Metatheriaan

Metatheria is a infraclass of mammals that, along with Eutheria (placental mammals) and Monotremata (monotremes), constitutes the class Mammalia. The defining characteristic of metatherians, commonly known as marsupials, is their reproductive strategy. Unlike placental mammals, which have a long gestation period where the fetus develops internally within the uterus nourished by a placenta, marsupials typically have a very short gestation period. The young are born in a relatively undeveloped state, often described as embryonic or fetal. After birth, these underdeveloped young crawl to a pouch, or marsupium, located on the mother's abdomen. Inside the pouch, they attach to a nipple and continue their development, receiving nourishment and protection until they are mature enough to survive outside.

The majority of extant marsupial species are found in Australia and surrounding islands, though a significant group, the didelphimorphs (opossums), are native to the Americas. This geographical distribution reflects their evolutionary history. Marsupials diverged from the lineage leading to placental mammals millions of years ago. While they possess mammary glands and hair, distinguishing them as mammals, their reproductive system differs notably from eutherians. For instance, female marsupials typically have two uteri and two vaginas, though the degree of fusion varies between species. Male marsupials often have a bifurcated penis.

The diversity within Metatheria is considerable, encompassing a wide range of forms from the small, insectivorous planigales to the large, herbivorous kangaroos. Despite their common name and the iconic image of the pouch, not all metatherians possess a well-developed pouch; in some species, it may be reduced or absent. The term "marsupial" is derived from the Latin word "marsupium," meaning pouch.