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 dinosaurs' 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 years 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.
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. 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.
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.
John Gittleman, a study co-author and director of the University of Georgia Institute of Ecology, said the researchers considered a range of previously 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 present-day mammals took off before the dinosaurs died out, Gittleman said. The cause of the later boom is also a mystery, he said.
The study's family tree includes 4,510 species, more than 99 percent of mammal species covered by 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.) To construct it, the researchers combined previously published work that relied on analysis of DNA, fossils, anatomy and other information.
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.