From “Evolutionary Tree of Life for Mammals Greatly Improved” (ScienceDaily, September 23, 2011), we learn:
Springer explained that the research team looked for spikes in the diversification history of mammals and used an algorithm to determine whether the rate of diversification was constant over time or whether there were distinct pulses of rate increases or decreases. The researchers found an increase in the diversification rate 80-82 million years ago, which corresponds to the time — specifically, the end of the Cretaceous Terrestrial Revolution — when a lot of different orders were splitting from each other.
“This is when flowering plants diversified, which provided opportunities for the diversification of small mammals,” Springer said.
Springer and colleagues also detected a second spike in the diversification history of mammals at the end of the Cretaceous — 65.5 million years ago, when dinosaurs, other large terrestrial vertebrates, and many marine organisms went extinct, opening up a vast ecological space.
Hmmm. Almost as if it were spring-loaded.