Fossil range: Early Cretaceous - Recent
Echidna - melbourne zoo
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Prototheria
Order: Monotremata
C.L. Bonaparte, 1837


Monotremes (monos, single + trema, hole; refers to the cloaca) are mammals that lay eggs instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). The subclass comprises a single order, Monotremata (though sometimes the subclass Prototheria is used). An infant monotreme is known as a puggle.

General characteristics

Like other mammals, monotremes are warm-blooded with a high metabolic rate (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; and have three middle ear bones (though this feature is now thought to have evolved independently in monotremes, see below).


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 (such as the Platypus, which can maintain its body temperature even while living in an icy mountain stream) maintain a constant body temperature in a wide variety of circumstances without difficulty. Early researchers were misled by two factors: monotremes maintain a lower average temperature than most mammals (around 32°C / 90°F , compared to about 35°C / 95°F for marsupials, and 38°C / 100°F for most placentals); secondly, the Short-beaked Echidna (which is much easier to study than the reclusive Platypus) only maintains normal temperature when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation.



A Platypus


A Long-beaked Echidna



Living monotremes lack teeth as adults. Fossil forms and modern platypus young have the "tribosphenic" (three-cusped) molars, 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 (Luo et al, 2001). The jaw of monotremes is constructed somewhat differently from those 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 synapsids; however, this feature, too, is now claimed to have evolved independently in monotremes and therians (Rich et al, 2005).

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 that 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.

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.



Fossil monotremes



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