Hippo history extracted from fossil teeth found in Kenya

Fossil teeth recently found in Kenya may help fill in some of the holes in the history of hippopotamuses.

The teeth, which are roughly 28 million years old, belong to a newly identified hippo ancestor named Epirigenys lokonensis. This extinct species links the oldest known family of hippos in Africa with their earlier Asia-dwelling ancestors, researchers from France and Kenya report online February 24 in Nature Communications.

Grooves in the teeth of E. lokonensis have similar patterns to those in teeth of anthracotheres, a family of extinct relatives of hippos and whales that lived about 40 million years ago in what is now Southeast Asia. But the enamel on E. lokonensis’ teeth is thicker and the points are blunter. The shape of the premolars is also more similar to that of hippo relatives that roamed Uganda about 21 million years ago.

This analysis of E. lokonensis’ teeth fills gaps in the story of how hippos got to Africa, the scientists say. The continent was isolated from other landmasses from nearly 110 million to 18 million years ago. Fossil evidence suggests that about 35 million years ago, small groups of primates and anthracotheres migrated from Asia to Africa. More species of mammals migrated again later, around 20 million to 18 million years ago, when a land bridge connected Asia and Africa. But E. lokonensis’existence in Africa 28 million years ago suggests that hippos are descendants of the first wave of anthracotheres — the earliest large terrestrial mammals — to invade Africa.

To get to Africa, the anthracotheres needed to be able to swim, suggesting that a semiaquatic lifestyle evolved early in hippos’ history, the researchers say. Knowing when this skill evolved in anthracotheres could help scientists reconstruct the common ancestor between hippos and their closest living relatives — whales, dolphins and porpoises.