Archaeologists have unearthed the skull of a giant prehistoric rat that roamed South America four million years ago.

The animal, believed to be the largest rodent ever to have walked the Earth, was bigger than a bull and weighed up to a tonne. The remains of the half-metre-long skull were discovered in a chunk of rock in Rio de La Plata on the coast of Uruguay. Experts led by Andres Rinderknecht at Montevideo's Museum of National History and Anthropology have named the new species Josephoartigasia monesi.

"When Andres showed me the fossil, I could not believe my eyes," said Ernesto Blanco, a biomechanics expert from the city's Institute of Physics, who studied the fossil. The rodent belongs to a family of creatures called dinomyids that appeared in parts of South America in the Oligocene epoch more than 20 million years ago and rapidly populated the regions that are now Argentina, Brazil, Venezuela and Colombia.

It lived alongside sabre-toothed tigers, ground sloths and giant armoured mammals and had small teeth and weak jaw muscles, suggesting it was not a carnivore. "This could imply a diet composed of soft vegetation and perhaps fruit," the authors write in Proceedings of the Royal Society B. The largest living rodent is the carpincho or capybara that also lives in regions of South America and can weigh up to 60kg.

Rats have always been big - yeah!