See text for more details Xenarthra (/zɛˈnɑːrθrə/; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas.

[2] They evolved and diversified extensively in South America during the continent's long period of isolation in the early to mid Cenozoic Era.

Females show no clear distinction between the uterus and vagina, and males have testicles inside the body, which are located between the bladder and the rectum.

The morphology of xenarthrans generally suggests that the anteaters and sloths are more closely related to each other than either is to the armadillos, glyptodonts, and pampatheres; this idea is upheld by molecular studies.

George Gaylord Simpson first suggested in 1931 that their combination of unique characteristics shows the group evolved from highly specialized early ancestors that lived underground or were nocturnal and dug with their forelimbs to feed on social insects like ants or termites.

[19] These extreme characteristics led to their confusion with unrelated groups that had similar specializations (aardvarks and pangolins), and obscures their relationships with other mammals.

Xenarthrans may have evolved from ancestors that had already lost basic mammalian dental features like tooth enamel and a crown with cusps; reduced, highly simplified teeth are usually found in mammals that feed by licking up social insects.

Several groups of xenarthrans did evolve cheek teeth to chew plants, but since they lacked enamel, patterns of harder and softer dentine created grinding surfaces.

[21] Currently, no living or extinct xenarthrans have been found to have the standard mammalian dental formula or crown morphology derived from the ancient tribosphenic pattern.

Additional points of articulation between vertebrae strengthen and stiffen the spine, an adaptation developed in different ways in various groups of mammals that dig for food.

PCR analysis determined that a mutation in a stem xenarthran led to long-wavelength sensitive-cone (LWS) monochromacy (single color vision), common in nocturnal, aquatic and subterranean mammals.

Analysis of the fossil South American Lujan fauna suggests far more large herbivorous mammals were present than similar contemporary environments can support.

South America had no placental predatory mammals until the Pleistocene, and xenarthran large-mammal faunas may have been vulnerable to many factors including a rise in numbers of mammalian predators, resource use by spreading North American herbivores with faster metabolisms and higher food requirements, and climate change.