Meet the Xenarthrans - Armadillos, Sloths, and Anteaters

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Armadillos, sloths, and anteaters, also known as xenarthrans (Greek for "strange joints"), can be distinguished from other mammals by (among other things) the unique joints in their backbones that endow them with the strength and support they need to pursue their climbing or burrowing lifestyles. These mammals are also characterized by their extremely few (or even no teeth), their relatively small brains, and (in males) their internal testicles.

As you'll know if you've ever seen a sloth in action, xenarthrans are also some of the slowest mammals on earth; they are technically warm-blooded, like other mammals, but their physiologies aren't nearly as robust as those of dogs, cats or cows.

Xenarthrans are an ancient group of placental mammals that once roamed across the expanse of Gondwana, before this giant continent of the southern hemisphere split up to form South America, Africa, India, Arabia, New Zealand, and Australia. The ancestors of modern armadillos, sloths and anteaters were initially isolated on the newborn continent of South America, but in the ensuing millions of years spread northward into areas of Central America and southern parts of North America. Although xenarthrans didn't make it into Africa, Asia, and Australia, these regions are home to unrelated mammals (like aardvarks and pangolins) that evolved the same general body plans, a classic example of convergent evolution.

One little-known fact about xenarthrans is that they were prone to gigantism during the Cenozoic Era, at a time when many mammals achieved dinosaur-like sizes thanks to temperate climates and an abundance of food. Glyptodon, also known as the Giant Anteater, could weigh up to two tons, and its hollowed-out shells were sometimes used by the early human inhabitants of South America to shelter from the rain, while the giant sloths Megatherium and Megalonyx were about the size of the largest bears on earth today!

There are about 50 species of xenarthrans extant today, ranging from the screaming hairy armadillo of South America to the pygmy three-toed sloth of the Panamanian coast.

Classification of Xenarthrans

Armadillos, sloths, and anteaters are classified within the following taxonomic hierarchy:

Animals > Chordates > Vertebrates > Tetrapods > Amniotes > Mammals > Armadillos, sloths and anteaters

In addition, armadillos, sloths, and anteaters are divided into the following taxonomic groups:

  • Anteaters and sloths (Pilosa)
  • Armadillos (Cingulata)