Image: Copyright Science / Painting by JOHN KLAUSMEYER
Hairless, legless and confined to the sea, whales make for unlikely mammals. But millions of years of evolution can yield surprising results. In the case of whales and their cetacean kin, it led to one of the most dramatic transformations known, producing fully aquatic mammals from terrestrial ones. For that reason, whale origins have long fascinated scholars.
The fossil record documents much of the whales' land-to-water transition. Determining which mammalian group gave rise to these leviathans, however, has proved difficult. Scientists agree that whales are actually highly specialized ungulates, or hoofed mammals. The question has been, to which ungulates are they most closely related? Traditionally, paleontologists have posited that whales descended from extinct hyenalike creatures called mesonychians, based on dental similarities between the two groups. But in recent years molecular biologists have put forth a different hypothesis¿based on DNA from living animals¿asserting that the ancestors of whales were instead artiodactyls¿a group whose extant members include hippopotamuses, pigs, camels and ruminants. Furthermore, several molecular studies have concluded that whales share a common artiodactyl ancestor with hippos and are thus more closely related to these animals than to any other living artiodactyl or to a mesonychian.
Now newly found fossils from Pakistan may resolve some of the confusion. According to two research teams, the ancient whale remains reveal telltale signs of artiodactyl ancestry. How the fossils bear on the proposed whale-hippo link, however, is less clear. The findings appear in the September 20 issue of the journal Nature and the September 21 Science.
Over the past decade paleontologists have unearthed a number of striking fossils from amphibious whales¿such as the bizarre-looking Ambulocetus, which moved easily between land and sea on well-developed limbs¿and wholly aquatic ones. The remains described in the Science report add substantially to that body of knowledge, revealing for the first time fully terrestrial whales.
J.G.M. Thewissen of the Northeastern Ohio Universities College of Medicine and his colleagues discovered the fossils in 50-million-year-old deposits in the Kala Chitta Hills of Punjab, Pakistan. The remains include several skulls, snout fragments and a number of postcranial bones representing two species of primitive whales known as pakicetids: wolf-size Pakicetus attocki and fox-size Ichthyolestes pinfoldi. Detailed analyses of the skull and locomotor skeleton, the team reports, reveal adaptations characteristic of land-dwelling mammals. In fact, many features, such as long, slender limb bones, indicate that the pakicetids were built to run.
Image: Courtesy of NORTHEASTERN OHIO UNIVERSITIES COLLEGE OF MEDICINE