Six distinct waves of mammal species diversity in North America over the last 65 million years were driven primarily by climate change, new research suggests.

Evolutionary biologists say that on each occasion warming and cooling periods, in two cases confounded by species migrations, marked the transition from one dominant grouping to the next.

"Although we've always known in a general way that mammals respond to climatic change over time, there has been controversy as to whether this can be demonstrated in a quantitative fashion," says professor Christine Janis of Brown University.

"We show that the rise and fall of these faunas is indeed correlated with climatic change - the rise or fall of global paleotemperatures - and also influenced by other more local perturbations such as immigration events."

Of the six waves of species diversity identified, four show statistically significant correlations with major changes in temperature. The other two still show a correlation with the pattern, but also correspond to periods when mammals from other continents happened to invade in large numbers, says Janis.

These transfers of dominance correlated with temperature shifts - reflected in past levels of atmospheric oxygen, which can be determined from the isotopes in the fossilized remains of deep sea microorganisms.

For example, after a warming episode about 20 million years in the early Miocene epoch, the dominant vegetation transitioned from woodland to a savannah-like grassland. As a result, many of the herbivores that dominated the following period hadhigh-crowned teeth that allowed them to eat the foods from those savannah sources.

The findings don't allow for detailed predictions of the future, says Janis, but do show the profound effects of climate change.

"Such perturbations, related to anthropogenic climatic change, are currently challenging the fauna of the world today, emphasizing the importance of the fossil record for our understanding of how past events affected the history of faunal diversification and extinction, and hence how future climactic changes may continue to influence life on earth," the researchers write.