Xenarthra and Pholidota (Armadillos, Anteaters, Sloths and Pangolins)

The mammalian order Xenarthra includes the armadillos, sloths and anteaters, and the extinct glyptodonts; the mammalian order Pholidota comprises the pangolins or scaly anteaters. Although they were once thought to be closely related, Xenarthra is now generally considered to represent one of the four primary divisions of placental mammals, with pangolins placed in a separate division. Xenarthrans are united by a suite of unusual anatomical features, primary among them the presence of extra joints in their backbones, whereas pangolin's most notable feature is their external covering of overlapping, horny scales. Both xenarthrans and pholidotans are typified by adaptations for digging and for feeding on ants and termites, though climbing forms are also common, and sloths and their relatives are herbivorous. Both orders are relatively small, with Xenarthra comprising 31 living species and Pholidota only 8, but Xenarthra also includes a very extensive extinct radiation of highly unusual mammals, including giant herbivorous sloths and the massively armoured, herbivorous glyptodonts. Xenarthrans are found in a variety of habitats throughout Central and South America, as well as southern North America, whereas pangolins are confined to the Old World tropics, from sub-Saharan Africa to southeast Asia, mostly in forested habitats. Key Concepts: The order Xenarthra includes the living armadillos, sloths and anteaters, along with unusual extinct forms like ground sloths and glyptodonts; the order Pholidota is comprised of the living pangolins or scaly anteaters. Xenarthra and Pholidota were once considered close relatives, but are now thought to be only distantly related among placental mammals. Xenarthrans are united by their possession of a suite of unusual anatomical features, in particular extra joints found in the thoracic and lumbar vertebrae; each subgroup has its own unique anatomical adaptations, for example, the complex bony carapace of the armadillos. Pholidotans are characterised by an external covering of overlapping, horny or keratinous epidermal scales. Both Xenarthra and Pholidota are small in terms of the living diversity of species, but Xenarthra is more diverse, with three very different subgroups. Xenarthrans and pholidotans are found in a variety of habitats throughout the New World and Old World tropics respectively, but both contain a significant number of threatened species, due largely to hunting pressure and habitat destruction. Most of the species in Xenarthra and Pholidota are characterised by adaptations for digging and for eating ants and termites, though both groups include climbing forms, and Xenarthra also includes a radiation of arboreal herbivores, the sloths. The modern xenarthran fauna is only an impoverished remnant of a once much more diverse radiation, one that included such extraordinary creatures as giant ground sloths, pampatheres (giant herbivorous armadillos) and glyptodonts (heavily armoured, herbivorous relatives of armadillos). Palaeanodonts are an extinct group of digging, ant-eating or insect-eating specialists that may be related to pangolins. Keywords: armadillo; sloth; anteater; pangolin; glyptodont; phylogeny; fossils; diversity