Whether the rise of placental mammals occurred before or after the mass-extinction event that eliminated the dinosaurs 66 million years ago is hotly debated. Fossils found so far suggest that it happened afterwards, but ‘molecular clock’ calculations — based on the rate of genetic mutations estimated from the DNA of modern animals — indicate an earlier start time.
Shaoyuan Wu and Scott Edwards at Jiangsu Normal University in Xuzhou, China, and their colleagues ran multiple molecular analyses using genome data from 82 mammalian species. Different molecular-clock models gave highly variable timing estimates for when the placental-mammal diversification spurt started.
But combining data from different analyses strongly suggests that the radiation started while dinosaurs were still alive, and continued steadily during and after their extinction. The radiation probably began in response to the earlier diversification of flowering plants, rather than the removal of dinosaurs, the authors say.