National Geographic News
Scientists have found fossil skeletons of two new species of primitive whales with well-developed limbs, fingers, and toes—supporting genetic evidence that hippos are the closest modern land-dwelling relatives to the giants of the sea.
It has long been known that whales are mammals that moved to the sea about 50 million years ago. But how they are related to other mammals is a controversial issue.
"Whales are warm-blooded animals like we are—that has been known for a long time," said University of Michigan paleontologist Philip D. Gingerich. "Yet they're so different from other warm-blooded, furry things that it's been a mystery both how they came to live in the sea and what ancestors they might have come from on land."
Gingerich, University of Michigan graduate student Iyad Zalmout, and researchers from the Geological Survey of Pakistan and the University of New Hampshire are co-authors of a paper in the September 21 issue of the journal Science that resolves some of the mystery. Their discovery of fossils of walking whales reveals important clues about how these animals got around and what they had in common with living and extinct land-living animals.
Some researchers use morphology (the study of an animal's structure and form) to suggest that whales are descended from mesonychians, an extinct group of meat-eating animals that resembled hyenas with hooves. Others use DNA, molecular, and genetic techniques to suggest that whales and hippos are more closely related to one another than either of them is to any other species.
The fossils found in Pakistan last year add weight to the second theory: that whales descended from the group of animals known as artiodactyls, whose members include sheep, cows, pigs, camels, deer, and hippos. Artiodactyla (Greek artios, entire or even numbered, and dactylos, finger or toe) are named for the even number of fingers and toes (two or four) found on each hand and foot.
The fossils found by Gingerich and the others are the first and only known specimens that have sheep-like ankle bones and archaic whale skull bones in the same skeletons. Some of the ankle bones have signature features that place the whales in the artiodactyls group.
Certain ankle bones show specialized features typically associated with adaptation to running. Such features are unique to artiodactyls, living and extinct.
The presence of artiodactyl-like ankles in the primitive whales strongly suggests common heritage rather than convergent evolution, said Kenneth D. Rose of the Program for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine in Baltimore, Maryland. Convergent evolution is the process by which different groups of organisms may evolve similar characteristics in response to particular environmental requirements.
"While ankles from primitive ancient whales have been discovered before, these are the first that are well-preserved enough to provide clues about whale ancestry," Rose said in a related article published in the same issue of Science.
Better Clues to Whales' Ancestors
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