Contents 1 General characteristics 1.1 Reproductive system 2 Physiology 3 Taxonomy 4 Fossil monotremes 4.1 Fossil species 5 References 6 Bibliography 7 External links

General characteristics[edit] 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.[5] 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.[6] Platypus Long-beaked echidna Diagram of a Monotreme Egg. 1) Shell; 2) Yolk; 3) Yolk Sac; 4) Allantois; 5) Embryo; 6) Amniotic Fluid; 7) Amniotic Membrane; and 8) Membrane 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,[7] although this is not well established.[8] Tooth loss in modern monotremes might be related to their development of electrolocation.[9] 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,[10] although, as with the analogous evolution of the tribosphenic molar, this is disputed.[11][12] Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians.[13] 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,[14] suggesting that the two sex chromosomes of marsupial and placental mammals evolved more recently than the split from the monotreme lineage.[15] 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.[16][17] 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.[18] L-ascorbic acid is synthesized only in the kidneys.[19] 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 venom in the male platypus. This venom is derived from b-defensins, proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of b-defensins, another trait shared with reptiles.[20] It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs.[21] Reproductive system[edit] This article may be expanded with text translated from the corresponding article in French. (August 2014) Click [show] for important translation instructions.  View a machine-translated version of the French article. Google's machine translation is a useful starting point for translations, but translators must revise errors as necessary and confirm that the translation is accurate, rather than simply copy-pasting machine-translated text into the English Wikipedia. Do not translate text that appears unreliable or low-quality. If possible, verify the text with references provided in the foreign-language article. You must provide copyright attribution in the edit summary by providing an interlanguage link to the source of your translation. A model attribution edit summary (using German): Content in this edit is translated from the existing German Wikipedia article at [[:de:Exact name of German article]]; see its history for attribution. You should also add the template {{Translated|fr|Monotremata#Appareil reproducteur}} to the talk page. For more guidance, see Wikipedia:Translation. Further information: Echidna § Reproduction, and Platypus § Reproduction The key anatomical difference between monotremes and other mammals gives them their name; monotreme means “single opening” in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Monotremes, like reptiles, have a single cloaca; marsupials also have a separate genital tract; while most placental mammal females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, the penis only carries semen, while urine is excreted through the cloaca.[22] The monotreme penis is similar to that of turtles, and is covered by a preputial sac.[23] Monotremes, like marsupials, produce larval young. Echidna "puggle" (a) compared to various "joeys": Virginia Opossum (b), Gray short-tailed opossum (c), Eastern quoll (d), Koala (e), Brushtail possum (f) and Southern brown bandicoot (g). Monotremes eggs are retained for some time within the mother and receive nutrients directly from her, and they generally hatch within 10 days after laying, much shorter than the incubation period of sauropsid eggs.[24][25] Newborn monotremes are larval and fetus-like, much like newborn marsupials (and perhaps all non-placental mammals[26]), and like them also bear relatively well developed forelimbs to crawl around. In fact, given that monotremes lack nipples, their puggles crawl about more frequently than marsupial joeys in search of milk, raising questions about the supposed development restrictions on marsupial forelimbs.[27] Monotremes lactate, excreting milk from their mammary glands via openings in their skin, rather than through nipples. All five extant species show prolonged parental care of infants, with low rates of reproduction and relatively long life-spans. Monotremes are also noteworthy in their zygotic development. Most mammal zygotes go through holoblastic cleavage, meaning that after fertilization, the ovum splits into multiple, divisible daughter cells. In contrast, the zygotes of monotremes, like those of birds and reptiles, undergo meroblastic (partial) division. This means the cells at the yolk's edge have cytoplasm continuous with that of the egg, which allows the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm.[28]

