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Temporal range: Paleocene –Recent, 59–0Ma
Scientific classification
Orders and suborders

The Xenarthra is a superorder of mammals. It is a group of placental mammals (infraclass Eutheria). They live at present only in the Americas, and are anteaters, tree sloths, and armadillos.

Their origins can be traced back as far as the Palaeogene (about 60-65 million years ago (mya), shortly after the Mesozoic) in South America.[1] Xenarthrans developed and diversified extensively in South America during its long period of isolation, invaded the Antilles by the early Miocene, and then spread to Central and North America as part of the Great American Interchange.[2]

Nearly all of the once abundant big xenarthrans, such as ground sloths, glyptodonts, and pampatheres went extinct at the end of the Pleistocene.

Xenarthrans differ from other placental mammals in several ways. 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 males lack external testicles, which are instead placed between the bladder and the rectum.[3] Also, xenarthrans have the lowest metabolic rates among the therians.[4][5]


  1. Archibald, J. David (August 2003). "Timing and biogeography of the eutherian radiation: fossils and molecules compared". Molecular Phylogenetics and Evolution 28: 350–359. PMID 12878471 . 
  2. Woodburne, Michael (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 . Retrieved 18 October 2011. 
  3. Kleisner, Karel; Richard Ivell & Jaroslav Flegr (March 2010). "The evolutionary history of testicular externalization and the origin of the scrotum". J. Biosc. 35 (1): 27–37. PMID 20413907 . 
  4. Elgar, M.A.; Harvey P.H. (1987). "Basal metabolic rates in mammals: allometry, phylogeny and ecology". Functional Ecology (British Ecological Society) 1 (1): 25–36. doi:10.2307/2389354 . 
  5. Lovegrove, B.G. (2000-08). "The zoogeography of mammalian basal metabolic rate". The American Naturalist (The University of Chicago Press) 156 (2): 201–219. doi:10.1086/303383 . PMID 10856202 .