利用者:Karasunoko/sandbox/3

Evolution[編集]

Reconstruction of ancient platypus relative Steropodon

The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around them—for example, that the monotremes were "inferior" or quasireptilian—still endure.[1] In 1947, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect.[1][2] In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups.[1][3] Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48 million years ago.[4]

 
 
 

Platypus

 
 

Echidnas

 
 
 live birth 
 

Marsupials

 
 true placenta 

Eutherians

 
 
 
Evolutionary relationships between the platypus and other mammals.[5]

The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period. The extinct monotremes Teinolophos and Steropodon were once thought to be closely related to the modern platypus,[2] but are now considered more basal taxa.[6] The fossilised Steropodon was discovered in New South Wales and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless). The molar teeth were initially thought to be tribosphenic, which would have supported a variation of Gregory's theory, but later research has suggested, while they have three cusps, they evolved under a separate process.[7] The fossil is thought to be about 110 million years old, making it the oldest mammal fossil found in Australia. Unlike the modern platypus (and echidnas), Teinolophos lacked a beak.[6]

Monotrematum sudamericanum, another fossil relative of the platypus, has been found in Argentina, indicating monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica (up to about 167 million years ago).[7][8] A fossilized tooth of a giant platypus species, Obdurodon tharalkooschild, was dated 5–15 million years ago. Judging by the tooth, the animal measured 1.3 meters long, making it the largest platypus on record.[9]

Platypus skeleton

Because of the early divergence from the therian mammals and the low numbers of extant monotreme species, the platypus is a frequent subject of research in evolutionary biology. In 2004, researchers at the Australian National University discovered the platypus has ten sex chromosomes, compared with two (XY) in most other mammals (for instance, a male platypus is always XYXYXYXYXY),[10] The sex chromosomes of the platypus have been found to have great homology to the bird Z chromosome.[11] The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation.[12][13] Though the platypus lacks the mammalian sex-determining gene SRY, a study found that the mechanism of sex determination is the AMH gene on the oldest Y chromosome.[14][15] A draft version of the platypus genome sequence was published in Nature on 8 May 2008, revealing both reptilian and mammalian elements, as well as two genes found previously only in birds, amphibians, and fish. More than 80% of the platypus' genes are common to the other mammals whose genomes have been sequenced.[12]

  1. ^ a b c Kirsch, John A. W.; Mayer, Gregory C. (29 July 1998). “The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory”. Philosophical Transactions: Biological Sciences 353 (1372): 1221–37. doi:10.1098/rstb.1998.0278. PMC 1692306. PMID 9720117. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692306/. 
  2. ^ a b Rauhut, O.W.M.; Martin, T.; Ortiz-Jaureguizar, E.; Puerta, P. (2002). “The first Jurassic mammal from South America”. Nature 416 (6877): 165–8. doi:10.1038/416165a. PMID 11894091. 
  3. ^ Messer, M.; Weiss, A.S.; Shaw, D.C.; Westerman, M. (March 1998). “Evolution of the Monotremes: Phylogenetic Relationship to Marsupials and Eutherians, and Estimation of Divergence Dates Based on α-Lactalbumin Amino Acid Sequences”. Journal of Mammalian Evolution (Springer Netherlands) 5 (1): 95–105. doi:10.1023/A:1020523120739. 
  4. ^ Phillips MJ; Bennett TH; Lee MS (2009). “Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas”. Proc. Natl. Acad. Sci. U.S.A. 106 (40): 17089–94. doi:10.1073/pnas.0904649106. PMC 2761324. PMID 19805098. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2761324/. 
  5. ^ Lecointre, Guillaume; Le Guyader, Hervé (2006). The Tree of Life: A Phylogenetic Classification. Harvard University Press. ISBN 978-0-674-02183-9. https://books.google.com/books?id=S4LxB9MRdzMC 2015年3月28日閲覧。 
  6. ^ a b Thomas H. Rich, James A. Hopson, Pamela G. Gill, Peter Trusler, Sally Rogers-Davidson, Steve Morton, Richard L. Cifelli, David Pickering, Lesley Kool, Karen Siu, Flame A. Burgmann, Tim Senden, Alistair R. Evans, Barbara E. Wagstaff, Doris Seegets-Villiers, Ian J. Corfe, Timothy F. Flannery, Ken Walker, Anne M. Musser, Michael Archer, Rebecca Pian and Patricia Vickers-Rich (2016). "The mandible and dentition of the Early Cretaceous monotreme Teinolophos trusleri". Alcheringa: An Australasian Journal of Palaeontology. in press. doi:10.1080/03115518.2016.1180034.
  7. ^ a b Pascual, R.; Goin, F.J.; Balarino, L.; Udrizar Sauthier, D.E. (2002). “New data on the Paleocene monotreme Monotrematum sudamericanum, and the convergent evolution of triangulate molars” (PDF). Acta Palaeontologica Polonica 47 (3): 487–492. http://www.app.pan.pl/archive/published/app47/app47-487.pdf. 
  8. ^ Folger, Tim (1993). “A platypus in Patagonia (Ancient life – 1992)”. Discover 14 (1): 66. 
  9. ^ Mihai, Andrei (2013年). “‘Platypus-zilla’ fossil unearthed in Australia”. ZME Science. Template:Cite webの呼び出しエラー：引数 accessdate は必須です。
  10. ^ Selim, Jocelyn (2005年4月25日). “Sex, Ys, and Platypuses”. Discover. 2008年5月7日閲覧。
  11. ^ Frank Grützner, Willem Rens, Enkhjargal Tsend-Ayush, Nisrine El-Mogharbel1, Patricia C. M. O'Brien, Russell C. Jones, Malcolm A. Ferguson-Smith & Jennifer A. Marshall Graves (16 December 2004). “In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes”. Nature 432 (7019): 913–7. doi:10.1038/nature03021. PMID 15502814. http://www.nature.com/nature/journal/v432/n7019/abs/nature03021.html. 
  12. ^ a b Warren, Wesley C. (8 May 2008). “Genome analysis of the platypus reveals unique signatures of evolution” (PDF). Nature 453 (7192): 175–183. doi:10.1038/nature06936. PMC 2803040. PMID 18464734. http://www.nature.com/nature/journal/v453/n7192/pdf/nature06936.pdf. 
  13. ^ “Beyond the Platypus Genome – 2008 Boden Research Conference”. Reprod Fertil Dev. 21 (8): i–ix, 935–1027. (2009). http://www.publish.csiro.au/nid/44/issue/4849.htm. 
  14. ^ Cortez, Diego; Marin, Ray; Toledo-Flores, Deborah; Froidevaux, Laure; Liechti, Angélica; Waters, Paul D.; Grützner, Frank; Kaessmann, Henrik (2014). “Origins and functional evolution of Y chromosomes across mammals”. Nature 508: 488–493. doi:10.1038/nature13151. 
  15. ^ Salleh, Anna (2014年5月5日). “Platypus Sex 'Master Switch' Identified”. Australian Broadcasting Corporation. http://www.abc.net.au/science/articles/2014/05/05/3994897.htm 
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