Mammals not such late developers, after all

The demise of the dinosaurs led to an evolutionary explosion of modern mammals - or so we thought. It now seems that most mammalian lineages were around well before the dinosaurs died out, perhaps being held in check by other mammals.

A second burst of diversification beginning about 50 million years ago may have resulted from the extinction of other, more primitive mammal groups, which may have suppressed the radiation of advanced mammals.