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Letters to Nature
Nature 381, 226 - 229 (16 May 1996); doi:10.1038/381226a0

Continental breakup and the ordinal diversification of birds and mammals

S. Blair Hedges*, Patrick H. Parker*, Charles G. Sibley & Sudhir Kumar*

* Department of Biology and Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
433 Woodley Place, Santa Rosa, California 95409, USA

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 ago1–3. Evidence is provided by the near-absence of fossils representing modern orders before the K/T boundary4,5. However, fossil-based estimates of divergence time are known to be conservative because of sampling biases6, and some molecular/time estimates point to earlier divergences among orders7–10. 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.

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