The word monotreme comes from the Greek words mono- and trema, meaning "one" and "hole." This name refers to the single opening monotremes have for excretion (solid and liquid) and reproduction.
Like other mammals, monotremes
- are "warm-blooded" with high metabolic rates (though not as high as other mammals, see below).
- have hair on their bodies
- produce milk to feed their young
- have a single bone in their lower jaw
- have three inner ear bones
The only surviving examples are all indigenous to Australia and New Guinea, though there is evidence that they were once more widespread. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina, (since named Monotrematum). In Australia, the earliest known marsupial fossils date to around 110 million years ago.
- Platypus has only one genus comprising one species: Ornithorhynchus anatinus.
- There are three species of echidna in two genera: Tachyglossus aculeatus, Zaglossus bruijnii and Zaglossus attenboroughi.
Similarly, it is still sometimes said that monotremes have less developed internal temperature control mechanisms than other mammals, but more recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty. (Consider the case of a platypus living in an icy mountain stream.) Early researchers were misled by two factors. Monotremes maintain a lower average temperature than most placentals (around 32 degrees, as compared with about 35 for marsupials, 38 for most placentals, or 41 for typical birds). Secondly, the echidna (which is much easier to study than the reclusive platypus) only maintains normal temperature when it is active: during cold weather, an echidna conserves energy by "switching off" its temperature regulation.
Monotremes lay eggs. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipples. All species are very long-lived, with low rates of reproduction and relatively prolonged parental care of infants.
Living monotremes lack teeth as adults. Fossil forms and modern platypus young have the "tribosphenic" (three-cusped) molarss which are one of the hallmarks of mammals. However, recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials.  The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in cynodonts and other pre-mammalian synapsidss.
However, the external opening of the ear still lies at the base of the jaw. The monotremes also have extra bones in the shoulder girdle, including an interclavicle, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful venom in the male platypus.
The physiology of monotremes is equally unique. Their metabolic rate is remarkably low by mammalian standards, although the extent to which this is a characteristic of monotremes, as opposed to an adaptation on the part of the small number of surviving species to harsh environmental conditions, is uncertain.