Like other mammals, monotremes are endothermic with a high metabolic rate (though not as high as other mammals see below) have hair on their bodies produce milk through mammary glands to feed their young have a single bone in their lower jaw and have three middle-ear bones. The name monotreme derives from the Greek words μονός ( monós 'single') and τρῆμα ( trêma 'hole'), referring to the cloaca. There is currently some debate regarding monotreme taxonomy. The extant monotreme species are the platypus and four species of echidnas.

The only surviving examples of monotremes are all indigenous to Australia and New Guinea although there is evidence that they were once more widespread, as Monotrematum is known from the Paleocene of South America. Monotremes are a subgroup of the Australosphenida, a clade which contains other related extinct mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia. In addition, they lay eggs rather than bearing live young, but, like all mammals, the female monotremes nurse their young with milk. Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts compared to the more common mammalian types. They are one of the three main groups of living mammals, along with placentals ( Eutheria) and marsupials ( Metatheria). Monotremes / ˈ m ɒ n ə t r iː m z/ are prototherian mammals of the order Monotremata.

Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right)

Platypus evolution how to level up