Twist in the tail: whales wolf-sized ancestors once walked the land
Fossils bridge gap between land mammals and whales.
Fifty million years ago, two mammals roamed the desert landscapes of what is now Pakistan. They looked a bit like dogs. They were, in fact, land-living, four-legged whales.
Their new-found fossils join other famous missing links, such as the primitive bird Archaeopteryx, that show how one group of animals evolved into another. And they undermine the two prevailing theories about which land mammals are most closely related to cetaceans - whales, dolphins and porpoises1.
Toeing the line
Because cetaceans are so unlike any land mammal, with their legs as paddles and their nostrils atop their heads, it has been immensely difficult to place them in the evolutionary scheme of things.
Earlier fossil studies related them to the mesonychians, an extinct group of meat eaters. The new discoveries send this idea "out the window", says Thewissen.
Less likely to be abandoned on the strength of Thewissen’s evidence is an alternative hypothesis, based on DNA sequence analysis. This is the idea that cetaceans are more like hippopotamuses (another even-toed ungulate) than any other land mammal.
"Almost all the molecular data place cetaceans within the living even-toed ungulates, as a sister group to hippopotamuses," says evolutionary biologist Ulfur Arnason of the University of Lund in Sweden.
Rapid evolutionary change, be it molecular, ecological or anatomical, is extremely difficult to reconstruct, and the speed with which cetaceans took to the water may make their bones an unreliable guide to their ancestry, he says
Arnason believes the two camps will remain divided, at least for now. "There’s no point trying to reach some sort of consensus based on compromise. It has often been very difficult to reconcile morphological and molecular opinions," he says.
JOHN WHITFIELD | Nature News Service
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