A study that examined the shape of hundreds of fossilized shark teeth suggests that modern shark biodiversity was triggered by the end-Cretaceous mass extinction event, about 66 million years ago.
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Seems reasonable - most of the mammal and bird groups formed pretty quickly at that time. Now, was it the opportunities offered by the mass extinction, or did the asteroid simply stir up a soup of evolution-friendly nutrients from the earth’s mantle?? 
Mass extinctions always create ooportunities!
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