Although today's awe-inspiritng African Bush Elephant (Loxodonta africana) might seem a mighty beast, it's a fraction of the size of ancient mammals that roamed the Earth 37 million to 2.7 million years ago.

The Eocene and Oligocene's Indricotherium measured in at more than five meters tall, and the Miocene and Pleistocene's Deinotherium likely weighted some 17,000 kilograms. But how did these mammals shoot up from their mostly miniature predecessors that lived during the Age of the Dinosaurs—and what kept them from getting even bigger?

A new analysis of fossils found across the globe has plotted the meteoric rise of the mammals, a class group that has been around for some 200 million years.

"For the first 140 million years of our evolutionary history we really did nothing—we were really kind of boring," Felisa Smith, an associate professor of biology at the University of New Mexico and coauthor of the new study, says of our shared mammalian line. Early mammals that lived alongside the dinosaurs tended to be modest, mouse-sized types (with a ranged from about 3 grams to 15 kilograms).

But across all of the major continents, during the first 25 million years after the dinosaurs were wiped out, mammals underwent an explosive growth spurt. By 42 million years ago, however, the researchers found, the intense growth had leveled off.

"There seemed to be remarkable consistency of body size across all different continents," Smith says. And that similarity was especially striking, she notes, "because the continents are really different." The findings were published online November 25 in Science.

She and her colleagues propose that the animals' body size was determined in large part by global climate and land area. "The largest mammals evolved when Earth was cooler and terrestrial land area was greater," Smith and her colleagues wrote in their paper. These two abiotic factors are not unrelated—with cooler climate translating into larger ice caps and thus more exposed land.

Another pattern they found (one "that kind of blew us away," Smith says) was a consistent difference between herbivores and carnivores. "The largest [mammalian] carnivores have never been more than a ton," Smith says, noting that this apparent size limit "might be more to do with how carnivores work" and often hunt in groups to bring down midsized rather than the largest herbivores.

So why did mammals never grow quite as massive as their former overlords the dinosaurs (the largest of which were nearly 10 times the size as of the largest mammal)?

It likely has to do with thermoregulation, Smith says. As endothermic mammals, we spend the majority of our energy keeping body temperature stable. And if at least some of the largest dinosaurs were exotherms, they could use more of their energy to grow—and "when you're 100 tons, you don't change temperature very fast," Smith points out.

Although the hulking Deinotherium died out some 2.5 million years ago, many of North America's megafauna persisted until close to 13,000 years ago. So though there might not been any wooly mammoths roaming the tundra these days, more research is helping scientists paint a better picture the evolutionary tale of mammals. Plus, Smith notes, "it's just kind of cool to envision a planet with all of these big mammals."

Image: Indricotherium, courtesy of Wikimedia Commons/Dmitry Bogdanova