The fossilised skull of a rat the size of a car has been unearthed. The creature lived about four million years ago, weighed about a tonne and ate mostly soft vegetation. It was so big that it probably spent much of its life semi-submerged in water, like a hippo, to reduce the stresses caused by its size.

Palaeontologists found the skull in rock deposits in Uruguay. It is believed to date back two to four million years to a time when giant wildlife was commonplace in South America.

The rodent, Josephoartigasia monesi, was uncovered by Andrés Rinderknecht and Ernesto Blanco. It has been nicknamed Mighty Mouse and is thought to have been similar to the capybara and pacarana, much smaller creatures that are still found in South America. Capybaras are the biggest living rodents at just over 60kg fully grown, while pacaranas weigh 15kg. The common rat weighs about 300g.

J. monesi is thought to have weighed about a tonne and the biggest specimens could have been more than 2.5 tonnes — about the same as hippopotamuses, which range from 1.4 tonnes to 3.2 tonnes.

The rodent was estimated to be about 3m (10 feet) long and 1.5m tall. Its huge incisors were more than 30cm (12in) long, of which 10cm would have been exposed.

The incisors puzzled the researchers because they were much tougher than necessary for an animal that ate only soft plants. Dr Blanco suggested that they could have been used to fell trees like beavers do, or to fight off predators or courtship rivals.

“It probably ate aquatic plants and fruits, and the environment probably was a forest near fresh water,” Dr Blanco said. “But much work is needed to have a definitive picture. We are working on biomechanical determination of the bite force to better understand this point.”

It shared the Earth with sabre-tooth cats and giant predatory birds that did not fly but could run down prey at terrifying speeds. Its great bulk would have protected it from the killer birds, but its young would have been at risk.

The near-complete skull delighted palaeontologists studying giant South American rodents. Before, they had been limited to bone fragments.

Other teeth in the skull were too small to have allowed the animal to have chewed food well. Researchers concluded that it must have consumed soft vegetation, including fruit.

Rodents make up about 40 per cent of all mammals but the new South American species was still about twice the size of the next biggest, a South American species called Phoberomys.

The skull came from a type of rodent, known as dinomyids, that include today’s pacaranas and which during the Miocene and Pliocene periods, from about 2 to 23 million years ago, underwent an evolutionary explosion creating many species in modern-day Argentina, Brazil, Uruguay, Venezuela, Bolivia and Colombia.

Dr Rinderknecht, of the National Museum of Natural History and Anthropology in Montevideo, the Uruguayan capital, and Dr Blanco, of the Institute of Physics in Montevideo, reported their findings in the journal Proceedings of the Royal Society B.

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