Marsupial eggs contain a moderate amount of yolk that nourishes the embryo during early development in the mother's reproductive tract. Young are born in an early stage of development and are small. The hindlegs are simple buds, but the forelimbs are strong enough for the young to climb from the female reproductive tract exit to the marsupium. In the marsupium, the young attaches to a teat and completes its development while nursing. Convergent evolution in Australian marsupials has produced a diversity of forms which resemble placental counterparts in all ecological roles.
Opossums are the only extant marsupials outside of the Australian region.
With isolation, their marsupial faunas diversified away from the placental mammals that began adaptive radiation on the northern continents. When North and South America joined at the isthmus of Panama, extensive migrations took place over the land bridge in both directions. Placental Mammals . In placental mammals, embryonic development is completed within the uterus where the embryo is joined to the mother by the placenta. Most mammalogists favor a genealogy that recognizes 4 main evolutionary lines of Placental mammals.