Imagine a rodent so big it weighs a ton and is the size of a bull.
Uruguayan scientists say they have uncovered fossil evidence of the biggest species of rodent ever found, one which roamed South America about 4 million years ago.
A herbivore, the beast may have been a contemporary — and possibly prey — of saber-toothed cats in a prehistoric version of Tom and Jerry.
Rodents are the most abundant group of living mammals on Earth and, with only a few exceptions, they're tiny.
But this one's skull was huge — more than half a meter (20 inches) long — suggesting an animal about three meters in length weighing more than a metric ton (1.1 U.S. tons).
The skull was originally found on a beach in the Uruguayan province of San Jose in 1987 by an Argentine fossil collector identified as Sergio Viera, who donated it to Uruguay's Natural History Museum, in the capital city of Montevideo.
Museum director Arturo Toscano told The Associated Press the fossil was found near the vast River Plate estuary — a muddy waterway separating Uruguay from Argentina that empties into the South Atlantic.
But it spent nearly two decades in a box at the museum before it was rediscovered by curator Andres Rinderknecht, who enlisted the help of fellow researcher Ernesto Blanco to study it.
Blanco told The AP he was shocked when he first came face to face with the fossil, saying it looked even bigger than a cow skull.
"It's a beautiful piece of nature," he said. "You feel the power of a very big animal behind this."
Blanco said the skull's shape and the huge incisors left no doubt they were dealing with a rodent, but warned that the estimate they made of the animal's bulk was imprecise — he said it weighed between 800 and 1,400 kilograms (1,700 and 3,000 pounds).
But he said the now-extinct rodent, named Josephoartigasia monesi, clearly outclassed its nearest rival, the Phoberomys, found in Venezuela and estimated to weigh between 400 and 700 kilograms (880 and 1,500 pounds).
Blanco said the animal's teeth pointed to a diet of aquatic plants, and the geological record suggested the rodent lived in forested areas close to fresh water.
The creature could 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.
Scientists uninvolved with the research agreed: 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, Pennsylvania.
She said it and other rodents grew bigger and bigger by filling the ecological niche taken elsewhere by rhinos and hippos.
"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.
Despite newspaper headlines hailing the "mighty mouse," researchers say that Josephoartigasia monesi was more closely related to guinea pigs and porcupines than it was to rats or mice.
"These are totally different from the rats and mice we're accustomed to," said Bruce Patterson, the curator of mammals at the Field Museum in Chicago, confirming that it was the biggest rodent he had ever heard of.
An artist's impression of the beast seemed to show a cross between a hippopotamus and guinea-pig.
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," Patterson said.
Little trace of the big rodent is left. Its closest surviving cousin, the pacarana, is endangered. The sharp-clawed 15 kilogram- (33 pound-) rodent lives in the hills around the Andes Mountains in South America.
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.
Both Blanco, the researcher, and Toscano, the museum director, said they hoped the find would attract more resources to museums in the developing world such as Uruguay's — which is so strapped for cash it has been unable to hold public exhibitions since 2000.
Rinderknecht and Blanco's research was published Wednesday in this week's issue of biological research journal, Proceedings of the Royal Society B.