Marsupials are pouch-bearing mammals such as opossums, kangaroos and wombats. After fertilization, the fetus initially grows within the uterus where it is nourished by a yolk sack (rather than a placenta); within 4-5 weeks, the immature fetus must leave the uterus and make its way to the maternal pouch where it attaches to a nipple for another month or more of development. This system imposes a much higher risk for the marsupial fetus compared with the fetal risk of a placental mammal; thus, in areas where they both exist, placental mammals tend to be more successful and often displace the marsupials.
Marsupials first appeared in the Cretaceous Period, some 100 million years ago. Whether they first evolved in North or South America remains a subject of controversy; nevertheless, the dirth of placental mammals in Gondwanaland (the attached land masses of South America, Antarctica, Australia, Madagascar and India) allowed the marsupials to thrive and disperse.
When marsupials first appeared, Africa had just separated from South America and the latter would break from Gondwanaland in another 25 million years. India also broke away about 85 million years ago, drifting northward with its cargo of marsupial fauna. Australia separated from Antarctica about 55 million years ago, moving toward the northeast and beginning its long history as an island continent.
Antarctica drifted to the South Pole, killing off the vegetation and all terrestrial fauna. India would begin crunching into southern Asia about 65 million years ago, forcing up the Himalayas and permitting an interchange of species between these land masses; as discussed above, the placental mammals would soon dominate. About 3 million years ago, South America connected with North America as the Isthmus of Panama drifted into place; the marsupials of South America would soon compete with eutherians (placental mammals) moving down from the north.
Australia thus remained the lone refuge for marsupials, allowing them to spread and diversify across the continent without competition. It was not until man arrived, about 60,000 years ago, that conditions changed; a combination of hunting and the use of fire to clear scrub are thought to have triggered the demise of the larger marsupial species. An additional threat was the introduction of dingos, which likely arose from domesticated dogs, within the last 10,000 years. The spread of urban centers, ranching and other human activities continue to pressure the native fauna, as evidenced by the extinction of the marsupial wolf in the 1950s. Nevertheless, Australia remains the last great refuge for marsupial life.
