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Whales by Land and by Sea
19 September 2001 7:00 pm
Four remarkable fossils have settled a long-standing evolutionary debate, showing that whales and even-toed hoofed mammals, such as cows and hippos, evolved from a common ancestor. Fossils of primitive whales that paddled with broad feet and hands and could haul themselves onto a beach like a sea lion are unveiled in the 21 September issue of Science, while a paper in the 20 September issue of Nature shows two wolf-sized whales that were complete landlubbers.
Paleontologists and molecular biologists have disagreed for years over how whales evolved. Molecular biologists argued that whales share a tight evolutionary kinship with hippos and other artiodactyls. Paleontologists, meanwhile, said that whales are most closely related to an extinct group of hoofed carnivores called the mesonychians. The molecular biologists must be mistaken, this group said, because no one had ever found a whale anklebone with key artiodactyl traits.
Now they have. Paleontologist Philip Gingerich of the University of Michigan, Ann Arbor, and colleagues from the Geological Survey of Pakistan unearthed relatively complete skeletons from 47-million-year-old rocks in Pakistan. The anklebones of the whales, Artiocetus clavis and Rodhocetus balochistanensis, display grooves and facets that clearly mark them as artiodactyl. Although the feet are too delicate to bear weight, Gingerich, writing in Science, thinks that the long toe bones might have been webbed for swimming. Hooves on the webbed fingers would have helped the animals clamber on the shore.
Also working Pakistan, a team led by Hans Thewissen of Northeastern Ohio Universities College of Medicine in Rootstown discovered a rich fossil bed. Thewissen and his colleagues report in Nature that they used carbon isotopes to show that the 49-million-year-old whale skulls belong with the other remains, which closely resemble those of artiodactyls. Both new taxa--Ichthyolestes pinfoldi and Pakicetus attocki--have bones built for walking rather than swimming. "Except for the head and tail, they looked like a dog and walked on their toes," Thewissen said. Again, the crucial anklebone shows that the whales are closely related to artiodactyls.
"It's a new era for cetacean research," said Christian de Muizon, a paleontologist at the Museum National d'Histoire Naturellein Paris, France. Paleontologist Kenneth Rose of Johns Hopkins University School of Medicine in Baltimore said the new fossils have forced him to abandon his earlier stance, and conclude that the artiodactyls, not the mesonychids, are whales' closest relative. Now scientists need to determine which particular artiodactyls are the sister group to whales.