THE classical hypothesis for the diversification of birds and mammals proposes that most of the orders diverged rapidly in adaptive radiations after the Cretaceous/Tertiary (K/T) extinction event 65 million years ago (1-3). Evidence is provided by the near-absence of fossils representing modern orders before the K/T boundary(4,5). However, fossil-based estimates of divergence time are known to be conservative because of sampling biases(6), and some molecular/time estimates point to earlier divergences among orders(7-10). In an attempt to resolve this controversy, we have estimated times of divergence among avian and mammalian orders with a comprehensive set of genes that exhibit a constant rate of substitution. Here we report molecular estimates of divergence times that average about 50-90% earlier than those predicted by the classical hypothesis, and show that the timing of these divergences coincides with the Mesozoic fragmentation of emergent land areas. This suggests that continental breakup may have been an important mechanism in the ordinal diversification of birds and mammals.