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A RARE fossil find has revealed the identity of the most likely early
ancestor of whales and dolphins. This “archaeopteryx of the oceans” offers a
unique insight into how some mammals made the transition back to the sea.

Scientist agree that cetaceans—the order of mammals comprising whales,
dolphins and porpoises—evolved from a terrestrial ancestor. But while DNA
analysis of living animals points to a hippopotamus-like creature, the fossil
evidence suggests they descended from a carnivorous ungulate—think of a
hyena with hooves.

Now Hans Thewissen, a palaeontologist at Northeastern Ohio University in
Rootstown, says that neither candidate passes muster. Some 50-million-year-old
skeletons found in Pakistan show that whales and dolphins are descended from a
fox-sized beast that looked like a cross between a wolf and a tapir.

The creature had a long, powerful tail and a tapir-like head. It is an
artiodactyl, a group to which goats, camels and cows also belong. Though hippos
are artiodactyls too, a family tree reconstructed by Thewissen suggests the two
are only distant relatives at best.

At first, the fossils from Pakistan threw up more questions than answers.
“Initially we had whale skulls and teeth, but no bones for them,” Thewissen
says. “And we had artiodactyl bones but no skulls for them. Slowly it started to
dawn on us that they could be from the same animal.”

To test the idea, Thewissen and his colleagues measured the ratio of carbon
isotopes in the fossils. They found that the cetacean skulls and teeth had the
same ratio of carbon-12 to carbon-13 as the artiodactyl bones, but a different
ratio from that found in other fossil mammals at the site. Animals absorb the
isotopes from their food, and since different food sources have different
carbon-12 to carbon-13 ratios, this finding suggests the cetacean skulls and
teeth came from the same animal as the artiodactyl bones.

These “intermediate” animals are a rare find that helps explain the dramatic
transition from terrestrial to marine mammals, says Christian de Muizon, a
palaeontologist at the National Museum of Natural History in Paris. Bones in
their inner ears were uniquely cetacean, and features of their ankle bones were
uniquely artiodactyl. “This contradicts the early hypotheses of palaeontologists
and molecular biologists,” he says.

But Wilfried de Jong, a molecular biologist at the University of Nijmegen in
the Netherlands who has studied cetacean evolution, says the new fossils are
hard to reconcile with the DNA evidence. “It’s a major issue to try to bring
together fossil morphological evidence and molecular evidence,” he says.