Marsupialia is a group of mammals characterized by giving birth to relatively undeveloped young, which typically continue to develop in a pouch on the mother's belly. This unique reproductive strategy allows the young to receive protection and nourishment while they grow. Common examples of marsupials include kangaroos, koalas, and opossums. Marsupials are primarily found in Australia and New Guinea, although some species, like the opossum, are native to the Americas. They exhibit a wide range of sizes and habitats, from the small quokka to the larger red kangaroo. Marsupials play important roles in their ecosystems, contributing to seed dispersal and pest control.