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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. Among living mammals they include the platypus and four species of echidnas (or spiny anteaters); there is debate regarding monotreme taxonomy (see below).

General characteristics

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.[2] 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.[3]

Platypus

Platypus

File:Long-beakedEchidna.jpg

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 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, who 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. Infant echidnas are sometimes known as puggles, alluding to their similarity in appearance to the Australian children's toy designed by Tony Barber.[4] The same term, though not generally accepted, is popularly applied to young platypuses as well.[5][6]

Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have "tribosphenic" molars (with the occlusal surface formed by three cusps arranged in a triangle), which are 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] The jaw of monotremes is constructed somewhat differently from that 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 pre-mammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and therians,[9] although, as with the analogous evolution of the tribosphenic molar, this is disputed.[10][11] 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,[12] suggesting that the two sex chromosomes of marsupial and placental mammals evolved more recently than the split from the monotreme lineage.[13] 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.[14][15]

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 that are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful venom in the male Platypus.

Physiology

File:Monotreme reproductive organs.png

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.[16] 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.[17][18]

It is still sometimes said[citation needed] that monotremes have less developed internal temperature control mechanisms than other mammals, but recent research shows that monotremes 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 (90 °F), compared to about 35 °C (95 °F) for marsupials, and 37 °C (99 °F) 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.

It was originally believed that the Echidna did not enter rapid eye movement sleep,[19] but a more recent study shows the Echidna does indeed enter REM sleep, amounting to about 15% of the total sleep time observed for 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).[20]

Taxonomy

Monotremes are conventionally treated as comprising a single order Monotremata, though a recent classification[21] 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 natural 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][22]

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 that this explains why monotremes retain a number of "primitive" (or basal) traits presumed to have been present in the Synapsid ancestors of later mammals, such as egg-laying (this does not mean they are more 'primitive' in any absolute sense; just as placental mammals have 'derived' features which would not have been present in the monotreme/placental common ancestor, such as live birth, monotremes have their own 'derived' features such as electroreception and snouts modified into 'beaks').[23][24][25] 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,[26] and this hypothesis continues to be the more widely-accepted one.[27]

The time at which 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.[28] Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. These fragments, from 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 gives a wide range of dates for the split between echidnas and platypuses, one survey putting the split at 19–48 million years ago,[29] another putting it at 17–89 million years ago.[30] 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 that the LCA of placentals and multituberculates was more recent.[31][32]

Fossil monotremes

The fossil record of monotremes is relatively sparse. Although biochemical and anatomical evidence suggests that 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 that monotremes had already diversified by that time.[33] A platypus tooth has been found in the Palaeocene of Argentina, so Michael Benton suggests in Vertebrate Palaeontology that monotremes arose in Australia in the Late Jurassic or Early Cretaceous, and that some subsequently migrated across Antarctica to reach South America, both of which were still united with Australia at that time.[34] However, a number of genetic studies suggest a much earlier origin in the Triassic.[28]

Fossil species

File:Steropodon jaw.jpg

Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils.

Media

File:French Island Echidna.ogg
File:Ornithorhynchus anatinus -Sydney Aquarium, Sydney, Australia -swimming-6a.ogv