Physiology[edit] Monotreme female reproductive organs Male Platypus Reproductive System. 1. Testes 2. Epididymis 3. Bladder 4. Rectum 5. Ureter 6. Vas Deferens 7. Genito - Urinary Sinus 8. Penis enclosed in a fibrous sheath 9. Cloaca 10. Opening in the ventral wall of the cloaca for the penis 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 placental mammals.[29][30] Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes.[31][32] Monotremes may have less developed thermoregulation than other mammals, but recent 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 when active; during cold weather, it conserves energy by "switching off" its temperature regulation. Understanding of this mechanism came when reduced thermal regulation was observed in the hyraxes, which are placental mammals. The echidna was originally thought to experience no rapid eye movement sleep.[33] However, a more recent study showed that REM sleep accounted for about 15% of 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).[34] Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the more septic manner of milk intake associated with the absence of nipples.[35]

Taxonomy[edit] Monotremes are conventionally treated as comprising a single order Monotremata, though a recent classification[36] 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.[7][37] 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.[38][39][40] 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,[41] and this hypothesis continues to be the more widely accepted one.[42] 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.[43] 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 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,[44] but another putting it at 17–89 million years ago.[45] 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 earliest echidna found to date is about 13 million years.[46] 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.[47][48] Play media An echidna building a defensive burrow on French Island ORDER MONOTREMATA Suborder Platypoda Family Ornithorhynchidae: platypus Genus Ornithorhynchus Platypus, O. anatinus Suborder Tachyglossa Family Tachyglossidae: echidnas Genus Tachyglossus Short-beaked echidna, T. aculeatus T. a. aculeatus T. a. acanthion T. a. lawesii T. a. multiaculeatus T. a. setosus Genus Zaglossus Sir David's long-beaked echidna, Z. attenboroughi Eastern long-beaked echidna, Z. bartoni Z. b. bartoni Z. b. clunius Z. b. diamondi Z. b. smeenki Western long-beaked echidna, Z. bruijni

Fossil monotremes[edit] See also: Evolution of mammals The fossil record of monotremes is relatively sparse. The first Mesozoic monotreme to be discovered was Steropodon galmani from Lightning Ridge, New South Wales.[49] 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 and Teinolophos, all from Australian deposits in the Cretaceous, suggesting monotremes had already diversified by that time.[50] 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.[51] However, a number of genetic studies suggest a much earlier origin in the Triassic.[52] Fossil species[edit] A 100-million-year-old Steropodon jaw on display at the American Museum of Natural History, New York City, USA Play media Platypuses swimming at Sydney Aquarium Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils. Genus Kryoryctes Species Kryoryctes cadburyi Family Steropodontidae – paraphyletic assemblage Genus Steropodon Species Steropodon galmani Genus Teinolophos Species Teinolophos trusleri – 123 million years old, oldest monotreme specimen Family Ornithorhynchidae Genus Ornithorhynchus – oldest Ornithorhynchus specimen 9 million years old Species Ornithorhynchus anatinus (platypus) – oldest specimen 10,000 years old Genus Obdurodon – includes a number of Miocene (5–24 million years ago) platypuses Species Obdurodon dicksoni (Riversleigh platypus) Species Obdurodon insignis Species Obdurodon tharalkooschild – middle and upper Miocene (15–5 mya) Species Monotrematum sudamericanum – 61 million years old, originally placed in separate genus, now thought an Obdurodon Family Tachyglossidae Genus Zaglossus – Upper Pleistocene (0.1–1.8 million years ago) Species Zaglossus hacketti Species Zaglossus robustus Genus Megalibgwilia Species Megalibgwilia ramsayi – Late Pleistocene Species Megalibgwilia robusta – Miocene

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Bibliography[edit] Ronald M. Nowak (1999), Walker’s Mammals of the World (6th ed.), Baltimore: Johns Hopkins University Press, ISBN 0-8018-5789-9, LCCN 98023686 

