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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). Monotremes were very poorly understood for many years, and to this day some of the 19th century myths that grew up around them endure. 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 (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. Physiology
Taxonomy The only surviving examples of monotremes are all indigenous to Australia and New Guinea, although there is evidence that they were once more widespread. Fossil and genetic evidence shows that the monotreme line diverged from other mammalian lines about 150 million years ago and that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. 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.) Fossil monotremes Excepting Ornithorhynchus anatinus, all the animals listed in this section are extinct. | |||||||||||||||||||||||||||||||||
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