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Temporal range: Paleocene –Recent, 59–0 Ma
Choloepus hoffmanni (Puerto Viejo, CR) crop.jpg
Hoffmann's two-toed sloth (Choloepus hoffmanni)
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Superorder: Xenarthra
Cope, 1889
Orders and suborders

See text for more details

The superorder Xenarthra is a group of placental mammals in the Americas and currently consists of anteaters, tree sloths, and armadillos. Xenarthrans originated in South America during the Paleocene about 59 million years ago.[1] Xenarthrans developed and diversified extensively in South America during the continent's long period of isolation in the early to mid Cenozoic Era. They spread to the Antilles by the early Miocene and, starting about 3 Mya, spread to Central and North America as part of the Great American Interchange.[2] Nearly all of the formerly abundant megafaunal xenarthrans, such as ground sloths, glyptodonts, and pampatheres, became extinct at the end of the Pleistocene.

Xenarthrans share several characteristics not present in other placental mammals, and are often considered to be among the most primitive order of placental mammals. The name Xenarthra, which means "strange joints", was chosen because their vertebral joints have extra articulations unlike other mammals. This trait is referred to as "xenarthry". Also, unlike other mammals, the ischium and sacrum are fused.[3] The males have internal testicles, which are located between the bladder and the rectum.[4] Xenarthrans have been determined to have single-color vision. Through PCR analysis, it was discovered that a mutation in a stem Xenarthran led to long-wavelength sensitive-con (LWS) monochromacy (single color vision) common in nocturnal, aquatic and subterranean mammals.[5] Further losses led to rod monochromancy in a stem cingulate and a stem pilosan pointing to a subterranean ancestry.[5] Furthermore, xenarthrans have the lowest metabolic rates among the therians.[6][7] They also seem to lack a functional pineal gland.[8]

Evolutionary relationships[edit]

Pink fairy armadillo (Chlamyphorus truncatus)

Xenarthrans were previously classified alongside the pangolins and aardvarks in the order Edentata (meaning toothless, because the members do not have incisors and lack, or have poorly developed, molars). Subsequently, Edentata was found to be a polyphyletic grouping whose New World and Old World taxa are unrelated, and it was split up to reflect their true phylogeny. Aardvarks and pangolins are now placed in individual orders, and the new order Xenarthra was erected to group the remaining families (which are all related). The name Xenarthra means "strange joints", and was chosen because their vertebral joints have extra articulations and are unlike those of any other mammals. The morphology of xenarthrans generally suggests that the anteaters and sloths are more closely related to each other than either is to the armadillos; this is upheld by molecular studies. Since its conception, Xenarthra has increasingly come to be considered to be of a higher rank than 'order'; some authorities consider it to be a cohort, while others consider it to be a superorder. Whatever the rank, Xenarthra is now generally considered to be divided into two orders: Cingulata, which contains the armadillos; and Pilosa, which contains the Vermilingua (anteaters) and Folivora (sloths; previously known as Tardigrada or Phyllophaga).[9]

Xenarthra may be most closely related to either Afrotheria[10] (in the group Atlantogenata), Boreoeutheria (in the group Exafroplacentalia), or Epitheria[11] (comprising Afrotheria and Boreoeutheria). In other words, it may be nested within Eutheria or it may be the basal extant group. A comprehensive phylogeny by Goloboff et al.[12] includes xenarthrans as a sister clade of Euarchontoglires within Boreoeutheria (Laurasiatheria+Euarchontoglires).


Below is a recent simplified phylogeny of the xenarthran families based on Slater et al. (2016)[13] and Delsuc et al. (2016).[14] The dagger symbol, "†", denotes extinct groups.

















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  4. ^ Kleisner, K; Ivell, R; Flegr, J (2010). "The evolutionary history of testicular externalization and the origin of the scrotum". Journal of Biosciences. 35 (1): 27–37. doi:10.1007/s12038-010-0005-7. PMID 20413907.
  5. ^ a b Emerling, Christopher A.; Springer, Mark S. (2015-02-07). "Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra". Proceedings of the Royal Society B: Biological Sciences. 282 (1800): 20142192. doi:10.1098/rspb.2014.2192. ISSN 0962-8452. PMC 4298209. PMID 25540280.
  6. ^ Elgar, M. A.; Harvey, P. H. (1987). "Basal Metabolic Rates in Mammals: Allometry, Phylogeny and Ecology". Functional Ecology. 1 (1): 25–36. doi:10.2307/2389354. JSTOR 2389354.
  7. ^ Lovegrove, Barry G. (2000). "The Zoogeography of Mammalian Basal Metabolic Rate". The American Naturalist. 156 (2): 201–19. doi:10.1086/303383. JSTOR 3079219. PMID 10856202.
  8. ^ The Pineal Gland and its Endocrine Role
  9. ^ McKenna, M.C.; Bell, S.K. (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. p. 93. ISBN 978-0-231-11013-6. OCLC 37345734.
  10. ^ Murphy, W. J.; Pringle, T. H.; Crider, T. A.; Springer, M. S.; Miller, W. (2007). "Using genomic data to unravel the root of the placental mammal phylogeny". Genome Research. 17 (4): 413–21. doi:10.1101/gr.5918807. PMC 1832088. PMID 17322288.
  11. ^ Kriegs, Jan Ole; Churakov, Gennady; Kiefmann, Martin; Jordan, Ursula; Brosius, Jürgen; Schmitz, Jürgen (2006). "Retroposed Elements as Archives for the Evolutionary History of Placental Mammals". PLoS Biology. 4 (4): e91. doi:10.1371/journal.pbio.0040091. PMC 1395351. PMID 16515367.
  12. ^ Goloboff, Pablo A.; Catalano, Santiago A.; Marcos Mirande, J.; Szumik, Claudia A.; Salvador Arias, J.; Källersjö, Mari; Farris, James S. (2009). "Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups". Cladistics. 25 (3): 211–30. doi:10.1111/j.1096-0031.2009.00255.x.
  13. ^ Slater, G., Cui, P., Forasiepi, A. M., Lenz, D., Tsangaras, K., Voirin, B., ... & Greenwood, A. D. (2016). Evolutionary relationships among extinct and extant sloths: the evidence of mitogenomes and retroviruses. Genome Biology and Evolution, evw023.
  14. ^ Delsuc, F., Gibb, G. C., Kuch, M., Billet, G., Hautier, L., Southon, J., ... & Poinar, H. N. (2016). The phylogenetic affinities of the extinct glyptodonts. Current Biology, 26(4), R155-R156.

External links[edit]