Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right)
Monotremes (/ˈ m ɒ n ə t r iː m z / ) are mammals of the order Monotremata. They are the only one of the three groups of living mammals that lay eggs, rather than bearing live young (the other two being plactals (Eutheria), and marsupials (Metatheria)). The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differces in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are differt from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk.
Monotremes have be considered members of Australosphida, a clade that contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America, and Australia, but this categorization is disputed and their taxonomy is under debate.
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All surviving examples of monotremes are all indigous to Australia and New Guinea, although they were also prest in the Late Cretaceous and Paleoce of southern South America, indicating that they were also prest in Antarctica, though remains have not be found there.
Like other mammals, monotremes are dothermic 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 plactal mammals is the primary communication route betwe the right and left brain hemispheres.
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The anterior commissure does provide an alternate communication route betwe the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from the neocortex, whereas in plactal mammals the anterior commissure carries only some of these fibers.
Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have a tribosphic 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 rect work suggests that monotremes acquired this form of molar indepdtly of plactal mammals and marsupials,
Monotreme jaws are constructed somewhat differtly from those of other mammals, and the jaw oping muscle is differt. 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 non-mammal cynodonts and other premammalian synapsids; this feature, too, is now claimed to have evolved indepdtly in monotremes and therians,
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The sequcing of the platypus gome has also provided insight into the evolution of a number of monotreme traits, such as vom and electroreception, as well as showing some new unique features, such as monotremes possessing 5 pairs of sex chromosomes and that one of the X chromosomes resembles the Z chromosome of birds,
This feature, along with some other getic similarities with birds, such as shared ges related to egg-laying, is thought to provide some insight into the most rect 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.
The presce of vitellogin ges (a protein necessary for egg shell formation) is shared with birds; the presce of this symplesiomorphy suggests that the common ancestor of monotremes, marsupials, and plactal mammals was oviparous, and that this trait was retained in monotremes but lost in all other extant mammal groups. DNA analyses suggest that although this trait is shared and is synapomorphic with birds, platypuses are still mammals and that the common ancestor of extant mammals lactated.
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The monotremes also have extra bones in the shoulder girdle, including an interclavicle and coracoid, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs on the sides of, rather than underneath, their bodies. The monotreme leg bears a spur in the ankle region; the spur is not functional in echidnas, but contains a powerful vom in the male platypus. This vom is derived from β-defsins, proteins that are prest in mammals that create holes in viral and bacterial pathogs. Some reptile vom is also composed of differt types of β-defsins, another trait shared with reptiles.
This section may be expanded with text translated from the corresponding article in Frch. (August 2014) Click [show] for important translation instructions.
The key anatomical differce betwe monotremes and other mammals gives them their name; monotreme means single oping in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Like reptiles, monotremes have a single cloaca. Marsupials have a separate gital tract, whereas most plactal mammalian females have separate opings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only sem passes through the pis while urine is excreted through the male's cloaca.
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Monotreme eggs are retained for some time within the mother and receive nutrits directly from her, gerally hatching within 10 days after being laid – much shorter than the incubation period of sauropsid eggs.
Are larval- and fetus-like and have relatively well-developed forelimbs that able them to crawl around. Monotremes lack nipples, so puggles crawl about more frequtly than marsupial joeys in search of milk, this differce raising questions about the supposed developmtal restrictions on marsupial forelimbs.
Rather than through nipples, monotremes lactate from their mammary glands via opings in their skin. All five extant species show prolonged partal care of their young, with low rates of reproduction and relatively long life-spans.
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Monotremes are also noteworthy in their zygotic developmt: Most mammalian zygotes go through holoblastic cleavage, where the ovum splits into multiple, divisible daughter cells. In contrast, monotreme zygotes, like those of birds and reptiles, undergo meroblastic (partial) division. This means that the cells at the yolk's edge have cytoplasm continuous with that of the egg, allowing the yolk and embryo to exchange waste and nutrits with the surrounding cytoplasm.
Male platypus reproductive system. 1. Testes, 2. Epididymis, 3. Bladder, 4. Rectum, 5. Ureter, 6. Vas Defers, 7. Gito-urinary sinus, 8. Pis closed in a fibrous sheath, 9. Cloaca, 10. Oping in the vtral wall of the cloaca for the pis.
Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 31 °C (88 °F) rather than the averages of 35 °C (95 °F) for marsupials and 37 °C (99 °F) for plactal mammals.
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Research suggests this has be a gradual adaptation to the harsh, marginal vironmtal niches in which the few extant monotreme species have managed to survive, rather than a geral characteristic of extinct monotremes.
Monotremes may have less developed thermoregulation than other mammals, but rect research shows that they easily maintain a constant body temperature in a variety of circumstances, such as the platypus in icy mountain streams. Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals; secondly, the short-beaked echidna, much easier to study than the reclusive platypus, maintains normal temperature only wh active; during cold weather, it conserves ergy by switching off its temperature regulation. Understanding of this mechanism came wh reduced thermal regulation was observed in the hyraxes, which are plactal mammals.
However, a more rect study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an vironmtal temperature of 25 °C (77 °F). Surveying a range of vironmtal temperatures, the study observed very little REM at reduced temperatures of 15 °C (59 °F) and 20 °C (68 °F), and also a substantial reduction at the elevated temperature of 28 °C (82 °F).
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Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compsate for the more septic manner of milk intake associated with the absce of nipples.
During the course of evolution the monotremes have lost the gastric glands normally found in mammalian stomachs as an adaptation to their diet.
Both the platypus and echidna species have spurs on their hind limbs. The echidna spurs are vestigial and have no known function, while the platypus spurs contain vom.
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Molecular data show that the main compont of platypus vom emerged before the divergce of platypus and echidnas, suggesting that the most rect common ancestor of these taxa was also possibly a vomous monotreme.
The traditional theria hypothesis states that the divergce of the monotreme lineage from the Metatheria (marsupial) and Eutheria (plactal mammal) lineages happed prior to the divergce betwe marsupials and plactal mammals, and this explains why monotremes retain a number of primitive traits presumed to have be prest in the synapsid ancestors of later mammals, such as egg-laying.
Most morphological evidce supports the theria hypothesis, but one possible exception is a similar pattern of tooth replacemt se in monotremes and marsupials, which originally provided the basis for the competing Marsupionta hypothesis in which the divergce betwe monotremes and marsupials happed later than the divergce betwe these lineages and the plactal mammals. Van Rheede (2005) concluded that the getic evidce favors the theria hypothesis,
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Monotremes are convtionally treated as comprising a single order Monotremata. The
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