Whether there is any relationship with the Hyracoidea cannot be determined until we are acquainted with the forerunners of Arsinoitherium, which is evidently a highly specialized type.
These and certain other facts referred to by the same author point to the conclusion that not only are the Sirenia and the Proboscidea derived from a single ancestral stock, but that the Hyracoidea - and so Arsinoitherium - are also derivatives from the same stock, which must necessarily have been Ethiopian.
It is now possible to define the suborder Hyracoidea as including ungulates with a centrale in the carpus, plantigrade feet, in which the first and fifth toes are reduced in greater or less degree, and clavicles and a foramen in the lower end of the humerus are absent.
Of the other suborders of ungulates, the Toxodontia and Litopterna are exclusively South American, and while the former may possibly be related to the Hyracoidea and Barypoda, the latter is perhaps more nearly akin to the Perissodactyla.
For many years extinct representatives of the Hyracoidea were unknown, partly owing to the fact that certain fossils were not recognized as really belonging to that group. The longest known of these was originally named Leptodon graecus, but, on account of the preoccupation of the generic title, the designation has been changed to Pliohyrax graecus.