Monotremes are 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 (or spiny anteaters). 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.〔Butler, Ann B., and William Hodos (2005). ''Comparative Vertebrate Neuroanatomy: Evolution and Adaptation'', (p. 361 )〕
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. Monotreme jaws are 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 premammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and therians, although, as with the analogous evolution of the tribosphenic molar, this is disputed.〔(【引用サイトリンク】 Comment on "Independent Origins of Middle Ear Bones in Monotremes and Therians" (I) )〕〔(【引用サイトリンク】 Comment on "Independent Origins of Middle Ear Bones in Monotremes and Therians" (II) )〕 The external opening of the ear still lies at the base of the jaw.
The sequencing of the platypus genome has also provided insight into the evolution of a number of monotreme traits, such as venom and electroreception, as well as showing some new unique features, such as the fact that monotremes possess 10 sex chromosomes and that their X chromosome resembles the sex chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved more recently than the split from the monotreme lineage.〔Veyrunes et al. ("Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes" ), Genome Res. 2008 June; 18(6): 965–973〕 This feature, along with some other genetic similarities with birds, such as shared genes related to egg-laying, is thought to provide some insight into the most recent common ancestor of the synapsid lineage leading to mammals and the sauropsid lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago during the Carboniferous.〔(【引用サイトリンク】title=Interpreting Shared Characteristics: The Platypus Genome | Learn Science at Scitable )〕 The presence of vitellogenin genes (a protein necessary for egg shell formation) is shared with birds, suggesting that when the common ancestor of mammals from ~225 million years ago split into monotremes, marsupials, and placental mammals, egg laying was retained in monotremes and lost in all other mammals. DNA suggests that while this trait is shared and is synapomorphic with birds, platypuses are still mammals and they evolved lactation with other mammals.〔5.
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