Monotremes
Five species of monotremes, or prototherians, are alive today and all live in Australia or New Guinea. These bizarre mammals, the duck-billed platypus and four species of echidnas, differ from other mammals in that they lack a placenta, lay eggs, and have legs that poke out to the side like reptiles. Monotremes nurse their young on milk, but they have no nipples on their mammary glands; the milk just oozes out and is lapped off the fur by their babies.
Marsupials
There are over 330 species of marsupials, or metatherians, alive today including kangaroos, possums, koalas and wombats. Following the complete break up of the ancient landmass of Gondwana into South America, Antarctica and Australia, the marsupials diverged into the two distinct groups seen today; one in the Americas and the other in Australia. Marsupials give birth to relatively undeveloped young which have to climb from the mother’s birth canal to the nipples in the pouch to complete their development. There are several cases of convergent evolution between marsupials and placental mammals, in which animals have evolved to fill the same ecological niche in different parts of the world. There are burrowing, grazing, gliding, and even long-snouted ant-eating forms which have evolved independently in the two groups.