| Xenarthra Temporal range: Middle Paleocene - Recent |
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| Hoffmann's Two-toed Sloth | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Theriiformes |
| Infraclass: | Eutheria |
| Superorder: | Xenarthra Cope, 1889 |
| Orders and suborders | |
See text for more details |
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The superorder Xenarthra is a group of placental mammals (infraclass Eutheria), extant today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene (about 60-65 Ma, 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 starting about nine million years ago, as part of the Great American Interchange[2] . Nearly all of the formerly abundant megafaunal xenarthrans, such as ground sloths, glyptodonts, and pampatheres went extinct at the end of the Pleistocene.
Xenarthrans share several distinctions from those of other placental mammals. 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]
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Xenarthrans were classified in the past together with the pangolins and aardvarks as the order Edentata (meaning toothless, because the members do not have front incisor teeth or molars, or have poorly-developed molars). It was subsequently realized that Edentata was polyphyletic—that it contained unrelated families and was subsequently 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 that the Xenarthra are outside the Epitheria, which contains all other known eutherians today. Some workers have even placed xenarthrans outside of placentals as a separate group Paratheria.[6]
The morphology of xenarthrans generally suggests that the anteaters and sloths are closest together within Xenarthra, which is upheld by molecular studies. The order Xenarthra is more and more often divided into two orders: Pilosa, containing the Vermilingua (anteaters) and Folivora (sloths; previously known as Tardigrada or Phyllophaga), and the separate order Cingulata (armadillos). Xenarthra now has the rank of cohort or super-order. The Xenarthra are part of the super-cohort Atlantogenata.
Xenarthra may be most closely related to either Afrotheria[7] (in the group Atlantogenata), or Epitheria[8] (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.[9] includes xenarthrans as a sister clade of Boreoeutheria (Euarchontoglires+Laurasiatheria) (in the Exafroplacentalia=Notolegia).
SUPERORDER XENARTHRA
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