World's Largest Rodent: Buffalo-Size Fossil Discovered

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Phoberomys was likely semi-aquatic and probably fed on sea grasses, much like the capybara (Hydrochaeris hydrochaeris), the largest living South American rodent, does today. The capybara can weigh as much as 150 pounds (70 kilograms).

Predators likely included several kinds of crocodiles that lived in the swamps, including some of the largest forms that ever existed such as the giant Amazonian crocodile Purussaurus brasiliensis, which was 40 feet (12 meters) long.

"Other possible predators that might have attacked mostly younger—and smaller—individuals could have included marsupial carnivores and phororhacids," said Horovitz. Phororhacids were large, flightless, predatory birds that evolved in South America when the continent was isolated from the rest of the world, including North America.

Heavy Rodent

The Phoberomys fossils recovered in Venezuela are not the first ever found, but they are the most complete. Based on earlier fossils from Brazil, scientists had speculated the rodent weighed an estimated 1,100 pounds (500 kilograms).

Given the completeness of the fossil skeletons recovered in Urumaco, Sánchez-Villagra and colleagues were able to determine a weight range of Phoberomys from 961 to 1,634 pounds (436 to 741 kilograms).

The weight range estimation is based on the relationship between the size of the limb bones that carry most of the animal's weight as compared to the known bone dimensions and weight of animals within the same group that are alive today.

Phoberomys belongs to the order of Rodentia. Living members of the group today include beavers, squirrels, guinea pigs, rats, mice, capybaras, and hundreds of other species, said Sánchez-Villagra.

"We find out what kind of correlation exists between size of the bones and weight for as many animals belonging to the group as possible," he said. "If we know what these correlations are and know the dimensions of the bones of Phoberomys, we can estimate its weight."

The lower end of the range is based on estimates from bones in the front limb bones and the high end from the rear limb bones.

Since Phoberomys' hind limbs were likely more important for walking and its front limbs for gathering food, the researchers conclude that body weight estimations based on the rear limb bones are more reliable: about 1,500 pounds (700 kilograms), or 10 times the size of the capybara—or as large as a modern buffalo.

Horovitz said the existence of a rodent this size in northern South America gives scientists a better picture of the large range of shapes and sizes of mammals that evolved in South America when it was isolated from the rest of the world.

"During this time, many groups of mammals that evolved in other continents were not present in South America, and at the same time the fauna in this continent took its own evolutionary path," she said. "Sometimes very different groups filled equivalent ecological roles in different parts of the globe."

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