Monotremes ("one hole," referring to their genitals) are members of Order Monotremata, the smallest of three groups of mammals (the others being marsupials and placentals), and the most distantly related to other living mammals. Monotremes split off from other mammals about 150 million years ago. By contrast, marsupials and placental mammals split from each other about 90 million years ago.
Although monotremes were once a larger group, today they only consist of just five species: the platypus, short-beaked echidna, Western long-beaked echidna, Sir David's long-beaked echidna, and the Eastern long-beaked echidna. All of them are only found in Australia and New Guinea, but there is evidence that at one point, monotremes were probably global in extent. Some taxonomists propose putting platypi and echnidnas into two separate orders.
Monotremes share many features of earlier amniotes, such as the ancestors of mammals: they have a single hole for doing all their business, a low metabolic rate by mammalian standards, and lay leathery eggs, unlike any other mammal. For a long time, monotremes were very poorly understood, and thought to be "inferior" mammals. In modern times, biologists have a less hierarchical view of the evolutionary process, and regard monotremes as just another adaptive group among mammals.
The body temperature of monotremes is around 32°C (90°F), relative to about 35°C (95°F) for marsupials, and 38°C (100°F) for placentals. A lower body temperature means monotremes benefit from slightly less food requirements but also have less energy available to power their muscles.
The reproductive mode of monotremes is an interesting cross between reptile-style egg-laying (although they are no more closely related to reptiles than other mammals) and conventional mammalian vivipary (live birth). They lay eggs, but the egg is retained within the mother's body and given nutrients for a time. Like other mammals, monotremes are covered in a coat of fur and produce milk for their young. But lacking nipples, female monotremes "sweat" milk for their young from a specialized patch on their bellies.
Monotremes are a quite mysterious and small order of mammals. When the platypus genome is sequenced in the near future, it will shed more light on this group and its relationships to extant mammalian orders.