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.
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 about 85 million years ago, with another between about 55 million and 35 million year ago, researchers report in Thursday's issue of the journal Nature.
The timing of that first period of evolutionary 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.
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.
But, "I don't think this is the final word," he said.
The study's approach for assigning dates was relatively crude, he said, 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, "I'm not sure that conclusion is well-founded," Murphy said.
S. Blair Hedges, an evolutionary biologist at Pennsylvania State University, said the new work "pushes the envelope in the methods and data, and that's really important."
He said the demise of the dinosaurs may have affected mammal evolution by influencing characteristics like body size rather than boosting the number of new species created. Such changes wouldn't be picked up by the new study, he noted.
