Xenarthra, order of mammals that includes armadillos, anteaters, and sloths. The order takes its name from xenarthrous processes—extra pairs of joints in certain vertebrae, or spinal bones, found only in members of this order. This unique adaptation helps support the back during digging.

Mammals in the Xenarthra order first appeared in South America during the Paleocene Epoch, about 65 million years ago. When a land bridge formed between North and South America in the Pliocene Epoch, roughly 5 million years ago, members of this order crossed to North America. Today these mammals can be found in Central and South America and the southern central and southeastern areas of North America.

Apart from xenarthrous processes, armadillos, anteaters, and sloths have few similarities. All three have a good sense of smell, but anteaters have poor eyesight, whereas sloths and armadillos have relatively good vision. Armadillos also have a keen sense of hearing, while anteaters and sloths have poor hearing. Anteaters are known for their tubular snout and long tongue, which they use to eat ants and termites; armadillos eat insects, small vertebrates, and plants; and sloths eat only plants. Armadillos are ground dwellers and are active during the day or night. Most anteaters and all sloths live in trees and are active primarily at night.

Mammals in the Xenarthra order were formerly referred to as Edentates, which means “toothless,” although only the anteaters are actually toothless. Armadillos and sloths have molars that in some species may lack the enamel covering found on the teeth of most other mammals. In this earlier classification, the Edentates also included scaly anteaters, aardvarks, and duck-billed platypuses. Comparative anatomy studies of these mammals revealed significant differences that distinguish them from the animals now in the Xenarthra order, including a lack of xenarthrous processes.

Scientific classification: The Xenarthra order is in the class Mammalia in the phylum Chordata. The armadillo makes up the family Dasypodidae; the anteater makes up the family Myrmecophagida; and the sloth is a member of the two-toed sloth family, Megalonychidae, or the three-toed sloth family, Bradypodidae.