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.

"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.

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.

Surviving an extinction event

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."

Once again, teeth told a story, this time revealing which mammals made it across the K-Pg boundary, and which did not.

Those with molars indicating a specialised diet -- only bugs or only plants, for example -- were less likely to weather the disaster than those with all-purpose chompers ready to eat whatever was available.

The findings, published in Proceedings of the Royal Society B, may hold a lesson for today's world, Grossnickle said.

Scientists say Earth is experiencing another mass extinction event, driven mainly by climate change -- only the sixth in the last half billion years, he pointed out.

"The types of survivors that made it 66 million years ago, mostly generalists, might be indicative of what will survive in the next hundred years, or the next thousand," Grossnickle said in a statement.

The Cretaceous-Tertiary extinction wiped out three-quarters of the plant and animal species on Earth, including all dinosaurs which could not fly.

With the exception of a few crocodiles and sea turtles, there is no evidence that tetrapods -- four-limbed vertebrates -- weighing more than 25 kilos (55 pounds) survived.

The discovery in the 1990s of the 180-kilometre (110-mile) wide Chicxulub crater straddling the Yucatan Peninsula and the Gulf of Mexico, pinpointed the likely spot where the asteroid hit.

After the K-Pg event, new forms of mammals such as horses, whales, bats, and primates emerged and spread in a dinosaur-free world.