Found: prehistoric rodent that was as big as a bull

The fossilised skull of a giant rodent that grew to the size of a bull has been discovered in South America, where it lived about four million years ago alongside sabre-toothed cats, huge flightless "terror" birds and giant ground sloths.

Scientists have found the almost complete skull of the extinct rodent, which weighed about a ton and grew about 5ft tall and about 9ft long.

Rodents, which include rats, mice and guinea pigs, are the most abundant group of living mammals. The latest discovery shows that they are also physically one of the most varied, ranging from a few grams – the size of a pygmy mouse – to about 1,000kg, the weight of the extinct South American specimen.

Scientists believe that the rodent may have lived on fruit and leaves, similar to the diet of its closest living relative, the pacarana, another South American rodent that grows to about 3ft long and weighs about 13kg.

Both the extinct rodent and the living pacarana belong to the Dinomyidae family of mammals.

The skull of Jospehoartigasia monesi, as the species has been named, is described in the "Proceedings of the Royal Society B" by Andres Rinderknecht of Uruguay's National Museum of Natural History and Ernesto Blanco of the Institute of Physics in Montevideo. "This species with estimated body mass of nearly 1,000kg is the largest yet recorded. The skull sheds new light on the anatomy of the extinct giant rodents of the Dinomyidae, which are known mostly from isolated teeth and incomplete mandible remains," the scientists said.

Scientists believe giant rodents may have become extinct because they were too big to dig burrows and too slow to outrun predators.