External links[edit] Find more aboutMonotremeat Wikipedia's sister projects Definitions from Wiktionary Media from Wikimedia Commons Taxonomy from Wikispecies The Wikibook Dichotomous Key has a page on the topic of: Monotremata UCMP Introduction to Monotremes v t e Extant mammal orders Kingdom Animalia Phylum Chordata Subphylum Vertebrata (unranked) Amniota Yinotheria Australosphenida Monotremata (Platypus and echidnas) Theria Metatheria (Marsupial inclusive) Ameridelphia Paucituberculata (Shrew opossums) Didelphimorphia (Opossums) Australidelphia Microbiotheria (Monito del monte) Notoryctemorphia (Marsupial moles) Dasyuromorphia (Quolls and dunnarts) Peramelemorphia (Bilbies and bandicoots) Diprotodontia (Kangaroos and relatives) Eutheria (Placental inclusive) Atlantogenata Xenarthra Cingulata (Armadillos) Pilosa (Anteaters and sloths) Afrotheria Afrosoricida (Tenrecs and golden moles) Macroscelidea (Elephant shrews) Tubulidentata (Aardvark) Hyracoidea (Hyraxes) Proboscidea (Elephants) Sirenia (Dugongs and manatees) Boreoeutheria Laurasiatheria Eulipotyphla (Hedgehogs, shrews, moles and relatives) Chiroptera (Bats) Pholidota (Pangolins) Carnivora (Dogs, cats and relatives) Perissodactyla (Odd-toed ungulates) Artiodactyla (Even-toed ungulates and cetaceans) Euarchontoglires Rodentia (Rodents) Lagomorpha (Rabbits and pikas) Scandentia (Treeshrews) Dermoptera (Colugos) Primates v t e Eggs List of egg topics Types Bird Fish and amphibian Monotreme Fossil record Cephalopod Fish Reptile (dinosaur) Pathology Biology Embryo Ichthyoplankton Ootheca Oviparity Spawn Zygote Components Yolk White Shell As food List of egg dishes Boiled Coddled Custard desserts Deviled Eggs Benedict Fried Omelette Poached Pickled Roe Scotch Scrambled Smoked Soufflé Tea egg Trophic egg In culture Easter egg Egging Fabergé egg Humpty Dumpty Oomancy Ovo vegetarianism Yoshi Category Commons v t e Extant Monotremata species by family Kingdom Animalia Phylum Chordata Class Mammalia (unranked) Australosphenida Tachyglossidae (Echidnas) Tachyglossus Short-beaked echidna (T. aculeatus) Zaglossus (Long-beaked echidnas) Sir David's long-beaked echidna (Z. attenboroughi) Eastern long-beaked echidna (Z. bartoni) Western long-beaked echidna (Z. bruijni) Ornithorhynchidae Ornithorhynchus Platypus (O. anatinus) Category v t e Members of Australosphenida Ambondro Asfaltomylos Ausktribosphenos Bishops Henosferus (?) Monotremata Taxon identifiers Wd: Q21790 ADW: Monotremata EoL: 1679 Fossilworks: 39737 GBIF: 791 ITIS: 179915 MSW: 10300001 NCBI: 9255 Authority control NDL: 00572666 Retrieved from "" Categories: MonotremesExtant Late Triassic first appearancesLiving fossilsHidden categories: CS1 maint: Multiple names: authors listArticles with 'species' microformatsArticles to be expanded from August 2014All articles to be expandedArticles needing translation from French WikipediaArticles containing video clips

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MammalsMioceneMesozoicSteropodonTeinolophosCretaceousPalaeoceneMichael BentonVertebrate Palaeontology (Benton)Late JurassicEarly CretaceousAntarcticaSouth AmericaTriassicEnlargeSteropodonEnlargeSydney AquariumKryoryctesKryoryctesSteropodontidaeSteropodonSteropodon GalmaniTeinolophosTeinolophos TrusleriOrnithorhynchidaeOrnithorhynchusOrnithorhynchus AnatinusObdurodonMioceneObdurodon DicksoniRiversleigh PlatypusObdurodon InsignisObdurodon TharalkooschildMonotrematum SudamericanumTachyglossidaeZaglossusPleistoceneZaglossus HackettiZaglossus RobustusMegalibgwiliaLate PleistoceneMioceneColin GrovesInternational Standard Book NumberSpecial:BookSources/0-801-88221-4OCLCDigital Object IdentifierDigital Object IdentifierDigital Object IdentifierInternational Standard Book NumberSpecial:BookSources/978-0-471-73383-6Digital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierDigital Object IdentifierPubMed IdentifierDigital Object 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