In evolutionary terms, the rhinoceros is a surviving member of one branch of an ancient line of animals, the ungulates, the first animals to develop hooves. The rhinoceros belongs to a group of ungulates known as the perissodactyls or odd-toed ungulates; this group also includes the horse and tapir. There are currently five extant species of rhinoceros in the world. Of these five, four are under severe threat of facing extinction. There are efforts underway to help preserve the rhinoceros, but those efforts may lack the time and resources necessary to insure their survival. The two species found in Africa are the white and the black rhinoceros. They are respectively classified as Ceratotherium simum and Diceros bicornis. The two African species are easily recognized by the differences in their lips; the white rhinoceros is square-lipped, the black rhinoceros is hooked-lipped.
