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Monotreme



         


Kollikodontidae (extinct)
Ornithorhynchidae - Platypus
Tachyglossidae - Echidnas
Steropodontidae (extinct)

</table> Monotremes are mammals that are best known for laying eggs, instead of giving birth to live young like marsupials and placental mammals (Eutheria). The subclass comprises a single order, Monotremata (though sometimes the subclass Prototheria is used). 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; and have three inner ear bones. Monotremes were very poorly understood for many years, and to this day some of the 19th century myths that grew up around them endure, particularly in the northern hemisphere. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals. It now seems plain that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the marsupial and placental groups. 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°C, as compared with about 35° for marsupials, 38° for most placentals, or 41° for typical birds). 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.
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Physiology

The key physiological difference between monotremes and other mammals is the one that gave them their name. Monotreme means 'single opening' in Greek, and comes from the fact that their urinary, excretory, and reproductive systems all open into a single duct, the cloaca. This structure is very simlar to the one found in reptiles. (In contrast to the single cloaca of monotremes, other mammals have separate openings for reproduction, urination and excretion: the vagina, urethra and the anus.)

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) 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 [1]. 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 synapsids.

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.

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.

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Taxonomy

The only surviving examples are all indigenous to Australia-New Guinea, though there is evidence that they were once more widespread. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. These fragments, from species Steropodon galmani, are the oldest known fossils of monotremes. Fossils from the genera Kollikodon, Teinolophos, and Obdurodon have also been discovered. In 1991, a fossil tooth of a 61-million- year-old platypus was found in southern Argentina (since named Monotrematum, though it is now considered to be an Obdurodon species). (See fossil monotremes below.)

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Fossil monotremes

Excepting Ornithorhynchus anatinus, all animals are extinct.

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