Kate Wong in Scientific American:
With its gaping maw, hairless body and eyes that sit high on its head, the semiaquatic hippo is one of the most distinctive members of Africa’s mammalian menagerie. Two species exist today–the common Hippopotamus amphibius and the smaller Liberian hippo, Choeropsis liberiensis–and 40 more are known from the fossil record. Experts agree that hippos belong to the mammalian order Artiodactyla, a group of even-toed, hoofed creatures whose extant representatives include camels, pigs and ruminants such as cows. But exactly where hippos sit on the artiodactyl family tree has proved devilishly difficult to discern.
Two hypotheses lead the pack. The first posits that the piglike peccaries, or tayassuids, are the closest relatives of the hippo. The second proposes that extinct swamp-dwelling creatures called anthracotheres own that distinction.
More here.
