Marsupial
20205
224187905
2008-07-07T18:48:49Z
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{{otheruses4|mammals|the marsupial frogs|Gastrotheca}}
{{Taxobox
| name = Marsupials<ref name=msw3a>{{MSW3 Gardner|pages=3-21}}</ref><ref name=msw3b>{{MSW3 Groves|pages=22-70}}</ref>
| fossil_range = Middle [[Cretaceous]] - Recent
| image = Kangaroo and joey03.jpg
| image_caption = Female [[Eastern Grey Kangaroo]] with a [[Joey (marsupial)|joey]] in her pouch
| image_width = 250px
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Mammal]]ia
| subclassis = [[Theria]]
| infraclassis = '''Marsupialia'''
| infraclassis_authority = [[Johann Karl Wilhelm Illiger|Illiger]], 1811
| subdivision_ranks = [[Order (biology)|Orders]]
| subdivision =
*[[Didelphimorphia]]
*[[Paucituberculata]]
*[[Microbiotheria]]
*[[Dasyuromorphia]]
*[[Peramelemorphia]]
*[[Notoryctemorphia]]
*[[Diprotodontia]]
*†[[Sparassodonta]]
*†[[Yalkaparidontia]]
}}
'''Marsupials''' are an [[infraclass]] of [[mammal]]s, characterized by a distinctive [[Pouch (marsupial)|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{{Fact|date=June 2008}}. 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|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 [[Gondwana]]n 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, hoofed mammals and other placentals.{{Fact|date=May 2007}}
On most continents, placental mammals were much more successful and no marsupials survived, though in South America the [[Didelphimorphia|opossums]] retained a strong presence, and the [[Tertiary]] saw the genesis of marsupial predators such as the [[Borhyaenidae|borhyaenid]]s 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]].{{Fact|date=June 2008}} As a result, native Australian placental mammals (such as [[hopping mouse|hopping mice]]) are more recent immigrants.
===Reproductive system===
Marsupials reproductive systems differ markedly from those of their placental mammal cousins (''[[Eutheria|Placentalia]]''). Females have two [[vagina]]e, 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.<ref name=DaMR/> The penis is used only for discharging [[semen]] into females, and is separate from the urinary tract.{{Fact|date=June 2008}} Both sexes possess a [[cloaca]]<ref name=DaMR>[http://www.biology.iastate.edu/intop/1Australia/Australia%20papers/Discoveries%20about%20Marsupial%20Rep Iowa State University Biology Dept. Discoveries about Marsupial Reproduction Anna King 2001. webpage] (note shows code, html extension omitted)</ref>, 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 (anatomy)|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: [[the Americas|American]] marsupials and the [[Australia]]n marsupials.<ref name=msw3a/><ref name=msw3b/> 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 ''[[opossum]]s'' is properly used to refer to the American species (though ''possum'' is a common diminutive), while similar Australian species are properly called ''[[possum]]s''.
[[Image:Koala climbing tree.jpg|thumb|250px|The [[Koala]] (''Phascolarctos cinereus'')]]
[[Image:Brushtail possum.jpg|thumb|right|250px|The [[Common Brushtail Possum]] (''Trichosurus vulpecula'')]]
[[Image:Sugar-Glider-at-Lone-Pine.jpg|thumb|right|250px|The [[Sugar Glider]] (''Petaurus breviceps'')]]
*Superorder [[Ameridelphia]]
** Order [[Didelphimorphia]] (93 species)
*** Family [[Didelphidae]]: opossums
** Order [[Paucituberculata]] (6 species)
*** Family [[Caenolestidae]]: [[shrew opossum]]s
*Superorder [[Australidelphia]]
** Order †[[Yalkaparidontia]]
** Order [[Microbiotheria]] (1 species)
*** Family [[Microbiotheriidae]]: [[Monito del Monte]]
** Order [[Dasyuromorphia]] (71 species)
*** Family †[[Thylacinidae]]: [[Thylacine]]
*** Family [[Dasyuridae]]: [[antechinus]]es, [[quoll]]s, [[dunnart]]s, [[Tasmanian Devil]], and relatives
*** Family [[Myrmecobiidae]]: [[Numbat]]
** Order [[Peramelemorphia]] (24 species)
*** Family [[Thylacomyidae]]: [[bilby|bilbies]]
*** Family †[[Chaeropodidae]]: [[Pig-footed Bandicoot]]
*** Family [[Peramelidae]]: [[bandicoot]]s and allies
** Order [[Notoryctemorphia]] (2 species)
*** Family [[Notoryctidae]]: marsupial moles
** Order [[Diprotodontia]] (137 species)
*** Family [[Phascolarctidae]]: [[Koala]]
*** Family [[Vombatidae]]: [[wombat]]s
*** 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]]
<small>† indicates [[extinction]]</small>
==See also==
* [[Metatheria]]
== References ==
{{reflist}}
{{Nofootnotes|date=February 2008}}
* [[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 p. 573-578
==External links==
{{sisterlinks|Marsupial}}
{{Wikispecies|Marsupialia}}
{{Wikibookspar|Dichotomous Key|Marsupialia}}
*[http://www.naturalworlds.org/marsupialring/index.htm The Marsupial Ring]
*[http://members.iinet.net.au/~foconnor/mammals/mammals.htm Western Australian Mammal Species]
*[http://www.genome.gov/25521146 Researchers Publish First Marsupial Genome Sequence] The National Institutes of Health May 2007
*[http://www.the-scientist.com/news/home/53187/ First marsupial genome released. Most differences between the opossom and placental mammals stem from non-coding DNA]
{{mammals}}
[[Category:Marsupials| ]]
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