The skull, which measures 53 cm long, suggests that the rodent could have been up to 3 metres long. It is believed to have roamed South America 4 million years ago, living alongside terror birds, saber-toothed cats, ground sloths and giant armored mammals among others.

A fossil collector found the skull in 1987 on a beach in the Uruguayan province of San Jose. He donated it to Uruguay's National History Museum, in the capital city of Montevideo.

One of the museum’s curator, Andres Rinderknecht, rediscovered the skull after two decades. He requested the help of fellow researcher Ernesto Blanco to study it.

"We can give an educated guess that the rodent would have been 3 metres long - assuming that it was similar to a Capybara (the largest rodent alive today) and taking it into account that large mammals generally have relatively smaller heads. Its tail probably was closer to the one of capybara or guinea pig (very short) and not like a rat," said author of the study, Dr Ernesto Blanco.

The scientists said that the rodent, who was determined to be a new species, Josephoartigasia monesi, could have reached to this impressive size in order to protect himself.

"If you are a rodent you cannot run so well so you would have had to fight with these predators," Blanco said. "It might have reached this size to protect itself."

The fossil is exceptionally well preserved and the researchers were able to speculate on how the rodent may have lived. They estimate that the rodent would have survived on soft plants and fruit not the usual diet of a rodent.