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Whales' Evolutionary Tale

By Sean Henahan, Access Excellence Science Updates

missing linkAnn Arbor, MI (9/24/01)- Although considered land-dwelling mammals, hippos have evolved numerous adaptations for a life spent mostly in the water. A combination of molecular biological sleuthing and good old fashioned paleontology now suggests that hippos and whales share a branch on the family tree, with one branch staying at least partly land based and the other heading for the water full time.

New research published by two separate groups in the journals Science and Nature provide surprising new information indicating that previous explanations of whale evolution were wide of the mark. Paleontologists have debated the time and manner in which mammals decided to return to the aquatic environment for many years. One school of thought maintained that the similarities in the teeth between ancient whales and an extinct line of carnivorous mammals known as Mesonychia suggested the two were related. The other school of thought held that whales shared common ancestry with Artiodactlya, the family of even-toed ungulates that includes pigs, camels, giraffes and hippopatami. The recent discoveries tend to support the latter hypothesis.

Image: Rodhocetus, a whale that lived 47 million years ago, visualized on the basis of new Eocene fossils from Pakistan. The ankle bones indicate a close relationship of early whales to hooved land mammals such as hippopotami and pigs. Forefeet retain hooves on the central digits, but hind feet with slender webbed toes indicate that Rodhocetus was predominantly aquatic. [Painting by John Klausmeyer, University of Michigan Exhibit Museum]

In the Science article, Dr. Philip D. Gingerich, Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, reports the discovery of the two new fossil mammal species, Artiocetus clavis and Rodhocetus balochistanensis, dating back some 47 million years ago. Dr. Gingerich has been toiling for years in the rough terrain of Balochistan Province in Northern Pakistan to find the missing link between whales and their earlier land-based ancestors. The fossil remains of these mammals suggest they lived primarily in the water, but where able to pull themselves up on the shore rather like modern day seals. Perhaps the most important aspect of the new fossil discoveries is that they include the bones from the fore and hind limbs including the astralagus and cuboid (ankle) bones. The astralagus was the real clincher. This bone is part of a unique 'double-pulley' skeletal structure seen only in artiodactyls and fossil whales. The presence of characteristic ear bone fossils provides further support for the family relationship, the researchers believe.

"For the first time, morphological evidence shows that artiodactyls are the closest relatives of the cetaceans. The new fossils superbly document the link between modern whales and their land-based forebears and should take their place among other famous 'intermediates,' such as the most primitive bird, Archaeopteryx, and the early hominid Australopithecus," comments Christian de Muizon of the Museum of Natural History in Paris, in Science.

JGM Thewissen, a former graduate student of Gingerich (now at Northeastern Ohio Universities College of Medicine) reports related research in Nature. His group discovered fossils of pakicetids, 50 million-year-old terrestrial whales in the Kala Chitta Hills of Punjab. In 1994 he discovered the fossil remains of Ambulocetus natans, a transitional aquatic mammal. In a new report in Nature he describes the discovery of two novel mammals, Ichthyolestes and the Pakicetus. Pakicetids are considered to be the first cetaceans.They lived on land, and may have fed while wading in shallow streams. While both species have the legs and other features of land-dwelling even-toed ungulates, they also have unique ear bones of the kind seen in whales.

Right: Hans Thewissen, Ph.D., examines some of the raw rock with whale fossils he brought back from Pakistan after a dig this past summer. In it, Hans and his team have found the 50-million-year-old bones of whale ancestors.

The two experts differ on the hippo-whale link. While Thewissen agrees that the new evidence supports the links between early ungulates and whales, he believes there is no special link with hippos. Gingerich in contrast believes that the combination of evidence from the fossil record and molecular biological studies suggests that hippos are the nearest living relative of the whales. The debate is far from resolved.

"Two other evolutionary transitions vital to our understanding of the relationship between whales and artiodactyls beg for elucidation: the precise ancestry of hippopotami and the origin of artiodactyls themselves. The answers seem likely to come only from an improved fossil record-perhaps from the same region that has yielded fossils showing that whales evolved from artiodactyls," notes Kenneth D. Rose of Johns Hopkins University, in a commentary of the Science report.

Both researchers would like to search for more evidence in the area where the fossils were found in a remote corner of northern Pakistan. This is the only part of the world where such transitional fossils have been discovered. Indeed, both Gingerich and Therwissen had been planning return visits to the Pakistani site in the near future. Both have canceled their trips in light of recent world events.

The new research appears in the Sep. 20, '01 issue of Nature and the Sep. 21,'01 issue of Science.

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