Marsupial

Marsupials[1][2] Fossil range: Early Cretaceous–Recent PreЄ Є O S D C P T J K Pg N
Female Eastern Grey Kangaroo with a joey in her pouch
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Infraclass:	Marsupialia Illiger, 1811
Orders
Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia †Sparassodonta †Yalkaparidontia

Present day distribution of marsupials. Distribution shown in blue.

Marsupials are an infraclass of mammals, characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy. In modern times, they are well-known for being the dominant group of mammals in Australia, though there are also a number of species found in the Americas, as well as on the island of New Guinea.

History

Isolated petrosals of Djarthia murgonensis, Australia's oldest marsupial fossils^[3]

It was once commonly believed that marsupials were a primitive forerunner of modern placental mammals, but fossil evidence conflicts with this assumption. Instead, both main branches of the mammal tree appear to have evolved concurrently toward the end of the Mesozoic era. 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.^[4]

Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago.^[5][6][7] This makes it almost contemporary to the earliest placental fossils, which have been found in the same area.^[8]

The discovery of ancestral marsupials in China might at first appear to support the idea that marsupials reached Australia via Southeast Asia.^[9] 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. These marsupials coexist with primates, hooved mammals and other placentals. However, due to the fact that in the early Cretaceous Australia and China were attached to southern continent of Gondwana and the northern continent of Laurasia, respectively, and separated by the wide Tethys Sea, the marsupial migration had to take a much more circuitous route. From their origin in east Laurasia (modern day China), they spread westwards into modern North America (still attached to Eurasia) and skipped across to South America, which was connected to North America up until around 65 MYA. Here they radiated into borhyaenids and shrew opossums, creating a unique fauna found in South America and Antarctica (which were connected until 35 MYA). Marsupials reached Australia via Antarctica about 50 MYA just after Australia had split off, suggesting a single dispersion event of just one species, related 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 Antarctic. In Australia, being the only mammals present (except for a few austrosphenids like echidnas and platypuses), they radiated into the wide variety we see today, island hopping some way through the Indonesian archipelago to almost complete a circumnavigation back to their homeland in China.^[10]

On the interconnected land masses of Afro-Eurasia and North America, placental mammals were much more successful and no marsupials survived. However, things were different on the isolated continental fragments of the former Gondwana.

In South America, the opossums retained a strong presence, and the Tertiary saw the genesis of 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. When the Isthmus of Panama rose and bridged North and South America three million years ago, South America's isolation finally came to an end. In the faunal exchange that followed, placental mammals from the north flooded the continent, causing both the sparassodonts and terror birds to go extinct. At the same time, however, a number of didelphimorphs invaded Central America, with one species reaching as far north as Canada.

In Australia, 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 given Australia's characteristic low fertility and aridity. As a result, extant native Australian placental mammals (such as hopping mice) are more recent immigrants.

A 2010 analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed that all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position (which is also consistent with morphological comparisons), followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials (whose relative branching order was not revealed). This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off.^[11][12]

Description

Koala
(Phascolarctos cinereus)

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.

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).

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.^[13]

Reproductive system

Marsupials' reproductive systems differ markedly from those of placental mammals (Placentalia). Females have two lateral vaginas, which lead to separate uteruses, 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.^[13] The males generally have a two-pronged penis, which corresponds to the females' two vaginas.^[14] 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.^[15][16] 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

Sugar Glider (Petaurus breviceps)

Common Brushtail Possum (Trichosurus vulpecula)

Squirrel Glider
(Petaurus norfolcensis)

Virginia Opossum (Didelphis virginiana), the only North American marsupial north of Mexico

Thylacine (Thylacinus cynocephalus) an extinct carnivorous marsupial found in Tasmania until the 1930s

Taxonomically, there are two primary divisions of Marsupialia: American marsupials and the Australian marsupials.^[1][2] 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

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 Journal of Zoology 1997 V. 45 pp. 573–578

External links

• Data related to Marsupialia at Wikispecies

• Marsupialia at Wikibooks.
• 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