Giant Rat Less Giant Than Originally Thought

By Andrew Moseman | May 22, 2008 9:13 am

The skull of Josephoartigasia monesiIf one Canadian researcher is right, the largest rodent ever found just lost about 1,300 pounds.

A biological brouhaha started this week over the fossils of the Josephoartigasia monesi, a giant rat that made its home a couple million years ago in what is now Uruguay. Unfortunately, only the fossilized skull survived — scientists never unearthed any of the remainder of the skeleton, so they had had to do a little guessing as to the rest of the creature’s proportions. Using the ratio of the size of a modern rat’s head to its body, the Uruguayan scientists who dug up the bones in January said the creature would have weighed a full ton — about 2,200 pounds, or 15 times heavier than the largest rodent roaming the earth today.

But their calculations may have been inaccurate, says Virginie Millien of McGill University in Montreal. By taking a wider sample of modern rodents, she now predicts that the rat could have been much smaller — as light as 770 pounds. But modern rats aside, you can still only tell so much from looking at a skull. Millien’s calculations have a range of possibilities extending as high as 3,300 pounds, though that size is rather unlikely. The Uruguay team was quick to point out that her mean figure — about 2,000 pounds — was only slightly lower than theirs.

If these details sound trivial, remember that the giant rat broke the previous rodent record, held by a 1,500 pound guinea pig discovered in Venezuela, so any weight reduction could knock the new find off its pedestal. Fortunately for J. monesi‘s pride, even at 770 pounds it would weigh in far heavier than its largest living relative. And, fortunately for those of us who are easily disgusted, it’s not coming back into existence anytime soon.

Image: Andres Rinderknecht & Ernesto Blanco © 2008

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