King rat: A reconstruction of the fossil skull with a modern-day rat for comparison.

Credit: Ernesto Blanco

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SYDNEY: Fossil hunters have uncovered the greatest rodent that ever lived – a one-tonne behemoth that bestrode the swamplands of South America some four million years ago.

Dubbed Josephoartigasia monesi, the monster rat was described from a 53-centimetre-long fossil found in the River Plate region of Uruguay. It offers important clues about the lifestyle and diet of an ancient group of rodents called the dinomyids.

"For the first time, we have the entire skull of the largest rodent ever reported," said palaeontologist Ernesto Blanco of the University of the Republic in Montevideo, Uruguay. Blanco is co-author of a report describing the find in the U.K. journal the Proceedings of the Royal Society B.

"It is exciting for people to know that a mouse heavier than two race horses [once lived] in South America," he said.

"Heavier than two race horses"

Most living rodents have body masses smaller than one kilogram. But this species makes the largest living rodent – the 60-kg capybara – look like a pygmy shrew in comparison.

The animal's size was estimated by comparing its skull to its closest living relatives and extrapolating. The giant would have been almost twice as heavy as the previous record holder; an ancient species called Phoberomys, which weighed in at 700 kg.

In life the rat would have had massive incisors several centimetres long. But, despite this fearsome look, the creature was not carnivorous and looked more hippo-like than rat-like, said the experts. Its small grinding teeth suggest it had only weak masticatory muscles for chewing food, and probably tucked into soft vegetation, fruit and squidgy aquatic plants.

The rat's massive bulk may have been its defence against the three-metre-tall, carnivorous 'terror birds' and sabre-toothed marsupials that also lived in South America at this time, said Blanco.

The researchers now plan to use CT (computed tomography) scans of the skull to study the species probable bite force. "Imagine how powerful the bite of a giant rodent would be," he added.