Marsupials are mammals in which the female typically has a pouch
(called the marsupium, from which the name 'Marsupial' derives)
in which it rears its young through early infancy. They differ from
placental mammals (Placentalia) in their reproductive traits. The
female has two vaginas, both of which open externally through one
orifice but lead to different compartments within the uterus. Males
usually have a two-pronged penis which corresponds to the females'
two vaginas. The penis only passes sperm. Marsupials have a cloaca
  that is connected to a urogenital sac in both sexes. Waste
is stored there before expulsion. The pregnant female develops a
kind of yolk sack in her womb which delivers nutrients to the embryo.
The embryo is born at a very early stage of development (at about
4-5 weeks), upon which it crawls up its mother's belly and attaches
itself to a nipple. It remains attached to the nipple for a number
of weeks. The offspring later passes through a stage where it temporarily
leaves the pouch, returning for warmth and nourishment.
Fossil evidence, first announced by researcher M.J. Spechtt in
1982, 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, and in the Tertiary marsupials produced predators
such as the borhyaenids and the saber-toothed Thylacosmilus. In
Australia placental mammals were not present throughout much of
the Tertiary and marsupials and monotremes dominated completely.
Native Australian placental mammals are more recent immigrants (e.g.,
the hopping mice).
The early birth of marsupials removes the developing young much
sooner than in placental mammals, and marsupials have not needed
to develop a complex placenta to protect the young from its mother's
immune system. 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 fetus to full-term
in bad seasons.
Because a newborn marsupial must climb up to its mother's nipples,
the otherwise minimally developed newborn has front limbs that are
much better developed than the rest of its body. This requirement
is responsible for the more limited range of locomotory adaptations
in marsupials than placentals; marsupials must retain a grasping
forepaw and cannot develop it into a hoof, wing, or flipper as some
groups of placental mammals have done.
There are about 334 species of marsupials, over 200 of them native
to Australia and nearby islands to the north. There are also many
extant species in South America and one species, the Virginia Opossum,
native to North America. Most marsupials are slow moving creatures
but kangaroos can reach speeds of up to 31mph (50km/h).
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