Researchers have unearthed fossil molars belonging to a hippopotamus ancestor in Kenya. The findings, published in Nature Communications this week, suggests that hippo ancestors were the very first large mammals to arrive in Africa, some 30 million years ago from Eurasia. Not lions, not rhinos, not giraffes.
Based on genetic data, hippos and cetaceans (whales, dolphins, and friends) are more closely related than they are to pigs and ruminants like cows. They form an exclusive group with one common ancestor believed to be a member of an extinct semi-aquatic, hippo-like group called the anthracotheres. And while we have spectacular fossils documenting whale evolution (such as whale ancestors that looked like hyenas), the origins of hippos are far more mysterious. Gaps in the fossil record have made it hard to understand the relationship between hippos and the ancestor they share with whales. "We know quite well the story of whales, because lots of people are looking for fossils of whales, and we have a complete evolutionary history of them," Fabrice Lihoreau of Université Montpellier II tells Washington Post. "But for the hippo, we only knew what was going on in the past 20 million years. Earlier than that, we couldn't recognize anything as a hippo."
Back in 1994, a lower jaw from a previously unknown anthracothere was uncovered in the Lokone Hills of Kenya within Oligocene deposits dated between 33.9 million and 23 million years ago. Lihoreau and colleagues went back in 2007, and several teeth and subsequent analyses later, the team discovered that the anthracothere features many functionally intermediate characteristics in hippo evolution—including changes that likely led to the unique teeth of modern hippos.
"Each species of mammal has a unique tooth morphology, very distinct from one species to another, so you can use them to find similarities between two species,” Lihoreau says. “This new species shows features very typical of hippos." They named it Epirigenys lokonensis. “Epiri” is hippopotamus in Turkana, Kenya, and “genys” is a play on ancient Greek words for origin, source, lineage, and for jaws.
Here's an illustration of the evolutionary transition of the upper molar from an anthracothere (left), Epirigenys (middle), and a primitive hippo (right). Black circles are the cusps (main relief of the tooth surface), black lines are the crests, and enamel islets are in orange.
Epirigenys was the most abundant large mammal at Lokone. At about 100 kilograms (220 lbs), they were much smaller than today’s hippos. "They are slender hippos, very thin hippos," Lihoreau tells Live Science. Furthermore, this new genus and species pushes back the evolutionary history of hippopotamids in Africa to the Paleogene, which ended around 23 million years ago. The work adds modern hippos to the long-standing record of creatures native to the continent. Compared to safari favorites like lions and antelopes, “this is really an African mammal,” he adds.
Below are the simplified phylogenetic relationships between hippos, anthracotheres, and cetaceans, as well as the position of Epirigenys:
Images: Fabrice Lihoreau/LPRP