Extinction of dinosaurs 'led to mammals growing 1000 times bigger'.
Mammals grew in size 1000-fold after being left a food bonanza by the extinction of the dinosaurs, research has shown.
From their origins as small shrew-like animals, some became enormously large - and the same pattern was seen all over the world.
They included giants such as Indricotherium, a hornless rhinoceros relative from Eurasia that stood 18 feet high and would have dwarfed an African elephant.
The largest land mammals that ever lived, Indricotherium and Deinotherium, which would have towered over the living African elephant (front)
Dr Jessica Theodor, from the University of Calgary in Canada, one of the study's authors, said: '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 a herbivore when you're big.'
The research, which looked at the fossil record of mammals around the world, showed that after the dinosaurs disappeared 65 million years ago ecosystems did not take long to re-adjust.
An African elephant would have been dwarfed by prehistoric mammals
'Within 25 million years the system is reset to a new maximum for the animals that are there in terms of body size,' said Dr Theodor. 'That's actually a pretty short timeframe, geologically speaking. That's really rapid evolution.'
When mammals shared the Earth with the dinosaurs they grew no heavier than about 10 kilograms, said the researchers.
But after the dinosaurs' exit they quickly exploded in size to a maximum of 17 tonnes.
'Nobody has ever demonstrated that this pattern is really there,' said Dr Theodor, one of 20 researchers from around the world who worked on the 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 scientists gathered data on Perissodactyla, odd-toed ungulates such as rhinos and horses, Proboscidea, which included elephants, mammoths and mastodons, Xenarthra, the anteaters, tree sloths and armadillos, as well a number of other extinct groups.
The results showed that mammals in colder climates grew the most since bigger animals are better at conserving heat.
The research appears today in the journal Science.