The beautiful image to the right (click on it for a full-size version) is taken from a widely-reported paper in Nature which debunks the “conventional” account of the mammals coming into existence only once the dinosaurs had been removed from the scene.
The chart shows species branching, with time increasing from the centre to the edge of the chart. We are at about “one minute before 6 o’clock”, at the bottom. The extinction of the dinosaurs (the “K/T boundary”, 65 million years ago) is marked by a dashed line.
As the summary on the Pharyngula blog explains, the researchers found that there are two principal periods of diversification among mammals: one around 20 to 35 million years before the extinction of the dinosaurs, the other some 10 to 15 million years after the dinosaurs’ disappearance. The first period in particular can be seen very clearly from the increased branching just inside the dashed line.
The Pharyngula post includes some fascinating charts showing how mammalian lineages continued to rise without a pause at the “K/T boundary” which marks the end of the age of the dinosaurs. The writer, PZ Myers, argues that our surprise at this is largely “a matter of perspective”:
Big animals are the most obvious creatures in a biome to our eyes, but they are not the most important or diverse – we have a perceptual bias for the charismatic megafauna.
The “charismatic megafauna”. Love it.
So the dinosaurs disappeared, but insects, bacteria and plants survived mostly unharmed, and small lizard-like or rodent-like animals continued to scamper cheerfully through the undergrowth:
We just didn’t pay much attention to them until some subset of their descendants grew large enough to trample us or eat us.
Update: Further information on the methodology and results of this study can be found here, in a post that also includes this fun summary of the usual account of the rise of the mammals:
The traditional view of the origin of mammals – the one you might have seen in a museum display or learned about in basic biology – basically said that mammals were the downtrodden proletariat of the Cretaceous, oppressed by the dinosaurean bourgeois and forced out of all of the good ecological niches. After the asteroid lined the dinosaurs up against the wall, the mammals were able to break the chains of their working class bondage and burst forth to fulfill their full ecological potential.