The demise of the dinosaurs kick-started a growth spurt in mammals that would see them become supersized within a mere 25 million years, new research has found.
The international team led by Felisa Smith from the University of New Mexico publish its findings today in the journal Science.
The mass extinction event at the end of the Cretaceous period, 65 million years ago, wiped out all the non-flying dinosaurs virtually overnight, as well as many other animals, plants and insects.
Suddenly there was room and resources for the mammals to flourish.
The researchers tracked how mammal body size changed over time by identifying the largest mammal from each geological period since the end of the Cretaceous. All major mammal groups including elephants, cats and horses were included.
For the extinct animals, often only fossil teeth were available. To work out how large the body might have been, researchers used modern day animals as a comparison. They calculated the ratio of tooth size to body size for these modern species, and then extrapolated this to extinct mammals.
When size was tracked over time, it was revealed that mammals eventually grew to a thousand times larger than they had been when they shared the Earth with dinosaurs.
The pinnacle of land mammal size was achieved by the bizarre Indricotherium, a hornless rhinoceros-like herbivore that lived around 34 million years ago. At 17 tonnes and standing five and a half metres at the shoulder, it would have dwarfed today's African elephant.
This pattern of increasing size in mammals after the demise of the dinosaurs repeated itself across all continents, including North America, Africa, Eurasia and to a lesser extent, South America, say the researchers.
So how did the dinosaurs keep the mammals at bay for so long?
Keeping mammals at bay
"It was most likely competition for resources [rather than direct predation]," says Dr Alistair Evans, a palaeontologist at Monash University in Melbourne and a co-author on the paper.
"The dinosaurs were there first, so they were able to fill the ecological niches very effectively, for example feeding on plants and carnivory: They could do it better than the mammals could," says Evans. "So there would have been limited opportunity for these smaller mammals to evolve into larger sizes."
He says, the researchers found that larger animals evolved whenever the Earth got cooler. A big body helps conserve heat, last longer without food and travel further to find it.
But mammals can't keep growing forever. The researchers say that mammal body size will always be limited by environmental temperatures and available land area.
Being big also means slower reproduction rates and a certain vulnerability to changes in the environment.
"We're talking over tens of millions of years, but it may be that if the world gets warmer in the future, the larger mammals may well go extinct again, because they are adapted to cooler climates," says Evans.