Marsupials are an ancient and diverse group of mammals, with about 272 species. They come in many different forms and live in many different ways. Marsupials include possums, opossums, wombats, koalas, kangaroos, bandicoots, wallabies, sugar gliders, Tasmanian devils, and Tasmanian wolves. They range in size from small, shrew-like animals to red kangaroos, which weigh up to 90 kg. Most marsupials are found in Australia, New Zealand, New Guinea, and Tasmania, and in Central and South America. One species of marsupial, the American opossum, has spread throughout much of North America.
Marsupials differ from other kinds of mammals (placental mammals) in many features. One of the most important is how their young develop. Placental mammals (which include all of the other kinds of mammals we are familiar with) have young that are fairly well developed when they are born, having spent a relatively long time developing within their mother. They then feed on milk from their mother for a relatively short period of time, though of course times differ widely among mammal species. Marsupial mothers are pregnant for only short periods of time, their young are born in an early stage of development and spend a relatively long period of time feeding on milk from their mother. It is during the nursing phase that most development of marsupial young occurs.