Washington ? They just needed some leg room: New research shows the great dinosaur die-off made way for mammals to explode 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. It makes the better-known woolly mammoth seem a bit puny.

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.”

Anyone who frequents natural history museums knows that the end of the dinosaurs 65 million years ago ushered in the age of mammals, and that some of them were gigantic. But the new study is the first comprehensive mapping of these giants in a way that helps explain how and why their size evolved.

“We didn’t have a clear idea of how the story went after the extinction of the dinosaurs,” explained Nick Pyenson, a curator at the Smithsonian Institution’s National Museum of Natural History, who wasn’t involved with the new research.

Previous theories suggested that species diversity drove increases in size, but the new study didn’t find that connection.