Xenarthrans have the lowest metabolic rates among the therians.
It includes the anteaters, tree sloths, and armadillos, as well as the extinct ground sloths, glyptodonts and pampatheres. In the past, these families were classified 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 thus invalid by cladistic standards. 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, morphological evidence suggests that the Xenarthra are outside the Epitheria, which contains all other known eutherians today.
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 and Folivora (previously Tardigrada), and the separate order Cingulata. Xenarthra now has the rank of cohort or super-order. The Xenarthra are part of the super-cohort Atlantogenata.