LONDON  Imagine a rodent that weighed a ton and was as big as a bull.

Uruguayan scientists say they have uncovered fossil evidence of the biggest species of rodent ever found, one that scurried across wooded areas of South America about four million years ago, when the continent was not connected to North America.

A herbivore, the beast may have been a contemporary, and possibly prey, of saber-toothed cats  a prehistoric version of Tom and Jerry.

Its huge skull, more than 20 inches long, suggested a beast more than eight feet long and weighing between 1,700 and 3,000 pounds.

Although British newspapers variously described it as a mouse or a rat, researchers say the animal, named Josephoartigasia monesi, actually was more closely related to a guinea pig or porcupine. “These are totally different from the rats and mice we’re accustomed to,” said Bruce Patterson, the curator of mammals at the Field Museum in Chicago, adding that it was the biggest rodent that he had ever heard of.