CALGARY - The extinction of dinosaurs 65 million years ago could have been a classic case of one door closing and another opening — with mammals being the beneficiaries.
A study published in the prestigious journal "Science" says researchers have demonstrated that after dinosaurs died out, mammals were able to grow in a way they had never been able to grow before.
Dr. Jessica Theodor, an associate professor in biological sciences at the University of Calgary, is a co-author of the study.
"We looked at how maximum body mass changed from the extinction of the dinosaurs to today," she said in an interview with The Canadian Press.
When mammals shared the land with dinosaurs, the largest mammal weighed about 10 kilograms, says Theodor. Over the next 25 million years, she says, the largest mammals grew to a maximum of 17 tonnes.
"We tend to think of evolutionary rates within species or within closely related species over shorter periods of time, but for an entire massive group like mammals to increase a thousandfold in 25 million years - that's actually pretty rapid."
Theodor says vegetation was an open source of food, and it's more efficient to be a herbivore if you're big. Once dinosaurs disappeared, mammals were able to get to the vegetation.
"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,” she says. "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."
Researchers collected data on the maximum size for major groups of land mammals on each continent, including Perissodactyla, odd-toed ungulates such as horses and rhinos; Proboscidea, which includes elephants, mammoth and mastodon; Xenarthra, the anteaters, tree sloths, and armadillos; and various other extinct groups.
"Basically the niches for very big animals had been taken up by the dinosaurs and when they were removed, mammals were able to evolve rapidly to fill the niches and they get much bigger," says Theodor. Every continent except Australia was included in the study.
She says there's a good reason why mammals don't get to the size of the biggest dinosaurs. "There are limits on how big they can get and those limits have to do in part with mammalian metabolism, which is very, very high."
The colder the climate, the bigger the mammals seem to get, she says, since larger animals conserve heat better.
Theodor says data suggest mammals wouldn't have increased in size had dinosaurs survived.
"Mammals are around in the fossil record for pretty much the entire history of dinosaurs, and they don't increase body mass until the dinosaurs are gone."
The article, "The Evolution of Maximum Body Size of Terrestrial Mammals," was written by Theodor and 19 other scientists around the world.
"I'm really excited because there's a lot of really interesting things about it. For the most part we already knew that mammals got bigger," she explained.
"But what we've been able to do is quantify it and show a) they got bigger really fast, b) they hit a maximum size by about 40 million years ago and really haven't changed that much since."