By Malcolm Ritter
March 29, 2007
NEW YORK â€“ Maybe you learned this in school: The big dinosaur die-off 65 million years ago was a liberation for mammals, and they quickly produced a bunch of new species that included ancestors of humans and other modern-day creatures.
Remember? Well, forget it, a new study says.
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.
â€œI was flabbergasted,â€ 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, about the size of shrews and cats. The long-standing idea has been that once the dinosaurs were gone, mammals were suddenly free to exploit new food sources and habitats, and they produced a burst of new species as a result.
The new study says that happened to some extent, but that the new species largely led to evolutionary dead ends. In contrast, no such explosion of species was found among the ancestors of modern-day mammals such as rodents, cats, horses, elephants and humans.
Instead, mammals showed an initial burst between about 100 million and 85 million years ago, with another between about 55 million and 35 million years ago, researchers report in today's issue of the journal Nature.
The timing of that first period of development generally agrees with the conclusions of some previous studies of mammal DNA, which argue for a much earlier origin of some mammal lineages than the fossil record does.
The second burst had shown up in the fossil record, MacPhee said. But he said the new study explains why scientists have been unable to find relatively modern-looking ancestors of the creatures known from that time: Without any evolutionary boost from the dinosaur demise, those ancestors were still relatively primitive.
Other experts praised the large scale of the new evolutionary tree, which used a controversial â€œsupertreeâ€ method to combine data covering the vast majority of mammal species. It 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.
William J. Murphy of Texas A&M University, who is working on a similar project, said no previous analysis had included so many mammal species. However, he said, â€œI don't think this is the final word.â€
Murphy said the study's approach for assigning dates was relatively crude, and some dates it produced for particular lineages disagree with those obtained by more updated methods.
So as for its interpretation of what happened when the dinosaurs died off, â€œI'm not sure that conclusion is well-founded,â€ Murphy said.
John Gittleman, study co-author and director of the University of Georgia Institute of Ecology, said the researchers considered a range of reported dates for when various lineages split. They found the overall conclusions of the study were not significantly affected by which dates they chose, he said.
Researchers should now look at such things as the rise of flowering plants and a cooling of the worldwide climate to explain why ancestors of modern-day mammals took off before the dinosaurs died out, Gittleman said.
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