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.
