The mainstream view is that the dinosaurs were killed some 65 million years ago at the end of the Cretaceous era, possibly by an asteroid that whacked into Earth, kicking up a pall of dust that cooled the planet and destroyed the vegetation on which the dinos depended.

When the "terrible lizards" shuffled off, the mammals that had been quietly waiting in the wings happily took over, according to this theory.

They leapt into freshly-vacated habitat niches and quickly diversified, bursting into the lineages that gave us the mammal species we see today. But a paper that appears on Thursday in the British journal Nature says that mammalian biodiversity grew in two significant phases. One took place millions of years before, and the other took place millions of years after the end of the Cretaceous.

Olaf Bininda-Emonds of the Technical University of Munich, Germany, and colleagues compiled a genetic "family tree" of almost all today's 4,500 mammal species.

Using a molecular clock based on a species' rate of evolution, they thew time in reverse, estimating when these mammals underwent genetic change.

Over a 160-million-year span, mammals underwent a first burst of diversification around 93 million years ago. The ancestors of today's primates, rodents and hoofed animals first appeared on the scene around 75 million years ago or later.

But far from universally benefitting from the end of the Cretaceous, mammals were badly hit by the catastrophe. Many mammalian species, like the dinosaurs, bit the dust.

A second diversification then happened about 35 million years ago, thus long after the twilight of the dinosaurs. This second burst was particularly important, for it yielded the lineages that became present-day mammals.

The cause for this second splurge of biodiversity is unclear, but it could be linked to a rise in global temperatures called the Cenozoic thermal maximum, say the authors.

"The big question now is what took the ancestors of modern mammals so long to diversify," said one of the authors, Ross MacPhee, curator of vertebrate zoology at the American Museum of Natural History.

"It's as though they came to the party after the dinosaurs left, but just hung around while their distant relatives were having a good time. Evidently we know very little about the macroecological mechanisms that play out after mass extinctions."

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