Mammals did not rapidly radiate after the K/T boundary.
That’s the punch line of a paper published in this week’s issue of Nature. This has been all over the news, including the New York Times twice (#1 and #2). You see, there’s this idea that when the dinosaurs (technically, the non-avian dinosaurs) disappeared, mammals quickly filled in the vacated niches. That means there should have been a rapid radiation of mammalian lineages following the dinosaur mass extinction — marked by the boundary between the Cretaceous and Tertiary periods (known as the K/T boundary).
The new study reveals that the extant mammalian lineages (those that are present today) did not originate immediately after the dinosaurs went extinct. PZ Myers points out that our concept of a mammalian radiation is biased by our attention to charismatic megafauna. The little mammals — which make up the majority of mammalian diversity — wouldn’t have been as affected by the K/T extinction event as large mammals, so we won’t see much of a phylogenetic signature of the radiation of the big guys. Also, the study focused on extant mammals, and would have missed any radiation of extinct lineages.
I want to focus on another point: that of morphological differentiation and lineage diversification. Oh, and there’s a phylogeny below the fold.
The authors of the study constructed a phylogeny using DNA sequences from over 4500 mammalian species — nearly every extant species — and showed that the major extant mammalian clades (shown in five different colors on the tree below) originated prior to the K/T boundary (shown as a dashed circle). They also show that there is no evidence for increased phylogenetic diversification after the dinosaur extinction by plotting the amount of lineages versus time.
But does this really address the issue of mammals filling the niches vacated by non-avian dinosaurs? Evolutionary lineages are different than morphological diversification. The authors showed that there wasn’t any rapid diversification of the sampled lineages. But these are genetic lineages, which do not correlate perfectly with morphological differentiation. It’s possible that many of the mammalian lineages were present prior to the K/T boundary, but they didn’t morphologically differentiate until after the dinosaur mass extinction.
What we have is evidence against rapid speciation after the dinosaur extinction. We do not have evidence that there wasn’t rapid evolution of phenotypes after the extinction (sorry for the double negative). These are two separate hypotheses which must be addressed using different sets of data.
Bininda-Emonds ORP, Cardillo M, Jones KE, MacPhee RDE, Beck RMD, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A. 2007. The delayed rise of present-day mammals. Nature 446: 507-511. doi: 10.1038/nature05634