Source, with picture of skull
Claire Truscott guardian.co.uk, Wednesday January 16 2008
The creature, nicknamed Mighty Mouse, is believed to have lived around 4m years ago, half submerged in water in order to reduce the stress caused by its vast size.
Its skull was discovered in a boulder that fell away from a cliff in the San Jose region of Uruguay.
However, the importance of the find was not immediately realised and the fossil was stored in the Montevideo national history museum for many years.
When palaeontologists Andreas Rinderknecht and Ernesto Blanco took a closer look last year, they realised it belonged to a creature that had never been seen before.
It is the first near-complete giant rodent skull to have been found. Researchers had previously been limited to studying bone fragments.
"It is a very impressive fossil," Blanco said. "From my point of view, it is very beautiful.
"We only have the skull, so it is not possible to be certain, but - looking at other large rodents - I guess that it was three metres long from the tip of its nose to the end of its very short tail."
Known by its Latin name, Josephoartigasia monesi, the vegetarian rat looked like a cross between a guinea pig and a beaver.
It is thought to have been similar to the capybara and pacarana, smaller creatures that can still be found in South America.
Capybaras are the biggest living rodents, growing to just over 60kg, while pacaranas weigh 15kg. The common rat weighs around 300g.
Although the Josephoartigasia monesi is thought to have had an average weight of around one tonne, its biggest examples could have weighed more than 2.5 tonnes - around the same as a medium-sized hippopotamus.
The rodent was estimated to be around three metres (10ft) long and 1.5 metres tall.
Its huge incisors - more than 30cm (12in) long – left researchers puzzled because its other, smaller, teeth showed that it ate only soft vegetation. Blanco believes the incisors could have been used to fight off predators or to fell trees in the same way beavers do.
At the time of its existence, the giant rat would have had to dodge attacks from huge meat-eating birds and sabre-toothed cats.
"It probably ate aquatic plants and fruits, and the environment probably was a forest near fresh water," Blanco added. "But much work is needed to have a definitive picture."
Until now, the largest prehistoric rodent to have been found was a 118-stone creature discovered in Venezuela in 2000 and believed to have lived 8m years ago.
The Mighty Mouse was roughly twice the size of the next biggest rodent of its time, a South American species called Phoberomys.
Rinderknecht, of the National Museum of Natural History and Anthropology in Montevideo, Uruguay, and Blanco, of the Institute of Physics, also in Montevideo, reported their findings in the journal Proceedings of the Royal Society B.