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​The Largest Rodent of All Time Also Had Crazy Death Tusks

Josephoartigasia monesi could mess you up, son.
​J. monesi, looking cool. Image: Andres Rinderknecht & Ernesto Blanco

In the Rob Reiner classic The Princess Bride, Westley and Buttercup end up scuffling with a Rodent of Unusual Size, which is basically a hog-sized rat. But as scary as the ROUS's sneak attack is, the fictional animal has nothing on the real life Josephoartigasia monesi, an extinct species that roamed South America from around four million to two million years ago.

Measuring 10 feet long and weighing about a ton—roughly the dimensions of a buffalo—this prehistoric animal was the largest rodent that ever lived. But its enormous size wasn't the only intimidating feature of J. monesi, according to a study published today in the Journal of Anatomy. The animal also boasted a lethal pair of incisors and a bite force comparable to a saber-toothed tiger. Make no mistake: this giant guinea pig could take you down.

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Size comparison of Josephoartigasia monesi with a human and a pacarana, its closest living relative. Image: Andres Rinderknecht and Ernesto Blanco

Only one fossil from the mega-rodent has ever been found; a nearly complete skull discovered in 1987 near Montevideo, Uruguay. The specimen wasn't described and identified as a new species until 2008, and today's study marks the first attempt to calculate the animal's bite force by performing finite element analysis on a CT scan of its skull.

Josephoartigasia monesi fossil. Image: Andres Rinderknecht and Ernesto Blanco

This technique suggested that J. monesi had a bite force of about 1,400 Newtons, which is actually more than what has been estimated for the jaws of the saber-toothed predators with which it shared its turf. On top of that, this massive rodent's incisors could withstand about 4,000 Newtons of force, making them a reliable tool for all kinds of adaptive skills.

"We concluded that Josephoartigasia must have used its incisors for activities other than biting, such as digging in the ground for food, or defending itself from predators," said lead author Philip Cox, who specializes in rodent cranial morphology, in a University of York statement. "This is very similar to how a modern day elephant uses its tusks."

Like elephants and many other tusked animals, J. monesi was a herbivore, and paleontologists think it likely subsisted on aquatic plants in South America's Pilocene river deltas.

Aside from saber-toothed tigers, the animal may have been prey to phorusrhacids, appropriately known as "terror birds," which were flightless, rampaging carnivores that dominated South America during this time. Considering how utterly petrifying predators of this time were, it's no wonder the species evolved its powerful incisors and massive frame. When it comes to warding off Cenozoic mega-hunters, it seems the best defense is a good offense.