PerspectiveEvolution

Fossils Versus Clocks

Science  08 Feb 2013:
Vol. 339, Issue 6120, pp. 656-658
DOI: 10.1126/science.1233999

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Summary

It's a great story, and one that most of us learned in grade school. Dinosaurs ruled Earth for eons, shaking the ground beneath them as their colossal forms roamed the dense tropical forests of the Mesozoic. Mammals were present but were minuscule by comparison, skulking about in the undergrowth as they foraged for insects. And so it went until a massive asteroid hit Earth about 66 million years ago, causing environmental havoc, climate change, and the worldwide extinction of non-avian dinosaurs [the Cretaceous-Paleogene (K-Pg) extinction event] (1). Only then did mammals begin to flourish and diversify into the myriad forms of today. It is a compelling tale, but one where timing is everything. On page 662 of this issue, O'Leary et al. (2) offer a fresh perspective on the pattern and timing of mammalian evolution drawn from a remarkable arsenal of morphological data from fossil and living mammals (see the figure).

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