A rough evolutionary
A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.[17] These groups lived in Eurasia and the discovery of Epirigenys in East Africa, which was likely in a sister group to Hippopotamidae, suggests that hippo ancestors entered Africa around 35 mya and were the earliest large mammals to colonise the continent.[18][19]
The oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to 8 million years ago. While hippopotamid species spread across Asia and Europe, no hippos have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago, an ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East.[20]
While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon—an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus—or of Choeropsis, an older and basal genus.[17][20]
Choeropsis madagascariensis skeleton with a modern hippopotamus skull.
Extinct species
Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippo, likely through the process of insular dwarfism.[21] Fossil evidence indicates many Malagasy hippos were hunted by humans, a likely factor in their eventual extinction.[21] Isolated members of Malagasy hippos may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippo.[22]
Three species of hippopotamus, the European hippopotamus (Hippopotamus antiquus), Hippopotamus major and Hippopotamus gorgops, ranged throughout continental Europe and the British Isles. All three species became extinct before the last glaciation. Ancestors of European hippos found their way to many islands of the Mediterranean Sea during the Pleistocene.[23] The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands, including Crete (Hippopotamus creutzburgi), Cyprus (the Cyprus dwarf hippopotamus, Hippopotamus minor), Malta (Hippopotamus melitensis), and Sicily (Hippopotamus pentlandi). Of these, the Cyprus dwarf hippo survived until the end of the Pleistocene or early Holocene. Evidence from an archaeological site, Aetokremnos, continues to cause debate on whether or not the species was encountered, and was driven to extinction, by man.[24][23] Across Eurasia, the hippopotamus became extinct between 50,000-16,000 years ago.[25]
