Monotremes (from the Greek μονός monos "single" + τρῆμα trema "hole", referring to the cloaca) are mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). The only surviving examples of monotremes are all indigenous to Australia and New Guinea, although there is evidence that they were once more widespread. Among living mammals they include the platypus and four species of echidnas (or spiny anteaters); there is debate regarding monotreme taxonomy (see below).
There are only five living monotreme species: the duck-billed platypus and four species of echidna (also known as spiny anteaters). All of them are found only in Australia and New Guinea. Monotremes are not a very diverse group today, and there has not been much fossil information known until rather recently.
In some ways, monotremes are very primitive for mammals because, like reptiles and birds, they lay eggs rather than having live birth. In a number of other respects, monotremes are rather derived, having highly modified snouts or beaks, and modern adult monotremes have no teeth. Like other mammals, however, monotremes have a single bone in their lower jaw, three middle ear bones, high metabolic rates, hair, and they produce milk to nourish the young.