It's long been thought that hippopotamuses share an evolutionary ancestor with whales, but gaps in the fossil record kept scientists from making the connection. In a new study in Nature Communications, researchers report on the discovery of a new species -- one identified by 30 million-year-old molars found in Kenya -- that reunites the modern hippo with its family tree.

Hippos and whales may both have reputations for lolling about in the water, but they're pretty different. Hippos (which come in two varieties these days, common and pygmy) keep to African lakes. Whales are widespread throughout the world's oceans. But DNA analysis suggested that these creatures all came from a common ancestor.

"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," said lead study author Fabrice Lihoreau of the University of Montpellier.

"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."

The new species (which is also part of a new genus of its own) is called Epirigenys lokonensis -- a play on words for "hippo origin" and a nod to the Lokone Hills of Kenya, where the fossil remains were found. The first molar was actually found in 1994 in an expedition led by Meave Leakey, but the new study's authors returned to the former African lake bed in 2007, after realizing that the specimen might be the missing link they'd been seeking.

"We imagined what the morphology of a hippo ancestor would be," Lihoreau said -- and then his team went looking for fossils to match it. "The Kenyan fossil was exactly what we were looking for."

Epirigenys lokonensis is a member of the anthracothere family -- an extinct family of animals not too unlike a hippo -- and scientists had already linked that family to a common ancestor with whales, swine, and ruminants like cows and goats.

But this anthracothere seems to be a clear transitional link between those previously discovered and the modern hippo.

"It has a very unique morphology," Lihoreau said of the molar and other fossils, which include other teeth and a piece of mandible. "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. This new species shows features very typical of hippos."

For now, the team only has teeth. These are often the last parts of a skeleton to survive. But because Lokone seems to have had an abundance of the creatures, Lihoreau and his colleagues are hopeful that they'll find a complete skeleton to study soon.