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Jessica Theodor, a paleontologist at the University of Calgary, holds a skull of Hyaenodon, a carnivorous mammal from a group known as the Creodonta, which lived between 15.9 and 42 million years ago. ((Riley Brandt/ University of Calgary))

Researchers have uncovered details of how giant rhinos, elephants, llamas and even rodents came to roam the Earth after the dinosaurs died out.

"When dinosaurs are around, the biggest mammals that are around are about 10 kilos," said University of Calgary paleontologist Jessica Theodor, one of 20 co-authors of the paper published online in the journal Science Thursday.

"And then, 25 million years after that, the largest-size mammals that are around are 1,000 times bigger — they're 10,000 kilos."

Dinosaurs died out around 65 million years ago. At that point, mammals had already been around for 140 million years, but never grew larger than a small dog such as a Scottish terrier. Over the next 25 million years, however, some mammals got exponentially larger.

One rhinoceros relative called Indricotherium — the largest land mammal that ever lived — hit a whopping 17 tonnes or 17,000 kilograms. (A male African elephant can grow as large as seven tonnes).

Where did giant mammals go?

Most giant mammals, including mammoths, went extinct toward the end of the last Ice Age, about 11,000 years ago. University of Calgary paleontologist Jessica Theodor said there are lots of arguments among scientists about whether climate change or human hunting was the main cause of their demise. Larger mammals are more vulnerable to those pressures, she added, because animals reproduce less frequently as they get bigger — elephants only have one baby every few years, compared to mice which can have up to 10 litters per year.

A gigantic size enabled mammals to take advantage of food and resources that dinosaurs no longer had a lock on, according to computer models created as part of the international study led by Felisa Smith at the University of New Mexico in Albuquerque.

"Most of the very largest mammals eat vegetation off the top of trees, and you have to be fairly big to be able to do that," Theodor said.

She added that animals must reach a certain minimum size to survive on leaves, as they aren't very nutritious compared with meat or fruit.

The computer models also suggested a previous theory — that the existence of large mammals was just a side-effect of an increase in the number and type of mammal species — was wrong.

Nor did it appear to be due to the fact that certain groups of mammals tend to be big, said Theodor, who thought that was the most interesting finding.

Rodent, hyrax took turns as biggest

Many different kinds took their turn as the biggest on their continent or in the world: "At some periods of time in Eurasia, for example, the biggest is a rhino. At other periods of time, it's an elephant relative. In South America, sometimes it's a rodent. In Africa in the early history, it's a giant hyrax."

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The largest land mammals that ever lived, Indricotherium and Deinotherium, would have towered over the living African Elephant. Indricotherium, which lived between 23 and 37 million years ago, weighed in at 15,000 kilograms, and Deinotherium, which lived between 2.7 and 8.5 million years ago, reached as much as 17,000 kilograms. ((IMPPS))

Modern-day hyraxes look like groundhogs and are about the size of a rabbit.

Theodor said paleontologists have long observed that mammals seemed to get bigger over time, but never had proof or details.

"Nobody had ever gone back and said, 'Okay, how fast do they get bigger? How big do they get?'"

To figure that out, the researchers painstakingly compiled fossil data from ancient mammals around the world. Theodor contributed data on hoofed animals — her area of specialty.

The data included measurements like tooth lengths, which correspond closely to body size. The researchers used the proportions of current-day animals to estimate the mass of each animal. Then the compiled data was reviewed to compare mammals from different continents and time periods when the climate, available land area and oxygen levels in the air varied.

In general, mammals grew larger during colder periods such as ice ages. That makes sense, because larger animals have less skin surface area compared to the volume of their bodies.

"Since you lose heat across your skin," Theodor said, "bigger mammals have an easier time staying warm."

The study also showed that the largest land mammals never got much bigger than around 10 to 17 tonnes — significantly smaller than the largest dinosaurs. The researchers believe that is because mammals need proportionately far more food due to their higher metabolic rate.

The largest carnivores were about 10 times smaller than the largest herbivores — as they are today — hitting a maximum mass of just 1000 kilograms.

Theodor said while the research confirms a lot of what paleontologists already thought, that is actually quite significant.

"That's how science works — you take what you think is right, and you go ahead and see if you can prove it's wrong."

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