Fossils of an Elusive Hippo Ancestor Found in Africa
Newly-studied fossils solidify the evolutionary link between hippos and whales.
Drawing and photograph illustrating the evolutionnary transition of the upper molar from an anthracothere (left), Epirigenys (middle), and a primitive hippo (right). Black circle are the cusps (main relief of the tooth surface), black line are the crests and in orange the styles (enamel islets). Image: Fabrice Lihoreau/LPRP
Hippos are strange creatures. For one, they're large herbivores that spend most of their time in water but feed on land, a behavior shared by few other animals except beavers and really large rodents called capybaras. Their morphology is strange too, particularly their teeth: they have thick, high-crowned molars that wear down into a clover shape and are not found in any other mammals.
The hippo's peculiar traits have made it difficult for researchers to pin down the animal's evolutionary history. Now, newly-identified 30-million-year-old tooth fossils shed light on the hippo's mysterious origins. The fossils, found at Lokone Hill in Kenya's Rift Valley, strengthen scientists' belief that hippos and whales share a common ancestor.
Described in a Nature Communciations paper released today, the discovery suggests that hippo ancestors migrated to Africa as early as 35 million years ago, making the hippo one of a small group of mammals with early evolutionary origins in Africa.
Scientists have suspected for awhile that hippos have ancient evolutionary roots in Africa. But the problem, according to Fabrice Lihoreau, a paleontologist at Montpellier University in France, was that researchers hadn't found fossils of hippo ancestors in the African fossil record.
"We therefore suspected what we call a ghost lineage," said Lihoreau, who led the fossil discovery, "that a group of mammals leading to hippos must be present but were never found."
The scientists have assigned the fossils to a new species they named Epirigenys lokonesis (a play on words meaning "hippo origin"). The species belongs to an extinct family of animals called anthracotheres, which scientists believe are a common ancestor of whales and hippos, as well as pigs, cows, and goats. Anthracotheres thrived in Africa and Eurasia and resembled something like a cross between a pig and a small hippopotamus.
The teeth and jawbone fossils Lihoreau's team found fit neatly with the dental oddities of modern hippos. "The new species is exactly what we expected to find at the right time, the right place and with the right morphology," said Lihoreau, whose team included researchers from Ethiopia and Kenya.
Because the researchers only found dental fossils, they can only speculate about other details of Epirigenys lokonesis's appearance. "We can say that this species was an herbivorous animal of 70-100 kg with a size close to that of a large sheep," said Lihoreau. He thinks Epirigenys likely looked like a small and slender hippo, and was likely at least semi-aquatic, if not fully aquatic.
"This research really helps fill in a gap in the fossil record," said Christine Janis, a paleontologist at Brown University. "You've got stronger arguments now supporting the idea of this early divergence between whales and hippos."
The fossil discovery helps scientists understand what happened after whales and hippos diverged, a story they are still trying to piece together. "We have a great fossil record of whales," said Janis, "so the whale side isn't a mystery. The hippo side is."
Scientists know that cetaceans, which include whales, dolphins and porpoises, originated in Southeast Asia about 50 million years ago. But until now, the earliest fossil hippo fossil only dated back to 20 million years ago in Africa.
Lihoreau's team believes that the species might have swum over to Africa some 30 million years ago from Asia. "Africa was an island—a large one—at this time," he said. This puts hippos in a selective group of mammals, others being elephants and small furry creatures called hyraxes, with a deep evolutionary history in Africa. Many animals that we think of as distinctly African, such as lions, antelopes and giraffes, actually migrated to the continent more recently, closer to 18 million years ago, said Lihoreau.
It is satisfying that this research not only strongly links anthracotheres and modern hippos, but in particular pins the existence of anthrocotheres to a specific time and place in Africa, said Jonathan Geisler, a paleontologist at the New York Institute of Technology. However, it's important to consider that the tooth anatomy of hippos and anthracotheres is really complicated, he said.
"This is a group of careful, meticulous scientists," said Geisler. "They've done a really good job of figuring how these different tooth forms evolved. But because the forms are so complicated, there's a chance that the picture's more complex than they realize. It's really important to get more fossils of these early hippos and anthracotheres, not just jaws and teeth, to really test their hypothesis."
Lihoreau and his colleagues hope to continue scouting for a more complete Epirigenys skeleton. He thinks this species is the key to telling the story of how whales came to be. "The evolutionary history of hippos [is] a challenge because we have to consider a huge degree of evolution that was not registered in the fossil record," he said. "Epirigenys [is] the fossil that we need to reconstruct the whole history."