The giant creature, nicknamed Mighty Mouse, is believed to have lived around 4 million years ago, half submerged in water in order to reduce the stress caused by its vast size. It is the largest rodent ever found.

The fossilized 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.

Cousin of the capybara

The vegetarian rat (pictured above), known to scientists by its Latin name Josephoartigasia Monesi, is thought to have 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 world’s biggest living rodents, growing to just over 60 kg whereas the common rat weighs a miniscule 300 grams.

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 3 metres long and 1.5 metres tall.