Nov 26, 2010 10:27 GMT  ·  By
The largest land mammals that ever lived, Indricotherium (grey) and Deinotherium (blue) would have towered over modern-day African elephants.
   The largest land mammals that ever lived, Indricotherium (grey) and Deinotherium (blue) would have towered over modern-day African elephants.

Dinosaurs once ruled our planet but after they disappeared 65 million years ago, it was the time for mammals to thrive, the proof being that there were some that even got a thousand times bigger than they used to be.

This is the first study that actually proves that there was a new pattern of increased body size in mammals, after the extinction of dinosaurs.

For finding out exactly how big mammals grew after what is called 'the competitive release' caused by dinosaur extinction, the researchers gathered data on the maximum size for major groups of land mammals on every continent.

They included Perissodactyla – odd-toed ungulates like horses and rhinos, Proboscidea – elephants, mammoths and mastodons, Xenarthra – the anteaters, tree sloths, and armadillos, and several other extinct groups.

This research showed what were the factors that limited mammals size on land, how much space did each animal have available and in what kind of climate did they live.

The team found out that the colder the climate, the bigger the mammal would be, because bigger animals seem to conserve heat better.

Also, it looks like there was no one group of mammals dominating the largest size class, the absolute biggest animal belongs to different groups over time and space.

Paper co-author Dr. Jessica Theodor, associate professor in the Department of Biological Sciences at the University of Calgary said that “basically, the dinosaurs disappear and all of a sudden there is nobody else eating the vegetation.

“That's an open food source and mammals start going for it, and it's more efficient to be an herbivore when you're big.”

Theodor said that not only did this study confirm the dramatic growth of mammals once there were no more dinosaurs, it also showed that the ecosystem can start over relatively fast.

“You lose dinosaurs 65 million years ago, and within 25 million years the system is reset to a new maximum for the animals that are there in terms of body size,” she said.

“That's actually a pretty short time frame, geologically speaking; that's really rapid evolution.”

Mammals are believed to have grown from a maximum weight of 10 kg (when dinosaurs existed) to a maximum of 17 tonnes afterwards.

“Nobody has ever demonstrated that this pattern is really there,” said Theodor, who is actually one of the 20 researchers worldwide that have been working on this study.

“People have talked about it but nobody has ever gone back and done the math.

“We went through every time period and said OK, for this group of mammals what's the biggest one?

“And then we estimated its body mass."

The research is published in the journal Science.