'Size of mammals grew dramatically after dinosaur extinction'
Toronto : The extinction of dinosaurs some 65 million years ago made way for mammals to grow in size – about a thousand times bigger than they had been, a new study has found.
The maximum size of mammals in the post-dinosaur era levelled off about 25 million years later, or 40 million years ago, as the ecosystem was able to reset itself relatively quickly, found the study published in the journal Science.
"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," said study co-author Dr Jessica Theodor, an associate professor at the University of Calgary in Canada.
"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. That's actually a pretty short time frame, geologically speaking," she said.
"That's really rapid evolution."
The researchers spent about three years searching the existing literature and an extensive fossil database for information about the evolution of mammal dimensions.
They found that many types of mammals living all over the world consistently experienced growth spurts followed by a plateau. By developing a model to explain the growth rate, the team discovered that the maximum size levelled off because of ecological constraints, such as the progressive decline in suitable habitats for colossal animals.
According to Theodor, mammals grew from a maximum of about 10 kilogrammes when they were sharing the earth with dinosaurs to a maximum of 17 tonnes afterwards.
"Nobody has ever demonstrated that this pattern is really there. People have talked about it but nobody has ever gone back and done the math," said Theodor, one of the 20 researchers from around the world who worked on the study.
"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 findings, she said, give clues as to what sets the limits on mammal size on land; the amount of space available to each animal and the climate they live in.
The colder the climate, the bigger the mammals seem to get, as bigger animals conserve heat better.
It also shows that no one group of mammals dominates the largest size class -- the absolute largest mammal belongs to different groups over time and space, she added.