Dasyuridae: antechinuses, quolls, dunnarts, Tasmanian Devil, and relatives
 * Family Myrmecobiidae: Numbat
 * Order Peramelemorphia (24 species)
 * Family Thylacomyidae: bilbies
 * Family †Chaeropodidae: Pig-footed Bandicoot
 * Family Peramelidae: bandicoots and allies
 * Order Notoryctemorphia (2 species)
 * Family Notoryctidae: marsupial moles
 * Order Diprotodontia (137 species)
 * Family Phascolarctidae: Koala
 * Family Vombatidae: wombats
 * Family †Diprotodontidae: diprotodon
 * Family Phalangeridae: brushtail possums and cuscuses
 * Family Burramyidae: pygmy possums
 * Family Tarsipedidae: Honey Possum
 * Family Petauridae: Striped Possum, Leadbeater's Possum, Yellow-bellied Glider, Sugar Glider, Mahogany Glider, Squirrel Glider
 * Family Pseudocheiridae: ringtailed possums and relatives
 * Family Potoridae: potoroos, rat kangaroos, bettongs
 * Family Acrobatidae: Feathertail Glider and Feather-tailed Possum
 * Family Hypsiprymnodontidae: Musky Rat-kangaroo
 * Family Macropodidae: kangaroos, wallabies, and relatives
 * Family †Thylacoleonidae: marsupial lions
 * Order †Sparassodonta

† indicates extinction