Monotremes
| Monotremes Temporal range: Late Triassic–Holocene, 210–0 Ma |
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| A short-beaked echidna, a platypus, a Steropodon reconstruction and a western long-beaked echidna | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Infraclass: | Australosphenida |
| Order: |
Monotremata C.L. Bonaparte, 1837 |
| Subgroups | |
Monotremes are basal mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). The only surviving examples of monotremes are all indigenous to Australia and New Guinea, although there is evidence that they were once more widespread. The existing monotreme species are the platypus and four species of echidnas. There is currently some debate regarding monotreme taxonomy.
The word monotreme comes from the Greek μονός, monos ("single") and τρῆμα, trema ("hole"), referring to the cloaca.
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 through mammary glands to feed their young; have a single bone in their lower jaw; and have three middle-ear bones.
In common with reptiles and marsupials, monotremes lack the connective structure (corpus callosum) which in placental mammals is the primary communication route between the right and left brain hemispheres. The anterior commissure does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from the neocortex, whereas in placental mammals the anterior commissure carries only some of these fibers.
Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have a "tribosphenic" form of molars (with the occlusal surface formed by three cusps arranged in a triangle), which is one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this is not well established. Toothloss in modern monotremes might be related to their development of electrolocation.
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