The mass extinction that wiped out dinosaurs and other life 65 million years ago apparently did not, contrary to conventional wisdom, immediately clear the way for the rise of today’s mammals.

In fact, the ancestral branches of most mammals, including primates, rodents and hoofed animals, emerged long before the global extinction and survived it more or less intact. But it was not until at least 10 million to 15 million years afterward that the lineages of living mammals began to flourish in number and diversity.

Some mammals did benefit from the extinction, but these were not closely related to extant lineages and most of them soon died off.

These are the surprising conclusions of a comprehensive study of molecular and fossil data on 4,510 of the 4,554 mammal species known to exist today. The researchers reporting the findings in today’s issue of the journal Nature said it was the first virtually complete species-level study of existing mammals.