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Dinosaur die-off let mammals get big

WASHINGTON — They just needed some leg room: New research shows the great dinosaur die-off made way for mammals to surge in size — some more massive than several elephants put together.

The largest land mammal ever: A rhinoceros-like creature, minus the horn, that stood 18 feet tall, weighed roughly 17 tons and grazed in forests in what is now Eurasia.

Tracking such prehistoric giants is more than a curiosity: It sheds new light on the evolution of mammals as they diversified to fill habitats left vacant by the dinosaurs.

Within 25 million years of the dinosaurs' extinction — fast, in geologic terms — overall land mammals had reached a maximum size and then leveled off, an international team of scientists reports today in the journal Science. And while different species on different continents reached their peaks at different points in time, that pattern of evolution was remarkably similar worldwide.

"Evolution can happen very quickly when ecology permits," said paleoecologist Felisa Smith of the University of New Mexico, who led the research. "This is really coming down to ecology allowing this to happen."

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