The prevailing theory that mammals only flourished after an asteroid strike wiped out the dinosaurs 66 million years ago is doubly wrong, according to a study published Wednesday.
Our warm-blooded predecessors thrived and spread over millions of years even as Tyrannosaurus and other flesh-ripping monsters lorded over the planet, researchers reported.
Moreover, these mammals took a big hit when the asteroid slammed into Earth, creating a hemispheric firestorm followed by a prolonged, bone-chilling drop in global temperatures.
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"The traditional view is that mammals were suppressed during the 'age of dinosaurs'," and thus held in check, said co-author Elis Newham, a doctoral student in evolutionary biology at the University of Chicago.
"However, our findings were that therian mammals -- the ancestors of most modern mammals -- were already diversifying considerably before the Cretaceous-Tertiary extinction event," also known as the K-Pg boundary.
The researchers pulled together dozens of studies that challenged and chipped away at the old theory.
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But key to the new conclusion, they said, was teeth.
An analysis of hundreds of molars from mammals alive during the 20 million years before the K-Pg boundary revealed a huge variety of shapes -- a telltale sign of varied diets and species diversity.
The scientists were surprised to find a sharp decline in the number of mammals after the asteroid crash.
"I didn't expect to see any sort of drop," said lead author David Grossnickle, also of the University of Chicago.
"It didn't match the traditional view that after the extinction, mammals hit the ground running."
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