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Temporal range: Early Jurassic – Holocene, 210–0Ma
C.L. Bonaparte, 1837
Temporal range: Early Jurassic – Holocene, 210–0Ma
C.L. Bonaparte, 1837
Monotremes (from the Greek μονός monos "single" + τρῆμα trema "hole", referring to the cloaca) 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.
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.
The key anatomical 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, defecatory, and reproductive systems all open into a single duct, the cloaca. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract), while most placental mammal females have separate openings for reproduction, urination, and defecation: the vagina, the urethra, and the anus.
Monotremes lay eggs. However, the egg is retained for some time within the mother, which actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipples, excreting the milk from their mammary glands via openings in their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants.
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. 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. 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.
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 their 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 venom in the male platypus.
Monotreme metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 32°C (90°F) rather than the 37°C (99°F) typical of placental mammals. Research suggests this has been a gradual adaptation to harsh environmental conditions on the part of the small number of surviving monotreme species rather than a historical characteristic of monotremes.
Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they maintain a constant body temperature in a wide variety of circumstances without difficulty (for example, the platypus while living in an icy mountain stream). Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals (around 32°C), compared to about 35°C (95°F) for marsupials and 37°C for most placentals); secondly, the short-beaked echidna (which is much easier to study than the reclusive platypus) maintains normal temperature only when it is active; during cold weather, it conserves energy by "switching off" its temperature regulation. Additional perspective came when reduced thermal regulation was observed in the hyraxes, which are placental mammals.
The echidna was originally thought to not enter rapid eye movement sleep. However, a more recent study showed that REM sleep accounted for about 15% of the total sleep time observed on subjects at an environmental temperature of 25°C (77°F). Surveying a range of environmental 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).
Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the less sterile manner of milk intake associated with the absence of nipples.
Monotremes are conventionally treated as comprising a single order Monotremata, though a recent classification proposes to divide them into the orders Platypoda (the platypus along with its fossil relatives) and Tachyglossa (the echidnas, or spiny anteaters). The entire grouping is also traditionally placed into a subclass Prototheria, which was extended to include several fossil orders, but these are no longer seen as constituting a group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a clade termed Australosphenida.
The traditional "theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria (marsupial) and Eutheria (placental mammal) lineages happened prior to the divergence between marsupials and placental mammals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying. Most morphological evidence supports the theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing "marsupionata hypothesis" in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. An analysis by Van Rheede in 2005 concluded that the genetic evidence favors the theria hypothesis, and this hypothesis continues to be the more widely accepted one.
The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. These fragments, from the 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.) Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago. All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.
The precise relationships between extinct groups of mammals and modern groups, such as monotremes, are somewhat uncertain, but cladistic analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates, with a number of analyses giving a more recent LCA for placentals and monotremes, but some also suggesting the LCA of placentals and multituberculates was more recent.
The fossil record of monotremes is relatively sparse. The first Mesozoic monotreme to be discovered was Steropodon galmani from Lightning Ridge, New South Wales.  Although biochemical and anatomical evidence suggests that the monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose, only a handful of monotreme fossils are known from before the Miocene epoch. The known Mesozoic monotremes are Steropodon, Kollikodon, and Teinolophos, all from Australian deposits in the Cretaceous, suggesting monotremes had already diversified by that time. A platypus tooth has been found in the Palaeocene of Argentina, so Michael Benton suggests in Vertebrate Palaeontology monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and some subsequently migrated across Antarctica to reach South America, both of which were still united with Australia at that time. However, a number of genetic studies suggest a much earlier origin in the Triassic.
Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils.
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