When I first met Hans Thewissen, he spending an afternoon standing on a table, pointing a camera at a fossil between his feet. He asked me to hold a clip light to get rid of some shadows. I felt like I was at a paleontological fashion shoot.
Thewissen was taking pictures of bones from a whale that walked. As I later wrote in my book At the Water’s Edge, Thewissen has discovered some crucial clues to the transitions that the ancestors of whales made from land to sea. In Pakistan, he discovered a 47-million-year-old fossil called Ambulocetus natans, that had an otter-like body. It was the first whale fossil ever found with functional legs. New fossils of other ancient whales have since surfaced. The bones Thewissen was photographing, for example, belonged to an even older, even more terrestrial relative of today’s whales, called Pakicetus.
When Thewissen and other have compared these fossils to those of other mammals, they’ve found that whales either evolved from even-toed ungulates (known as artiodactyls) or a close relative of artiodactyls. Meanwhile, other scientists have been comparing the genes of whales to other mammals, and they’ve found that one kind of artiodactyl–hippos–is the closest living relative to whales. But the fossil record of hippos is pretty sketchy. The oldest member of the family is only 15 million years old.
So I was pretty excited to read the newest paper from Thewissen and his colleagues, published in tomorrow’s issue of Nature. They’ve identified what they believe is the closest fossil relative of whales. It’s a raccoon-sized beast named Indohyus that lived 48 million years ago in Kashmir. Analyzing the bones of Indohyus, the scientists discovered that it shares some–but not all–of the traits previously considered unique to cetaceans from Pakicetus to today’s whales and dolphins.
Even more intriguing is the evidence suggesting that Indohyus was fairly aquatic. The evidence comes from isotopes in the fossils, as well as from the structure of the bones. Living mammals that spend a lot of time underwater tend to have heavy bones that they use to keep them from floating up to the surface of the water. So does Indohyus. Its teeth appear adapted for eating vegetation. It might have eaten underwater, like muskrats do today, or on land, as hippos do. Its adaptations to water may have helped it find refuge from predators on land. (The inimitable Carl Buell, who illustrated walking whales for me in At the Water’s Edge, has painted this portrait of Indohyus.)
If Thewissen’s right, then a key step in the origin of whales was the transition from eating plants to eating meat. (Pakicetus and other early whales show signs of having been meat-eaters.) But that transition came after the ancestors of whales had already started to take the plunge.
For more, watch Thewissen talks about Indohyus and whales on this video
Source: Thewissen et al, “Whales originated from aquatic artiodactyls in the Eocene epoch of India,” Nature, http://www.nature.com/doifinder/10.1038/nature06343