High Quality Content by WIKIPEDIA articles! The superorder Xenarthra is a group of placental mammals (infraclass Eutheria), extant today only in the Americas. THe origins of the order can be traced back as far as the early Tertiary (about 60 million years ago, shortly after the Mesozoic era). XEnarthrans developed and diversified extensively in South America during its long period of isolation, invaded the Antilles by the early Miocene, and then spread to Central and North America starting about nine million years ago, as part of the Great American Interchange. NEarly all of the formerly abundant megafaunal xenarthrans went extinct at the end of the Pleistocene.