The remains of a giant rat, estimated to weigh about a ton, was discovered in Uruguay proving that mammoth rodent once roamed the face of the earth some four million years ago.

Archaeologists said they have unearthed the half-meter long skull of the giant prehistoric rat, which by any standard, is bigger than a bull, in a chunk of rock in Rio de La Plata on the coast of Uruguay.

Archaeological experts led by Andres Rinderknecht at Montevideo's Museum of National History and Anthropology have named the new specie, Josephoartigasia monesi.

"When Andres showed me the fossil, I could not believe my eyes," exclaimed Ernesto Blanco, a biomechanics expert from the city's Institute of Physics.

According to archaeologists, the rodent is of the same family of animals called dinomyids which inhabited parts of South America in the Oligocene epoch more than 20 million years ago and spread to the regions now known Argentina, Brazil, Venezuela and Colombia.

While the giant rats co-existed with saber-toothed tigers, ground sloths and giant armored mammals, it had small teeth and weak jaw muscles, prompting experts to surmise it was not a carnivore.

"This could imply a diet composed of soft vegetation and perhaps fruit," the authors stated in the Proceedings of the Royal Society B.

Prior to its discovery, the South American carpincho or capybara, which weighs up to 60 kg when fully grown, is considered as largest living rodent.