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New theory raised on rise of mammals

Refutes view on post-dinosaur era

NEW YORK -- Scientists published the first virtually complete family tree of all living mammals yesterday, saying it revealed new details in the story of how mammals became so successful in a world once dominated by dinosaurs.

The work challenges the view that the end of the age of dinosaurs -- brought on by a fiery meteor some 65 million years ago -- cleared the way for mammals and, eventually, humans. Scientists who constructed a massive evolutionary family tree for mammals found no sign of such a burst of new species at that time among the ancestors of present-day animals.

Only mammals with no modern-day descendants showed that effect.

"I was flabbergasted," said study co-author Ross MacPhee, curator of vertebrate zoology at the American Museum of Natural History in New York.

At the time of the dinosaur demise, mammals were small, ranging in size between shrews and cats.

The long-held view has been that once the dinosaurs were gone, mammals were suddenly free to exploit new food sources and habitats, and as a result they produced a burst of new species.

The new study says that happened to some extent, but that the new species led to evolutionary dead ends.

In contrast, no such burst was found for the ancestors of modern-day mammals like rodents, cats, horses, elephants, and people.

Instead, they showed an initial burst between 100 million and about 85 million years ago, with another between about 55 million and 35 million year ago, researchers report in today's issue of the journal Nature.

The study also argues for a much earlier origin of some mammal lineages than the fossil record shows.

The new tree challenges paleontologists to find new fossils that can shed light on mammal history, said Greg Wilson, curator of vertebrate paleontology at the Denver Museum of Nature & Science.

But critics said that the study used flawed techniques, and that the dates it generated were bound to be contested.

"I have got a lot of problems with it," said Mark Springer, a biologist at the University of California, Riverside, and a leading specialist on evolutionary family trees.

Some experts praised the large scale of the new evolutionary tree, which used a controversial "supertree" method to combine data covering the vast majority of mammal species.

To construct it, the researchers combined previously published work that relied on analysis of DNA, fossils, anatomy, and other information.

The family tree includes 4,510 species, more than 99 percent of mammal species included in an authoritative listing published in 1993.

Nearly 300 species have since been added to the listing, but the researchers said that doesn't affect their study's conclusions.

William J. Murphy, professor at Texas A & M University who is working on a similar project, said no previous analysis had included so many mammal species.

But, "I don't think this is the final word," he said.

Gareth Cook of the Globe staff contributed to this report.