Giant Fossil Rodent

The reconstructed Josephoartigasia skull used in the study by Cox et al., 2015. New York City rats are beasts of legendary size and ferocity. That reputation probably has more to do with East Coast attitude more than anything else. Even the largest of the city’s rats look puny in comparison to South America’s capybara – at sizes over 130 pounds, the largest rodent in the world. But even the capybara is a lightweight next to the largest rodent of all time. At an estimated weight of over a ton, Josephoartigasia monesi was truly a rodent of unusual size.

First described by paleontologists Andrés Rinderknecht and R. Ernesto Blanco in 2008, Josephoartigasia is only known from a 1.7-foot-long skull found in the 4-2 million year old rock of Uruguay. That lone cranium was enough to know that the mammal was new to science and the title holder for biggest rodent of all time.

Rather than preying upon adventurers, though, these real ROUS’ munched plants in prehistoric forests and deltas. They were more guinea pig than rapacious rat. But what exactly did they eat?

There are a few ways to detect the diet of a prehistoric creature. Gut contents certainly help, as well as cololites and coprolites (feces preserved inside and outside the body, respectively). For Josephoartigasia, however, paleontologists only have a cranium. Clues about what was on the rodent’s menu have to be drawn from the skull.

One method is to look at the damage to a prehistoric creature’s teeth. Worn edges, pits, scratches, and other jots and tittles can reflect an animal’s diet close to the time of its death. This is often paired with geochemical analyses that can match carbon isotopes to certain plant types. And then there’s Finite Element Analysis. Borrowing some techniques from engineering, paleontologists can estimate bite forces and see how skulls coped with the stresses of feeding. This latter method is what University of York anatomist Philip Cox, Rinderknecht, and Blanco chose to see what Josephoartigasia could have fed on.

The first step was completing the skull. After all, a bite requires both upper and lower jaws. As a stand-in, Cox and colleagues picked the mandible of the closest-living relative to Josephoartigasia – the plains viscacha. With a 3D scan of the living rodent’s jaw to complement that of its extinct cousin, the researchers simulated how Josephoartigasia bit at different positions along the jaw.

Josephoartigasia had an especially powerful bite. At the incisors, the rodent could chomp with a force of over 300 pounds. A bite at the molars was an even more powerful 936 pounds. While the researchers stress that these are maximum values and don’t represent a typical bite, they forces are still comparable to those of carnivores like bone-cracking dogs and large crocodylians. Josephoartigasia wasn’t restricted to soft foods, but could bite through some really tough stuff.

But what makes Josephoartigasia strange is that its incisor teeth were incredibly strong. These chisel-like teeth, Cox and colleagues write, “could resist much greater forces than could ever be generated by the masticatory muscles.” The question is why.

While stress-resistant choppers don’t have to be a direct adaptation and could have more to do with large body size, Cox and colleagues suggest that such strong incisors could have been useful for digging through the ground for roots or even defense. If this was the case, the researchers speculate, Josephoartigasia was “behaving in an elephant-like manner, using its incisors like tusks, and processing tough vegetation with large bite forces at the cheek teeth.” If so, this would only add to the evidence that Josephoartigasia was one mammoth rodent.