Fossil of rodent that weighed as much as a one-tonne truck discovered

US researchers have discovered a fossil of a rodent in South America, which might be the largest rodent known to have existed.

In a boulder on the southern coast of Uruguay, the 53 centimetre-long skull of the new species, Josephoartigasia monesi, was discovered.

The researchers, based on the age of the rock, estimated that the skull is between two and four million years old.

"This is a remarkable finding because of the size of the animal and the fortunate completeness of the skull," Nature quoted evolutionary biologist Ines Horovitz of the University of California, Los Angeles, as saying. The scientists couldn't access other skeletal measures, such as the length of the limb bones that are traditionally used to estimate the size of an animal, because they found only the skull of the rodent.

However, the research team developed an equation relating skull length to body mass using information from 13 of J. monesi 's closest living relatives.

The calculation gave an estimate of 1,008 kilograms. Adding information from 6 other measures of skull size gave an estimate of 1,211 kilograms, with a likely range of 468 to 2,586 kilograms.

The relatively small grinding teeth in the skull suggested that J. monesi was a member of the Dinomyidae family, which contains just one living example, the pakarana. This rodent lives in South America and weighs roughly 15 kilograms.

The resemblance in molars and premolars suggest the bull-sized J. monesi had a relatively weak jaw and lived on roughly the same diet as its modern-day cousin: soft vegetation and fruit.