Scientists at the University of New Mexico making a survey of big-mammal body size found mammal groups around the world tended to give rise to giant species at about the same time, ScienceNews.org reported Thursday.

Such supersizing took about 20 million years after the disappearance of the dinosaurs, paleoecologist Felisa A. Smith says.

The few mammal species that survived the catastrophe that wiped out dinosaurs started small, "about the size of a baseball, certainly not as big as a football," Smith says.

But with dinosaurs gone, mammal groups diversified to fill new niches, and sizes shot up.

It peaked with creatures like the Indricotherium, a rhinoceros-like mammal that weighed about 17 tons, and Deinotherium, tipping the scales at more than 18 tons.

But by 40 million years ago sizes reached a plateau, the scientists say.

Body size is limited roughly by the amount of land area available and by temperature, the researchers say, making them the major constraints that set a maximum body size.

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