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

See text for more details

The superorder Xenarthra is a group of placental mammals, extant today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced as far back as the Paleocene, as early as 59 million years ago in South America.[1] Xenarthrans developed and diversified extensively in South America during its long period of isolation in the early to mid Cenozoic Era. They invaded the Antilles by the early Miocene and, starting about 9 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. 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, there is fusion of the ischium and sacrum.[3] The males have internal testicles, which are located between the bladder and the rectum.[4] Furthermore, xenarthrans have the lowest metabolic rates among the therians.[5][6]

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 front incisor teeth 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. Because they lack characteristics believed to be present in the common ancestor of other known eutherian mammals, some weak morphological evidence suggests Xenarthra is outside Epitheria, the group that contains all other eutherians known today. Some even place xenarthrans outside of placentals, in the separate group Paratheria.[7]

The morphology of xenarthrans generally suggests 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).

Xenarthra may be most closely related to either Afrotheria[8] (in the group Atlantogenata), Boreoeutheria (in the group Exafroplacentalia), or Epitheria[9] (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.[10] includes xenarthrans as a sister clade of Euarchontoglires within Boreoeutheria (Laurasiatheria+Euarchontoglires).




  1. ^ O'Leary, M. A.; Bloch, J. I.; Flynn, J. J.; Gaudin, T. J.; Giallombardo, A.; Giannini, N. P.; Cirranello, A. L. (2013). "The placental mammal ancestor and the post–K-Pg radiation of placentals". Science 339 (6120): 662–667. doi:10.1126/science.1229237. PMID 23393258. 
  2. ^ Woodburne, Michael O. (2010). "The Great American Biotic Interchange: Dispersals, Tectonics, Climate, Sea Level and Holding Pens". Journal of Mammalian Evolution 17 (4): 245–264. doi:10.1007/s10914-010-9144-8. PMC 2987556. PMID 21125025. 
  3. ^ Delsuc, Frédéric; Catzteflis, François M.; Stanhope, Michael J.; Douzery, Emmanuel J. P. (August 2001). "The evolution of armadillos, anteaters and sloths depicted by nuclear and mitochondrial phylogenies: implications for the status of the enigmatic fossil Eurotamandua" (PDF). Proc. R. Soc. Lond. B 268 (1476): 1605–15. doi:10.1098/rspb.2001.1702. PMC 1088784. PMID 11487408. 
  4. ^ Kleisner, K; Ivell, R; Flegr, J (2010). "The evolutionary history of testicular externalization and the origin of the scrotum" (PDF). Journal of biosciences 35 (1): 27–37. doi:10.1007/s12038-010-0005-7. PMID 20413907. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ Gaudin, Timothy J.; Wible, John R.; Hopson, James A.; Turnbull, William D. (1996). "Reexamination of the morphological evidence for the cohort Epitheria (Mammalia, Eutheria)". Journal of Mammalian Evolution 3: 31–79. doi:10.1007/BF01454253. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. 

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