Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central America, and one in North America, north of Mexico.

History
The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists. Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes. Most genetic and molecular evidence also supports grouping marsupials and placental mammals as a single clade, subclass Theria.

Marsupials and placental mammals split from the monotremes during the Cretaceous Period. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs. Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago (mya). This makes it almost contemporary to the earliest eutherian fossils, which have been found in the same area.

The oldest metatherian fossils (Metatheria being a larger clade that groups marsupials with some of their extinct relatives) are found in present-day China, and there are a few species of marsupials presently living in Indonesia as far west as Sulawesi, which is sometimes considered to be in an Asian ecozone. However, these modern marsupials appear to be have reached the islands relatively recently via Australia. About 100 mya, the supercontinent Pangaea was in the process of splitting into the northern continent Laurasia and the southern continent Gondwana, with what would become China and Australia already separated by the Tethys Ocean. Marsupials spread westward into modern North America (still attached to Eurasia) and then to South America, which was connected to North America until around 65 mya. Laurasian marsupials eventually died off, possibly due to competition from placental mammals for their ecological niches.

In South America, the opossums retained a strong presence, and the Tertiary saw the evolution of shrew opossums (Paucituberculata) and metatherian predators such as the borhyaenids and the saber-toothed Thylacosmilus. South American niches for mammalian carnivores were dominated by these marsupial and sparassodont metatherians. While placental predators were absent, the metatherians did have to contend with avian (terror bird) and terrestrial crocodilian competition. South America and Antarctica remained connected until 35 mya, as shown by the unique fossils found there. North and South America remained disjointed until about three million years ago, when the Isthmus of Panama formed. This led to the Great American Interchange. Competition from placental mammals from the north drove sparassodonts to extinction, while didelphimorphs (opossums) invaded Central America, with the Virginia opossum reaching as far north as Canada.

Marsupials reached Australia via Antarctica about 50 mya, shortly after Australia had split off. 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). This progenitor may have rafted across the widening, but still narrow, gap between Australia and Antarctica. In Australia, they radiated into the wide variety we see today, island hopping some way through the Indonesian archipelago. A 2010 analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials. This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off.

In Australia, terrestrial placental mammals 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. Extant native Australian terrestrial placental mammals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from southeast Asia.

Early development
An infant marsupial is known as a joey. Marsupials have an extremely short gestation period (about 4–5 weeks), and the joey is 'born' essentially in a fetal state. The blind, furless, miniature newborn, the size of a jelly bean, crawls across its mother's fur to make its way into the pouch, where it latches onto a teat for food. It will not re-emerge for several months, during which time it develops fully. After this period, the joey begins to spend increasing lengths of time out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens it will seek refuge in its mother's pouch for safety.

Joeys stay in the pouch for up to a year in some species, or until the next joey is born. A marsupial joey is unable to regulate its own body temperature, and thus relies upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature between 30 - 32 C must be constantly maintained.

An early birth removes a developing marsupial from its parent's body much sooner than in placental mammals, and thus marsupials have not developed a complex placenta to protect the embryo from its mother's immune system. Though early birth places the tiny newborn marsupial at a greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to full-term in bad seasons.

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. 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.

Some common structural features can be found among marsupials. Ossified patellae are absent. Marsupials (and also monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemisphere.

Reproductive system
Marsupials' reproductive systems differ markedly from those of placental mammals (Placentalia). Females have two lateral vaginas, which lead to separate uteri but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent. The males generally have a two-pronged penis, which corresponds to the females' two vaginas. The penis is used only for discharging semen into females, and there is instead a urogenital sac used to store waste before expulsion.

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 on the underside of the mother either inside a pouch called the marsupium or open to the environment. To crawl to the nipple and attach to it, the marsupial must have well developed forelimbs and facial structures. This is accomplished by accelerating forelimb and facial development in marsupials compared to placental mammals. As a result, there is decelerated development of such structures as the hindlimb and brain. 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, protection and nourishment.

Taxonomy
Taxonomically, 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.


 * Order †Sparassodonta (formerly viewed as marsupials, now as a sister group of metatherians)
 * Superorder Ameridelphia
 * Order Didelphimorphia (93 species)
 * Family Didelphidae: opossums
 * Order Paucituberculata (6 species)
 * Family Caenolestidae: shrew opossums
 * Superorder Australidelphia
 * Order Microbiotheria (1 species)
 * Family Microbiotheriidae: monito del monte
 * Order †Yalkaparidontia
 * 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

† indicates extinction