nisäkkään

Nisäkkään, the Finnish term for marsupial, refers to a group of mammals that give birth to relatively undeveloped young, which typically finish developing in a pouch or on the mother's teats. The scientific name for the clade is Marsupialia, within the infraclass Placentalia. Marsupials are distinguished by the presence of a marsupium, or pouch, in which the embryo continues to develop after a very brief gestation period, usually lasting only a few days to a couple of weeks. The young lodge into the pouch and attach to a nipple, where they remain for several months to a year before becoming independent.

Marsupials are exclusively found in the Southern Hemisphere, with the vast majority residing in Australia, New Guinea, and surrounding islands. A small number also occur in the United States, primarily on the west coast, in species such as the Virginia opossum. In total, the group comprises about 330 species, grouped into 12 families, including the Macropodidae (kangaroos and wallabies), Dasyuridae (marsupial carnivores such as quolls and Tasmanian devils), and the Phalangeridae (cuscus and brushtail possums). Many marsupials are herbivorous, feeding on vegetation, whereas others are carnivorous or omnivorous.

The evolutionary history of marsupials can be traced back to the Paleocene of the Cretaceous–Paleogene boundary, around 120 million years ago. Fossil evidence indicates that the earliest marsupials evolved in North America, after which they spread to South America and later to Australia via Antarctica, following the breakup of Gondwana. Phylogenetic studies show two primary clades: the "Australidelphia," consisting of most Australian marsupials, and the "Didelphoidea," containing New World opossums. Throughout their history, marsupials have adapted to diverse habitats, ranging from deserts and rainforests to alpine tundra, and continue to play a key ecological role in many ecosystems.