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

Abstract

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

Figure 1.

Living edentates and pholidotans. (a) Greater long‐nosed armadillo, Dasypus kappleri; (b) three‐toed sloth, Bradypus variegatus; (c) giant anteater, Myrmecophaga tridactyla; (d) long‐tailed pangolin, Phataginus tetradactyla and (e) giant pangolin, Smutsia gigantea. (a)–(c) from Eisenberg and (d) and (e) from Kingdon .

Figure 2.

Skeleton and life restoration of the glyptodont Glyptotherium arizonae. From Gillette and Ray .

Figure 3.

(a) Cladogram summarising relationships within the Cingulata, as proposed by Gaudin and Wible . Animals depicted at the right of the figure, starting at the top: Glyptodon (Pleistocene, SA), Chaetophractus (hairy armadillo), Tolypeutes (three‐banded armadillo), Dasypus (long‐nosed armadillos). Skulls depicted in left lateral view at the far right of the diagram, starting at the top: Propalaeohoplophorus (glyptodont, Miocene, SA), Holmesina (pampathere, Pleistocene, NA and SA), Proeutatus (Miocene, SA), Euphractus (yellow armadillo), Tolypeutes, Dasypus, Peltephilus (Miocene, SA). Abbreviations: NA=North America, SA=South America. Figure from Gaudin and McDonald . (b) Cladogram summarising relationships within the Folivora (=Tardigrada, Phyllophaga), as proposed by Gaudin . Sloths are divided into four monophyletic families, the Megalonychidae, Megatheriidae, Nothrotheriidae and Mylodontidae. The relationship of certain basal, Miocene megatherioid sloths to the three main megatherioid families (Megalonychidae, Megatheriidae and Nothrotheriidae) is not resolved. The living Bradypus is placed as the sister taxon to all other sloths, whereas living Choloepus is incorporated in the family Megalonychidae. Animals depicted at the right of the figure from top to bottom: Choloepus (two‐toed sloth), Megatherium (Pleistocene, SA), Bradypus (three‐toed sloth). Skulls depicted in left lateral view at the far right of the diagram, from top to bottom: Acratocnus (Pleistocene, WI), Choloepus, Eremotherium (Pleistocene, NA and SA), Nothrotheriops (Pleistocene, NA), Hapalops (Miocene, SA), Paramylodon (Pleistocene, NA), Bradypus. Abbreviations: NA, North America; SA, South America and WI, West Indies. From Gaudin and McDonald .

Figure 4.

Skeleton of the Pleistocene megatheriid ground sloth Nothrotheriops shastensis (Stock, ).

Figure 5.

Skull and life restoration of the late Eocene palaeanodont Xenocranium pileorivale (Rose and Emry, ).

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Further Reading

Gardner AL (2008) Mammals of South America. Volume 1. Marsupials, Xenarthrans, Shrews, and Bats. Chicago, IL: University of Chicago Press.

Loughry J and Vizcaíno S (eds) (2008) Biology of the Xenarthra. Gainesville, FL: University of Florida Press.

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Redford KH and Eisenberg JF (1992) Mammals of the Neotropics. The Southern Cone, vol. 2. Chicago, IL: University of Chicago Press.

Rose KD (2006) The Beginning of the Age of Mammals. Baltimore, MD: Johns Hopkins University Press.

Simpson GG (1980) Splendid Isolation. New Haven, CT: Yale University Press.

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Rose, KD, and Gaudin, TJ(Sep 2010) Xenarthra and Pholidota (Armadillos, Anteaters, Sloths and Pangolins). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001556.pub2]