Thursday, 29 March 2007

Present day mammalian ancestors walked in the time of dinosaurs

Scientists have long thought that the mass extinction of the dinosaurs around 65 millions years ago opened the door for modern mammal species to proliferate. But a new, complete 'tree of life' tracing the history of all 4,500 mammals on Earth shows that they did not diversify as a result of the death of the dinosaurs, researchers claim.

The study, conducted by an international team of researchers from Munich, the U.S., the U.K. and Australia, contradicts the previously accepted theory that the Mass Extinction Event (MEE) that wiped out the dinosaurs 65 million years ago prompted the rapid rise of the mammals we see on the earth today.

“This is the first time such a large history of mammal evolution has been constructed,” said team member Marcel Cardillo, a visiting fellow at the School of Botany and Zoology at The Australian National University.

By comparing data from a range of sources including the fossil and molecular records, the researchers show that most existing mammal orders first appeared between 100 and 85 million years ago, well before the meteor impact that is thought to have killed the dinosaurs.

However, throughout the Cretaceous epoch, when dinosaurs walked the earth, these mammal species were relatively few in number, and were prevented from diversifying and evolving in ecosystems dominated by dinosaurs, scientists say.

The tree of life shows that after the MEE, certain mammals did experience a rapid period of diversification and evolution. However, most of these groups have since either died out completely, such as Andrewsarchus (an aggressive wolf-like cow), or declined in diversity, such as the group containing sloths and armadillos.

The researchers believe that our 'ancestors', and those of all other mammals on earth now, began to radiate around the time of a sudden increase in the temperature of the planet – ten million years after the death of the dinosaurs.

“Our study shows that modern mammal ancestors appeared millions of years before the dinosaurs disappeared, and then chugged along at low rates of species diversity for many more millions of years before exploding into the multitude of species we see today,” Cardillo said.

Cardillo said it would take many more years of study to find out the reasons why modern mammal ancestors were around for so long before they diversified, but suggested that it could be the result of complex relations between mammals and other animal species, and variations in the Earth’s climate, geology and atmosphere.

More information is available from the Australian National University’s press release.

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