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New international research co-led by Utah State University has determined that land mammals, once out from under the heel of dinosaurs, quickly ballooned in size, some achieving weights 1,000 times greater than the largest Cretaceous period mammals.

This expansion in body size happened uniformly across the land masses and mammalian groups, suggesting global evolutionary forces were at play, said Morgan Ernest, a USU associate professor of biology and co-director of the National Science Foundation-funded project.

For the first 140 million years of their existence, mammals did not evolve much. Meanwhile, their reptilian contemporaries ruled the planet and achieved vast body sizes and diversity before a meteor strike snuffed out most large bodied animals.

"We were the little vermin scurrying around the feet of the dinosaurs," said Felisa Smith, an associate professor of biology at the University of New Mexico, referring to the mammalian line which includes humans.

After dinosaurs were wiped out in a cataclysmic event that dramatically altered habitats 65 million years ago, mammals soon evolved gargantuan bodies as they filled ecological niches formerly occupied by reptiles, according to the study published last week in the journal Science.

"In addition to the strong consistency in the patterns across continents, I was particularly struck by the relationship between the evolution of size in carnivores and herbivores," said Ernest, who specializes in the field of macroecology. "The evolution of maximum size in carnivores tracked increases in size in herbivores, though carnivores remained 10 times smaller than their potential prey."

The study, the fruit of a five-year $500,000 NSF grant, brought together paleontologists, evolutionary biologists and macroecologists from 12 universities around the world and the Smithsonian. The team examined the mammal fossil record represented by teeth, which not only preserve well, but are a reliable indicator of body size, according to Ernest.

During the dinosaur reign, mammal sizes ranged from five grams to 15 kilograms, or the size of a large border collie. During the ensuing Paleocene and Eocene periods, some land mammals became monstrous, much larger than any living today — although the blue whale remains the largest animal to have ever lived.

The researchers concluded mammals experienced an "exponential rise" in body size that plateaued after 42 million years, sometime in the Oligocene period.

"That plateau is set by environmental temperatures as well as the effective land area," Smith said. "Moreover, the patterns are the same on every continent, despite the fact that Africa, North America, Eurasia, and South America have very different geological histories and the animals on those continents were very different."

Indricotherium transouralicum, a hornless Eocene cousin to the rhino, weighed 17 tons, stood 18 feet at the shoulder and roamed Eurasia almost 34 million years ago. They would dwarf today's African elephants, now the world's largest land mammal. Indricotherium have been huge, it was a puppy next to its dinosaur counterparts, the sauropods that were sometimes longer than 100 feet and weighed more than 100 tons.

No one knows why land mammals never achieved dino proportions, but "a favorite hypothesis is mammals and birds have much higher metabolic demands than lizards and fish," Ernest said. That means these creatures would need more food to maintain warm-blooded bodies, but many paleontologists suspect dinosaurs were warm-blooded, too, unlike today's cold-blooded reptiles.

The larger mammal species, of course, also died out with dinosaurs and those that survived were small burrowers.

"They came out into this world that had no large animals in it and that allowed enormous ecological opportunity to diversify in size and occupy a whole range of terrestrial and aquatic habitats," Smith said.

The next step is to figure out what drove land mammal evolution toward bigness and why they stopped getting bigger, never coming close to matching the size of dinosaurs or their marine contemporaries. Did the increase in mammal biodiversity play a role?

"The more species you have the more likelihood you have of picking up a really big mammal. But we did not find a relationship," Ernest said.

Furry and frenetic

After the extinction of dinosaurs 65 million years ago, land mammals evolved into much larger creatures, achieving maximum body sizes that were 1,000 times heftier than before the Cretaceous Period, according to international research that was co-led by Utah State University. The team analyzed the mammalian fossil record and documented a pattern toward largeness across the continents and mammal taxonomical groups.