Aquatic mammals – dolphins, whales, otters, seals – tend to be especially beloved by us, their distant terrestrial cousins.

Because they’re beloved, they feel familiar. Because they’re familiar, we don’t often stop to think about what strange niches they inhabit: millions of years after shuffling out of the primordial seas and acquiring fur and warm blood, these creatures ended up back in the water.

Paleontologists do stop to think about this. And to them, one aquatic mammal has been an evolutionary black box — the hippopotamus.

On Tuesday, French and Kenyan scientists reported the discovery of a fossil found in Kenya that colours in an important piece of hippo history, and draws scientists a little bit closer to resolving a longstanding puzzle: how hippos and whales branched off from a single common ancestor.

“It’s a really intriguing specimen,” said Jessica Theodor, a paleontologist at the University of Calgary who was not involved in the research.

The research contributes to “the hunt to figure out where hippos come from and how are they related — and it still leaves a lot of interesting, open questions.”

In the 1990s, paleontologists were stunned by DNA and blood protein analysis showing that the closest living relatives of cetaceans – whales, dolphins and porpoises – are hippos.

“The reason everyone’s so hot on where hippos came from is because of the molecular data that linked them to whales,” said Theodor.

A succession of strange and beautiful cetacean fossils stretching back 53 million years helped scientists see how whales evolved. But the hippo fossil story has been a frustrating blank, with the oldest known hippo ancestor only 17 million years old. That has made it difficult to guess what the common ancestor of the two groups looked like, and even led some scientists to question the link.

Now, in a study published in the journal Nature Communications, a team of scientists led by Fabrice Lihoreau at the University of Montpellier has described a set of 30-million-year-old fossilized teeth belonging to a species of anthracothere, an extinct group of hoofed mammals that once roamed from Eurasia to what is now Saskatchewan. Based on both animals’ unusual three-lobed molars, Lihoreau and his colleagues believe the ancient creature was an ancestor of modern hippos.

Most scientists already believed that hippos derived from anthracotheres, but the new study puts paleontologists on firmer footing further back in evolutionary time: the African anthracotheres came from Asia, which puts them much closer to when cetaceans split off.

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But both Lihoreau and Theodor said they wanted to see more remains of this anthracothere species — a skull, at the very least — to say more about the morphology of hippos, which is peculiar even among aquatic mammals: hippos spend most of their time in freshwater but eat grass, and don’t swim so much as trudge along the bottom.

More fossils, perhaps eventually that of the whale-hippo missing link, will help show how cetaceans evolved into fully aquatic mammals while hippos remained partially bound to land.

“When you look through all of the mammals, there is nothing that looks like hippos,” said Lihoreau.