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Taxonomy
Mammals form two major groups. The Theria subclass includes all the mammals that give birth, including marsupials. The other subclass, the Protheria, is much smaller. These are the monotremes, which are egg-laying mammals. Examples include the duck-billed platypus and the echidna. Theria is further divided into the placental and the marsupial orders.
Physical Characteristics
One of the fundamental differences between marsupials and placentals is how their offspring develop. Marsupial babies are born as tiny, nearly embryonic animals and they continue their development in their mother's pouch. A newborn red kangaroo is only an inch long and struggles to the pouch with the aid of its relatively large front legs. In contrast, placental mammals are well developed when they are born and the young of monotremes obviously develop in eggs.
Ecology
Australian marsupials fill much the same ecological niches as their Old World counterparts. Kangaroos and wallabies, for example, have the role of grazing animals, which is mostly filled by hooved placental mammals such as antelopes elsewhere. Having the same niche has led to superficial resemblances between many marsupials and placentals, although they are not closely related. Arboreal gliding possums fill the role that flying squirrels occupy elsewhere. The almost-extinct (as of this article's publication) Tasmanian tiger had a similar appearance to large placental carnivores. Small marsupials fill the niches of rodents and shrews, and look much like them. Marsupial moles burrow, and there are marsupials with long snouts that have a similar niche to anteaters.
Conservation
One of the main conservation problems marsupials face, especially in Australia, is the arrival of introduced placental mammals. Some, such as rabbits, compete for the same food supply as the endemic marsupials. Other introduced species, including foxes and domestic cats, see smaller marsupials as prey items. Habitat destruction is another major threat.