Marsupials (Metatherians) are one of the three groups of mammals living on the earth today; the others are the placental mammals (Eutherians) and the monotremes (Prototherians).
Notable differences between marsupials and placental mammals are:
Young marsupials are born very early in their development
and make a perilous journey from the birth canal to the teats
of the mammary glands, from which they suckle milk. Many marsupials,
like kangaroos, have a well-defined pouch covering the teats and
in which they carry their young. Some species only have a flap of skin
to cover the teats.
Marsupials are believed to have diverged from placental mammals about 90
million years ago. Since then the two groups of mammals have
evolved independently of one another. Marsupials used to occur
throughout the world, but now only inhabit a few regions.
The two maps below show the distribution of marsupials
in the Late Cretaceous amd their present distribution. Most marsupials
occur in Australasia, although some inhabit
Although many marsupial species are now endangered due to the
destruction of their habits and the introduction of exotic predators,
populations of certain species have increased to the extent
where they are now serious pests.
There is still no universal agreement on the classification of the
marsupials. The 18 families of extant marsupials have recently been
placed into two cohorts: Cohort Ameridelphia (the American marsupials)
and Cohort Australidelphia (the Australian marsupials).