The superorder Xenarthra is a group of placental mammals, extant today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced as far back as the Paleocene, as early as 59 million years ago in South America. Xenarthrans developed and diversified extensively in South America during its long period of isolation in the early to mid Cenozoic Era. They had invaded the Antilles by the early Miocene and, starting about 9 Mya, spread to Central and North America as part of the Great American Interchange. Nearly all of the formerly abundant megafaunal xenarthrans, such as ground sloths, glyptodonts, and pampatheres, became extinct at the end of the Pleistocene.
Xenarthrans share several characteristics not present in other placental mammals. The name Xenarthra, which means "strange joints", was chosen because their vertebral joints have extra articulations unlike other mammals. This trait is referred to as "xenarthry". Also, unlike other mammals, the ischium and sacrum are fused. The males have internal testicles, which are located between the bladder and the rectum. Furthermore, xenarthrans have the lowest metabolic rates among the therians.