The newly discovered skull is considerably larger than a modern-day rat (Image: Blanco) An artist’s impression of J. monesi‘s headshows how closely it might have resembled the pakarana (Image: Gustavo Lecuona / Blanco)

A 1-tonne rodent has been discovered by scientists in Uruguay. But there is no need to worry, Josephoartigasia monesi is around 2 million years old and fossilised.


J. monesi‘s skull, a whopping 53 centimetres long, was discovered in a broken boulder on the coast of Uruguay by Andrés Rinderknecht of the National Museum of Natural History and Anthropology, and Ernesto Blanco of the Institute of Physics, both in Montevideo.

By comparing the skull’s dimensions to the body sizes of existing rodents, the researchers determined that its owner probably weighed about 1000 kilogrammes, making it the world’s largest known rodent.

However, unlike today’s rodents, the relatively small size of the animal’s teeth suggests it did not have a great deal of chewing power and might have fed on soft vegetables and fruit.

Rodent explosion

The discovery displaces Phoberomys pattersoni, a 700 kg cousin of the guinea pig that roamed the Orinoco delta in Northern Venezuela some 8 million years ago. When a nearly intact P. patersoni skeleton was described in 2003, its size was compared to that of a cow and it was hailed as the world’s largest rodent.

“There no doubt whatsoever that this was a huge and exceptional animal because of its size,” says Marcelo Sánchez of the University of Zurich, Switzerland, who discovered the P. patersoni skeleton.

Sánchez says that while the latest discovery is “incredible”, it is only a matter of time before another, larger rodent is unearthed.

This is because South America saw a huge explosion in the diversity of rodents after the continent split from North America and became an island some 65 million years ago. Dinosaurs had just been wiped out and many animal groups were filling the void they left behind.

Without competition from other mammals which were diversifying on the other side of the water in North America, rodents of all sizes emerged in South America.

Just one giant species remains today of the rodent explosion: a 50 kg guinea pig known as the capybara which lives in much of South America and is the largest living rodent.

Competition and climate

Around the time that the recently discovered J. monesi was alive, the two Americas were joined once more.

Sánchez speculates that the connecting land bridge may have helped bring about the demise of the giant rodents. Animals, among them the sabre-toothed cat, crossed the bridge in both directions bringing diseases, and competition for food and territory.

It is likely that changes in the climate will have also rendered the rodents’ home less hospitable. J. monesi was found in what is now an arid region, but was then lush and forested.

“Our work suggests that 4 million years ago in South America, ‘mice’ that were larger than bulls lived with terror birds, sabre-toothed cats, ground sloths, and giant armoured mammals,” say the Uruguayan researchers.

Both J. monesi and P. patersoni are closely related to the pacarana, a rare, metre-long rodent with a white striped coat that still lives in the tropical forests of the western Amazon River basin.

Journal reference: Proceedings of the Royal Society B (DOI: 10.1098/rspb.2007.1645)

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