Mammal size exploded after dinosaurs

A team of international scientists have shown that when the dinosaurs became extinct some 65 million years ago, mammals began to get bigger - a lot bigger!

The study published this week in the journal Science, shows that mammals became a thousand times bigger than they had been once the dinosaurs were out of the way.

"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," says paper co-author Dr. Jessica Theodor, associate professor in the Department of Biological Sciences at the University of Calgary.

The mammals considered as part of the research includes Indricotherium transouralicum, a hornless, rhino-like herbivore that weighed about seventeen tonnes and stood about 18 feet high at the shoulder. That animal lived in Eurasia almost 34 million years ago.

The researchers gathered data on the maximum size for the major groups of land mammals on each continent, including Perissodactyla (odd-toed ungulates like horses and rhinos), Proboscidea (which includes elephants, mammoths and mastodon), Xenarthra (anteaters, tree sloths and armadillos), as well as a number of other extinct groups.

"that's really rapid evolution"Theodore says the results confirm that ecosystems can reset themselves relatively quickly after a major disruption: "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 says. "That's really rapid evolution."

The scientists found that mammals grew to a maximum of about 10 kg when they shared the earth with dinosaurs but up to 17 tonnes once the dinosaurs were gone.

The research, funded by a National Science Foundation Research Coordination Network grant, was led by scientists at the University of New Mexico, who brought together paleontologists, evolutionary biologists and macroecologists from universities around the world.

John Gittleman from the University of Georgia in the US was also involved in the research and says that there is a much better fossil record for mammals than for many other groups. "That's partly because mammals' teeth preserve really well. And as it happens, tooth size correlates well with overall body size" says Gittleman.

"During the Mesozoic, mammals were small," said Gittleman. "Once dinosaurs went extinct, mammals evolved to be much larger as they diversified to fill ecological niches that became available. This phenomenon has been well-documented for North America; we wanted to know if the same thing happened all over the world."

 Image: The largest land mammals that ever lived, Indricotherium (left) and Deinotherium (middle), would have towered over the living African elephant (right). [Credit: Alison Boyer/Yale University]