This article is about mammals. For frogs, see Marsupial frog.
Temporal range: Late Jurassic[3][4] - Present, 160–0Ma
Present day distribution of marsupials.

Marsupials are an infraclass of mammals living primarily in the Southern Hemisphere. A distinctive characteristic, common to most species, is that the young are carried in a pouch. Well-known marsupials include kangaroos, wallabies, the koala, possums, opossums, wombats and the Tasmanian devil. Less well-known species of marsupials include the numbat, bandicoots, bettongs, the bilby and quolls.



The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists.[6] 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, as does most genetic and molecular evidence.[7]

The ancestors of marsupials split from those of placental mammals during the Jurassic period.[8] 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.[9] Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago (mya).[10][11][12] This makes it almost contemporary to some early eutherian fossils which have been found in the same area.[13]

The oldest metatherian fossils (Metatheria being a larger clade that groups marsupials with some of their extinct relatives) are found in present-day China.[14] About 100 million years ago (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. Metatherians 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 were disconnected 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.


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.[15] Extant native Australian terrestrial placental mammals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.[16]

Genetic analysis suggests a divergence date between the marsupials and the placentals at The ancestral number of chromosomes has been estimated to be 2n = 14.


Early development





Reproductive system

Marsupials' reproductive systems differ markedly from those of placental mammals.[20][21] The female develops a kind of yolk sac in her womb which delivers nutrients to the embryo. Embryos of some marsupials additionally form placenta-like organs that connect them to the uterine wall, although it is not certain that they transfer nutrients from the mother to the embryo.[22] Pregnancy is very short, typically 4 to 5 weeks, and the embryo is born at a very young stage of development.



Most male marsupials, except for , have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.[23][24][21][25][26][27][28][29][30][31] The penis is used only for discharging semen into females, and is separate from the urinary tract. When not erect, it is retracted into the body in an S-shaped curve.[26] Neither marsupials nor monotremes possess a baculum.[25] The shape of the glans penis varies among marsupial species.[26][32][33][34] A male koala's foreskin contains naturally occurring bacteria that play an important role in fertilization.[35]

The male thylacine had a pouch that acted as a protective sheath, covering the male's external reproductive organs while he ran through thick brush.[36]


Female marsupials 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.[37] The definitive placenta in all marsupials is generated by the yolk sac.[38] Among three fetal membranes in mammals, the yolk sac, allantois, and amnion, only the first two form a placenta.[39] The evolution of placentation in vertebrates is linked to the evolution of viviparity, a reproductive system in which the females retain their eggs to give birth to their young. Marsupials give birth at a very early stage of development (about four to five 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.[40][41] This is accomplished by accelerating forelimb and facial development in marsupials compared to placental mammals, which results in 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.






† indicates extinction

See also



Further reading

  • ISBN 0-7301-0422-2
  • Tim Flannery, Country: a continent, a scientist & a kangaroo, pages 196–200. ISBN 1-920885-76-5

External links

  • Western Australian Mammal Species