Marsupials

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.
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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.
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Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals.
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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.
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marsupials: the mammals whose embryos develop within the mother's uterus for a short period of time before birth.
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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.
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monotremes
and marsupials). Embryo and fetus are nourished by a placenta. They are viviparous (producing young alive rather than lying eggs), have a long period of gestation and the young is born immature.
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Shared derived characters that are regarded as homologous are called synapomorphies, e.g. the presence of mammary glands in all three lineages of mammals (monotremes, marsupials and placentals).
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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.
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