"From what we can tell, we know it was a herbivore that lived on the shores of rivers or alongside streams in woodland areas," Rinderknecht told the AP. "Possibly it had a behavior similar to other water-faring rodents that exist today, such as beavers, which split their time between land and water."

But he said the rodent appears to have had no tail, adding that follow-up studies are being planned to better determine its diet and other traits.

The creature may have been a contemporary to the saber-toothed cats and giant carnivorous birds that roamed the area millions of years ago, but Blanco said it was not clear whether such predators had the power necessary to bring down the huge beast.

"This investigation began about a year and a half ago but it's still not complete," Rinderknecht said, adding that the next step may be a CT scan of the skull "to better determine its interior dimensions."

The research by Rinderknecht and Blanco was published Wednesday in this week's issue of biological research journal, Proceedings of the Royal Society B.

Scientists uninvolved with the finding agreed that this was one really big rodent.

"I think it's a very important discovery — it is certainly an immense animal," said Mary Dawson, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh. She said it and other rodents grew bigger by filling the ecological niche taken elsewhere by rhinoceroses and hippopotamuses.

"They got large taking the role of some herbivores that were not present at that time — South America was still an island continent," she said. But when North and South America were linked about 3 million years ago, the rodents were swamped by North American animals and eventually died out.

"It's too bad they're extinct, I'd love to see those things," she said.

Patterson said its discovery gave scientists more insight into the fauna of the prehistoric South American continent, when it hosted creatures such as marsupial predators and hoofed animals known to scientists as archaic ungulates.

"These were things with trunks on their noses, huge claws on their hands, they look like somebody just made them up," he said.

Little trace of big rodent is left. Its closest surviving cousin, the pacarana, is endangered. The sharp-clawed 33-pound rodent lives in the hills around the Andes Mountains. It is considered among the largest living rodents, but its slow rate of reproduction — and reputation among humans as a tasty treat — means its prospects are grim.

Blanco said he was thrilled with the discovery of the huge rodent after so many years.