Marsupial

BIRD: linking the biodiversity community

Category: Marsupials

Marsupials make up one of the two major groups within the mammal family tree. They are most readily distinguished from placental mammals by the presence (in most species) of the marsupium or pouch, within which the female rears its young.

Fossil evidence does not support the once-common belief that marsupials were a primitive forerunner of the placental mammals: both main branches of the mammal tree appear to have evolved at around the same time, toward the end of the Mesozoic era, and have been competitors since that time. In most continents, placentals were much more successful and no marsupials survived; in South America the opossums retained a strong presence; in Australia's harsh climate the placentals died out and only marsupials and monotremes survived.

It is understood (although not certain) that the first marsupials evolved in present-day North America, and spread from there to occupy most of the world. At about the same time, placental mammals also arose and spread. (It is not known where placentals first developed. It used to be assumed that it was somewhere in the northern hemisphere, but recent evidence suggests that this may not be so.)

Many of the most obvious differences between marsupials and placentals concern the reproductive arrangements. Female marsupials have two vaginas, each leading to a different compartment in the uterus; males typically have a two-pronged penis. The marsupial placenta is simple: with birth taking place at a very early stage of development, there is no need to protect developing young from the mother's immune system in the way that placental mammals must.

After birth, the tiny newborn marsupial crawls to the pouch and finds a nipple to attach itself to, usually becoming fused to that nipple for some weeks. The front limbs develop early to aid the journey. This need for strong, grasping forelimbs in infancy limits the possible range of adaptations, meaning that marsupials cannot develop specialised forelimbs in the way that some placentals have: wings, hooves, and flippers are out of the question.

After leaving the pouch for the first time, the offspring typically returns for warmth, sleep and nourishment until it is well advanced. Early birth places the tiny new-born marsupial at greater risk, but significantly reduces the risks associated with pregnancy, as there is no need to carry a large foetus to full-term in bad seasons.

There are between 260 and 280 modern marsupial species, almost 200 of them native to Australia and nearby islands to the north. Nearly all the remaining species are South American. The solitary modern North American marsupial, the Virginia Opossum, is a relatively recent addition: it arrived from South America just a few million years ago.

Present-day marsupials are classified in two primary subdivisions: the American group (superorder Ameridelphia) which contains all except one of the New World species, and the Australasian group (superorder Australidelphia) which contains all others, including a single South American species.

• CLASS MAMMALIA
• Subclass: Monotremata monotremes
• Subclass: Marsupialia
  • Superorder Ameridelphia
  • Order Didelphimorphia (American opossums, 63 species in one family)
  • Order Paucituberculata (American shrew opossums, 6 species in one family)
  • Superorder Australidelphia
  • Order Microbiotheria (1 species)
    • Family Microbiotheriidae: Monito del Monte
  • Order Dasyuromorphia (63 species)
    • Family Thylacinidae: Thylacine extinct
    • Family Dasyuridae: marsupial carnivores
    • Family Myrmecobiidae: Numbat
  • Order Peramelemorphia (21 species)
    • Family Peroryctidae rainforest bandicoots
    • Family Peramelidae: bandicoots and bilbies
  • Order Notoryctemorphia (2 species)
    • Family Notoryctidae: marsupial moles
  • Order Diprotodontia (117 species)
    • Family Phascolarctidae: Koala
    • Family Vombatidae: wombats
    • Family Phalangeridae: brushtail possums and cuscuses
    • Family Burramyidae: pygmy possums
    • Family Tarsipedidae: Honey Possum
    • Family Petauridae: gliders and allies
    • Family Pseudocheiridae: ringtailed possums and relatives
    • Family Potoroidae: potoroos, rat kangaroos, bettongs
    • Family Acrobatidae: Feathertail Glider
    • Family Hypsiprymnodontidae: Musky Rat Kangaroo
    • Family Macropodidae: kangaroos, wallabies, and relatives
• Subclass: Placentalia placental mammals