Published online 15 January 2008 | Nature | doi:10.1038/news.2008.441

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Rodent of unusual size discovered

Uruguayan skull points to 'rat' that weighed a tonne.

Artist's impressions of the giant rat, based on portions of the skull found in Uruguay.ROYAL SOCIETY

It’s definitely not a rodent you’d want to wrestle with. A fossil found along the South American coast hints that a beast weighing more than 1,000 kilograms once roamed there. Researchers say that it is the largest rodent known to have existed.

The largest living rodent, the capybara, roams the forests of South America and can weigh upwards of 60 kilograms.

The 53 centimeter-long skull of the new species (Josephoartigasia monesi) was discovered in a boulder on the southern coast of Uruguay. Based on the age of the rock, the researchers estimate that the skull is between two and four million years old. Andrés Rinderknecht of the Natural History Museum in Montevideo, Urguay, and Ernesto Blanco of the University of the Republic, also in Montevideo, report their find in Proceedings of the Royal Society B1.

“This is a remarkable finding because of the size of the animal and the fortunate completeness of the skull,” says evolutionary biologist Inés Horovitz of the University of California, Los Angeles.

Horovitz has analysed the remains of the previous rodent heavyweight champion, Phoberomys pattersoni, which weighed in at an estimated 700 kilograms2. Although J. monesi might win in a head-to-head competition, Horovitz says that it might be too soon to decide. The size of the new animal has been estimated in a different way from P. pattersoni, she notes, making a direct comparison of the two creatures difficult.

"If you put both skulls side by side there is no doubt about which animal was larger," Blanco wrote to Nature News in an e-mail.

Big head

Because the researchers found only a skull, they couldn’t access other skeletal measures, such as the length of the limb bones, that are traditionally used to estimate the size of an animal.

Instead, the team developed an equation relating skull length to body mass using information from 13 of J. monesi’s closest living relatives. This gave an estimate of 1,008 kilograms. Adding information from 6 other measures of skull size gave an estimate of 1,211 kilograms, with a likely range of 468 to 2,586 kilograms.

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Relatively small grinding teeth in the skull suggest that J. monesi was a member of the Dinomyidae family, which contains just one living example, the pakarana. This rodent also lives in South America and weighs roughly 15 kilograms.

The similarity in molars and premolars suggest the bull-sized J. monesi had a relatively weak jaw and lived on roughly the same diet as its modern-day cousin: soft vegetation and fruit. That would perhaps make it less vicious than its size would suggest.

The team next plans to make a biomechanical model to determine the strength of J. monesi’s bite. 

  • References

    1. Rinderknecht, A. & Blanco, R. E. Proc. R. Soc. B 10.1098/rspb.2007.1645 (2008).
    2. Sánchez-Villagra, M. R., Aguilera, O. & Horovitz, I. Science 301, 1708-1710 (2003). | Article | PubMed | ChemPort |
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