marsupial, member of the order Marsupialia, or pouched mammals. With the exception of the New World opossums and an obscure S American family (Caenolestidae), marsupials are now found only in Australia, Tasmania, New Guinea, and a few adjacent islands. They are generally distinguished from placental mammals by the absence of a placenta connecting the embryo with its mother, although in a few forms the female has a rudimentary placenta that functions for a short time. The embryo is nourished during its brief gestation by a fluid secreted by the mother's uterus. The young are born in a very undeveloped state; at birth the great gray kangaroo is about 1 in. (2.5 cm) long and the opossum about 11/2 in. (3.8 cm) long. Immediately after birth the young crawl to the mother's nipples and remain attached to them while continuing their development. As they are still too helpless to suckle, milk is squirted into them by the periodic contraction of muscles over the mother's mammary glands. In nearly all marsupials the female's nipples are covered by a pouch, or marsupium, formed by a fold of abdominal skin. Even after the suckling stage the young return at times to the pouch for shelter and transportation. In many species the young are carried on the mother's back after the suckling stage. In addition to having a less efficient reproductive system than the placental mammals, marsupials are of generally lower intelligence. They were once widespread over the earth, but were displaced in most regions as the more successful placental mammals evolved. The Australian region, which has been isolated from contact with other regions since the Cretaceous period, had almost no native placental mammals, and the marsupials were able to continue their evolution there without competition. They underwent an adaptive radiation in Australia comparable to that of placental mammals in the rest of the world, evolving many forms that superficially resemble various placental mammals and fill the same ecological niches. Thus, there are animals known as Tasmanian wolves (see thylacine), marsupial moles, marsupial mice, and native cats (see dasyure), which live very much like the correspondingly named placental mammals and, in many cases, are strikingly similar in appearance. See bandicoot, numbat, phalanger, Tasmanian devil, wombat.

Any mammal of the infraclass Marsupialia, characterized by premature birth and continued development outside the womb. The young remain attached to the mother's teats for a period corresponding to the late stages of fetal development of a placental mammal. More than 170 species (e.g., bandicoots, kangaroos, koalas, wombats) are found in Australia, New Guinea, and nearby islands. About 65 species of opossum occur in the Americas and seven species of ratlike marsupials in South America. Many species have a pouch (marsupium), a fold of skin covering the nipples on the mother's lower belly, where the young continue their development.

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Marsupials are an infraclass of mammals, characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy.

History

It was once commonly believed that marsupials were a primitive forerunner of modern placental mammals, but fossil evidence, first presented by researcher M.J. Spechtt in 1982, conflicts with this assumption. Instead, both main branches of the mammal tree appear to have evolved concurrently toward the end of the Mesozoic era. The earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago. This makes it almost contemporary to the earliest placental fossils, which have been found in the same area.

Some scientists believe that the marsupials evolved in North America and dispersed from there, via Europe, to Asia and Africa. This diaspora would have also reached South America before it became an island continent. This theory suggests that marsupials passed from South America through Antarctica to Australia (via Gondwanan land connections), a continent already occupied by placental mammals. Alternately, another theory posits that marsupials originated in Australia and traveled, via Antarctica and South America, to North America.

The discovery of Chinese marsupials appears to support the idea that marsupials reached Australia via Southeast Asia. However, marsupial fossils found in New Guinea are younger than those in Australia, evidence which presents a problem for this theory. There are a few species of marsupials still living in Asia, especially in the Sulawesi region of Indonesia. These marsupials coexist with primates, hooved mammals and other placentals.

On most continents, placental mammals were much more successful and no marsupials survived, though in South America the opossums retained a strong presence, and the Tertiary saw the genesis of marsupial predators such as the borhyaenids and the saber-toothed Thylacosmilus. In Australia, however, marsupials displaced placental mammals entirely, and have since dominated the Australian ecosystem. Marsupial success over placental mammals in Australia has been attributed to their comparatively low metabolic rate, a trait which would prove helpful in the hot Australian climate. As a result, native Australian placental mammals (such as hopping mice) are more recent immigrants.

Reproductive system

Marsupials reproductive systems differ markedly from those of their placental mammal cousins (Placentalia). Females have two vaginas, both of which open externally through one orifice but lead to different compartments within the uterus. Males generally 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. Both sexes possess a cloaca, which is connected to 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 inside the marsupium. 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 and nourishment.

Description

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. All 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.

There are about 334 species of marsupial, and over 200 are native to Australia and neighboring northern islands. There are also 100 extant American species; these are centered mostly in South America, but the Great American Interchange has provided Central America with 13 species, and North America with one (the Virginia Opossum).

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

See also

• Metatheria

References

• Tim Flannery (1994),The Future Eaters: An Ecological History of the Australasian Lands and People, pages 67–75. ISBN 0-8021-3943-4 ISBN 0-7301-0422-2
• Tim Flannery, Country: a continent, a scientist & a kangaroo, pages 196–200. ISBN 1-920885-76-5
• Austin, C.R. ed. Reproduction in Mammals. Melbourne: Cambridge University Press,1982.
• Bronson, F. H. Mammalian Reproductive Biology. Chicago: University of Chicago Press, 1989.
• Dawson, Terrence J. Kangaroos: Biology of Largest Marsupials. New York: Cornell University Press, 1995.
• Frith, H. J. and J. H. Calaby. Kangaroos. New York: Humanities Press, 1969.
• Gould, Edwin and George McKay. Encyclopedia of Mammals. San Diego: Academic Press, 1998.
• Hunsaker, Don. The Biology of Marsupials. New York: Academic Press, 1977.
• Johnson, Martin H. and Barry J. Everitt. Essential Reproduction. Boston: Blackwell Scientific Publications, 1984.
• Knobill, Ernst and Jimmy D. Neill ed. Encyclopedia of Reproduction. V. 3 New York: Academic Press, 1998
• McCullough, Dale R. and Yvette McCullough. Kangaroos in Outback Australia: Comparative Ecology and Behavior of Three Coexisting Species. New York: Columbia University Press, 2000.
• Taylor, Andrea C. and Paul Sunnucks. Sex of Pouch Young Related to Maternal Weight in Macropus eugeni and M. parma. Australian Jounal of Zoology 1997 V. 45 pp. 573–578

External links

• The Marsupial Ring
• Western Australian Mammal Species
• Researchers Publish First Marsupial Genome Sequence The National Institutes of Health May 2007
• First marsupial genome released. Most differences between the opossom and placental mammals stem from non-coding DNA