Metatheria is a group of mammals that includes marsupials, such as kangaroos, koalas, and opossums. These animals are characterized by giving birth to relatively undeveloped young, which typically continue to grow and develop in a pouch on the mother's body. This unique reproductive strategy distinguishes them from Eutheria, or placental mammals. Metatherians are primarily found in Australia and the Americas. They have adapted to various environments, showcasing diverse forms and behaviors. Their evolutionary history dates back to the late Cretaceous period, making them an important group for understanding mammalian evolution.