A new anthracothere fossil, which has been identified as the ancestor of the hippopotamus, is reported online this week in Nature Communications. The fossil, from Lokone in Kenya, helps unravel the mysterious origins of the hippopotamus, placing them with the first large mammals to invade Africa approximately 30 million years ago.
Hippopotamuses and cetaceans (whales, dolphins and porpoises) form a distinct group with one common ancestor believed to be an anthracothere (a member of a family of extinct hippopotamus-like creatures). Despite numerous specimens documenting cetacean evolution, the relationship between hippopotamuses and these ancestors remains poorly substantiated by the fossil record and the ancestor of the modern day hippopotamus has not been identified.
Fabrice Lihoreau and colleagues describe an anthracothere fossil, discovered in deposits dated to the Oligocene epoch (from about 33.9 to 23 million years ago). This anthracothere is the most abundant large mammal at Lokone and shows many functionally intermediate characteristics in hippopotamus evolution, including evolutionary changes that may have led to the unique hippopotamid dental morphology. The authors attribute it to a new Oligocene genus and species, which pushes back the evolutionary history of hippopotamids in Africa to the Paleogene period, adding the living hippotamuses to the long standing record of creatures endemic to the continent.