References

  1. ^ Groves, C. (2005). Wilson, D. E.; Reeder, D. M, eds. Mammal Species of the World (3rd ed.). Baltimore: Johns Hopkins University Press. pp. p. 1–2. OCLC 62265494. ISBN 0-801-88221-4. 
  2. ^ "Animal Bytes — Order Monotremata". 
  3. ^ Butler, Ann B., and William Hodos (2005). Comparative Vertebrate Neuroanatomy: Evolution and Adaptation, p. 361
  4. ^ "Questions about The Lost Forests". Retrieved 2009-03-22. 
  5. ^ "An Echidna Puggle". Fourth Crossing Wildlife. Retrieved 2007-10-21.  External link in |publisher= (help)
  6. ^ "Platypus Fact File: Background and Naming". Australian Platypus Conservancy. Retrieved 2008-03-26. 
  7. ^ a b Luo, Z.-X.; Cifelli, R.L.; Kielan-Jaworowska, Z. (2001). "Dual origin of tribosphenic mammals". Nature. 409: 53–57. doi:10.1038/35051023. 
  8. ^ "Mammalian evolution: Relationships to chew over.". Nature. 409: 28–31. 2001-01-04. doi:10.1038/35051199. 
  9. ^ Rich, T. H.; Hopson, J. A.; Musser, A. M.; Flannery, T. F.; & Vickers-Rich, P. (2005). "Independent origins of middle ear bones in monotremes and therians.". Science. Science. 307 (5711): 910–914. PMID 15705848. doi:10.1126/science.1105717.  Cite uses deprecated parameter |coauthors= (help)
  10. ^ "Comment on "Independent Origins of Middle Ear Bones in Monotremes and Therians" (I)". Science Magazine. Retrieved 2007-10-21. 
  11. ^ "Comment on "Independent Origins of Middle Ear Bones in Monotremes and Therians" (II)". Science Magazine. Retrieved 2007-10-21. 
  12. ^ "Platypus Genome Explains Animal's Peculiar Features; Holds Clues To Evolution Of Mammals". Sciencedaily.com. 2008-05-07. Retrieved 2011-06-09. 
  13. ^ Veyrunes et al. "Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes", Genome Res. 2008 June; 18(6): 965–973
  14. ^ "Interpreting Shared Characteristics: The Platypus Genome | Learn Science at Scitable". Nature.com. Retrieved 2011-06-09. 
  15. ^ "Genome analysis of the platypus reveals unique signatures of evolution". 453 (7192). PMC 2803040Freely accessible. PMID 18464734. doi:10.1038/nature06936. Retrieved 2011-06-09. 
  16. ^ "Thermal Biology of the Platypus". Davidson College. 1999. Retrieved 2006-09-14. 
  17. ^ J.M. Watson and J.A.M. Graves (1988). "Monotreme Cell-Cycles and the Evolution of Homeothermy". Australian Journal of Zoology. CSIRO. 36 (5): 573–584. doi:10.1071/ZO9880573. 
  18. ^ T.J. Dawson, T.R. Grant and D. Fanning (1979). "Standard Metabolism of Monotremes and the Evolution of Homeothermy". Australian Journal of Zoology. CSIRO. 27 (4): 511–515. doi:10.1071/ZO9790511. 
  19. ^ Siegel JM, Manger PR, Nienhuis R, Fahringer HM, Pettigrew JD. (1998-07-29). "Monotremes and the evolution of rapid eye movement sleep". Philosophical Transactions of the Royal Society of London, Series B, Bio-Sciences. Retrieved 2012-05-13. 
  20. ^ SC Nicol, NA Andersen, NH Phillips, RJ Berger (March 31, 2000). "The echidna manifests typical characteristics of rapid eye movement sleep". Neuroscience letters. US National Library of Medicine National Institutes of Health. PMID 10729631. Retrieved 2012-01-10. 
  21. ^ McKenna, Malcolm C., and Susan K. Bell (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. 631 pp. ISBN 0-231-11013-8
  22. ^ Luo, Z.-X.; Cifelli, R.L.; Kielan-Jaworowska, Z. (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontologia Polonica. 47: 1–78. 
  23. ^ Terry A. Vaughan, James M. Ryan, Nicholas J. Czaplewski: Mammalogy, Fifth Edition (2010), p. 80
  24. ^ "Introduction to the Monotremata". Ucmp.berkeley.edu. Retrieved 2011-06-09. 
  25. ^ http://www.webpages.uidaho.edu/~jacks/Lecture3.pdf
  26. ^ Van Rheede, Teun: "The Platypus Is in Its Place: Nuclear Genes and Indels Confirm the Sister Group Relation of Monotremes and Therians". Molecular Biology and Evolution, Volume 23, Number 3, pp.587-597, 2005
  27. ^ "Monotremes". Tolweb.org. doi:10.1073/pnas.94.4.1276. Retrieved 2011-06-09. 
  28. ^ a b "Hedges.indb" (PDF). Retrieved 2011-06-09. 
  29. ^ Phillips, MJ; Bennett, TH; Lee, MS. (2009). "Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas". PNAS. 106: 17089–17094. PMC 2761324Freely accessible. PMID 19805098. doi:10.1073/pnas.0904649106. 
  30. ^ "Hedges.indb" (PDF). Retrieved 2011-06-09. 
  31. ^ Benton, Michael J. Vertebrate Palaeontology (2004), p. 300
  32. ^ Carrano, Matthew T., and Richard W. Blob, Timothy J. Gaudin, and John R. Wible (2006). Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles, p. 358.
  33. ^ "Fossil Record of the Monotremata". Ucmp.berkeley.edu. Retrieved 2011-06-09. 
  34. ^ Benton, M.J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 303–304. ISBN 0-412-73810-4. 

External links

[1]Template:Monotremata Template:Australosphenida nav


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