BioEd Online

Researchers have analysed fossils of what seems to be the missing link between whales and land-based mammals.

The fossils help to show how cetaceans, including whales, dolphins and porpoises, might have evolved from artiodactyls — even-toed hoofed mammals — some 50 million years ago. The teeth hint that the wading creature still preferred terrestrial plants over fish, indicating that it probably took to water to get away from predators rather than in search of food.

Hans Thewissen, at Northeastern Ohio Universities College of Medicine in Rootstown, and his colleagues examined fossils of a small, stocky artiodactyl belonging to a group called the raoellids that were found in Kashmir, India. The creature — Indohyus — looked similar to a mini deer and probably waded in the water like a hippo. That doesn’t sound much like a whale, but Indohyus has similarities to whales and other cetaceans that have not been seen before in raoellids or in other artiodactyls.

Specifically the fossil hosts a thick covering of bone over the middle-ear space (called the involucrum). Before this, the involucrum had only ever been seen in cetaceans. The findings are published this week in Nature¹.

Vegetarian diet

Thewissen’s team also looked at the teeth of Indohyus, to find out what it ate. There has been debate about whether cetacean teeth had adapted to meat-eating before or after they became aquatic.

The levels of different carbon and oxygen isotopes in the tooth enamel of land-dwelling animals differ from those in aquatic animals because of the different isotope compositions in the food and water that they ingest. Indohyus’s teeth have higher levels of the carbon-13 isotope than is typical for water-foraging whales from the Eocene, suggesting that it fed on land-based plants instead. "We'd like to know in more detail what it ate," says Thewissen. "Isotopes found in the teeth indicate that it was not submerged vegetation. We'll be studying that in the future."

Another clue as to how Indohyus lived can be found in its limb bones, which were thickened and heavy in the same way that a hippo’s are. This suggests the animal was a wader, with heavy bones to help stop it from floating.

Safety in the water

Based on this evidence, Thewissen suggests that the ancestors of whales took to the water as a predator-avoidance mechanism, and didn't develop specific aquatic feeding behaviour until much later.

Palaeontologist Jonathan Geisler, from Georgia Southern University in Statesboro, had previously identified a link between raoellids and whales, but his evidence was based only on small fragments of teeth. This new work solidifies the link, he says.

"What is really important about these fossils is that they seem to confirm the hypothesis that the ancestor of cetaceans became semi-aquatic before evolving teeth specialized for eating fish," says Geisler.