Xenarthra

Xenarthra (from Greek xeno, meaning "strange", and arthron, meaning "joints") is an order of mammals that includes anteaters, armadillos, and sloths. This order is considered to be one of the youngest and most primitive orders of mammals, having evolved in South America during the Oligocene. Xenarthrans differ from other mammals in that they have an additional articulation (joint) between the vertebrae of their backbone. This provides them a greater degree of flexibility, allowing them to curl up into a ball when threatened. Xenarthrans also have significant differences from other mammals in their digestive and immune systems, as well as their teeth, claws, and fur. They are important indicators of the biodiversity of ecosystems, and their unique adaptations are keystones for conservation.