The odd-toed ungulates are browsing and grazing mammals. They are usually large to very large, and have relatively simple stomachs and a large middle toe. South America once had a great diversity of ungulates of native origin, but these dwindled after the interchange with North America, and disappeared entirely following the arrival of humans. Sequencing of collagen from fossils of one recently extinct species each of notoungulates and litopterns has indicated that these orders comprise a sister group to the perissodactyls.