The platypus and four species of echidna or spiny anteater are the only living monotremes. They are native to Australia and New Guinea. Fossil evidence suggests that they may have lived in South America as well.
Monotremes lay eggs rather than give birth but produce milk from mammary glands and have hair as do all other mammals. They also have other common mammal characteristics including four-chambered hearts, single lower jaw bones, sets of three middle ear bones and the ability to regulate their own body temperature. Evolutionary biologists believe that their ancestors diverged from those of other mammals about 220 million years ago.