The marsupials and placentals diverged from a pantotheran stem stock in the late Cretaceous. The first marsupials appeared in North America approximately 80 million years ago, e.g.
Marsupials
Marsupials do not produce a shelled egg. The egg, which is poorly supplied with yolk, is retained for a time within the reproductive tract of the mother. The embryo penetrates the wall of the uterus.
marsupials Pouched mammals. The young develop internally, but are born while in an embryonic state and remain in a pouch on the mother's abdomen until development is complete; this group includes kangaroos, koalas, and opossums.
The Marsupials
The marsupial mammals occupy Australia, and dither from placental mammals because they bear their young inside a pouch. Long ago, the land mass of Earth consisted of one single continent, Pangaea, where all animals existed.
[edit] Marsupials
Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals.
Subclass Metatheria: Marsupials (such as the koala, opossum, and kangaroo) are born while in an embryonic stage and finish development outside the mother's body, often in a pouch.
There are many more marsupials than monotremes. Kangaroos, koalas, bandicoots, and possums. You'll find a lot of them in Australia. Australia is an island continent. Because of its isolation, placental mammals didn't take hold in their ecosystems.
In marsupials, however, the paternal X chromosome is always inactivated. Inactivation is achieved by methylation of the X chromosome DNA, a common way in which the cell silences particular genes.
A subclass of the Class Mammalia (others are monotremes and marsupials). Embryo and fetus are nourished by a placenta.
Another interesting phenomenon is called convergent evolution whereby different species of different ancestry come to resemble one another closely because of adaptation to similar environments. The marsupials of Australia resemble placental mammals ...
The evolution from a relatively primitive and unspecialized type of organism to several divergent forms specialized to fit numerous distinct and diverse ways of life, as occurred in Darwin's finches or the marsupials of Australia.
A mammal that gives live birth to young that have gestated for only a short period of time. The young usually crawl into a pouch (the marsupium) or protected area and attach to their mother's teat to finish developing. Examples of marsupials include ...
cells from having twice as many gene products from the X chromosomes as males, one copy of the X chromosome in each female cell is inactivated. In placental mammals, the choice of which X chromosome is inactivated is random, whereas in marsupials it ...
 See also: Mammals, Human, Species, Organ, Placenta
  
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