Bones Boost Whale-Hippo Link
HAYAMA, JAPAN--A whale may be just an overgrown hippopotamus with an unusual lifestyle. A new analysis of early whale fossils reported here last week at the International Symposium on the Origin of Mammalian Orders could tip the balance in a dispute about which land mammals went back to the water to become whales. The new fossil evidence gives tentative support to a conclusion reached by researchers who study DNA to learn how animals are related: Cetaceans--whales, dolphins, and porpoises--are descendants of an ancestor related to the even-toed ungulates, or artiodactyls, which include cows, pigs, and hippos, perhaps the closest living relative of whales.
Paleontologists have long rejected that conclusion, insisting that cetaceans descended from extinct hyenalike mammals called mesonychians. But at the meeting, whale evolution expert Hans Thewissen, a paleontologist at Northeastern Ohio Universities College of Medicine in Rootstown, said that analyses of new specimens of early whales and whale ancestors his team collected in Pakistan weaken the mesonychian link. The teeth of a 50-million-year-old whale called Pakicetidae, said Thewissen, are not as highly evolved as those of the mesonychians, making it unlikely that whales are the descendants of that group.
The fossil whales give mixed evidence about whether the cetaceans belong among the ungulates. Thewissen says that five traits of the early whales, including features of the skull, upper teeth, and feet, are "not inconsistent" with the hippo hypothesis. But the last molar on the lower jaw, which has three sections in artiodactyls, has just two in whales. And Thewissen recently discovered an anklebone from an early whale ancestor that still had legs. It lacks the rounded head characteristic of an ungulate anklebone, although it is similar in other respects.
Thewissen thinks his findings open the door to a tentative link between whales and ungulates. Several paleontologists at the meeting agreed that the whale-hippo link is looking more plausible, and Norihiro Okada, a molecular biologist at the Tokyo Institute of Technology, thinks the case will soon get stronger: "I think paleontologists may discover more [features common to early cetaceans and early hippos] in the near future."