LONDON (AP) - Imagine a rodent so big it weighs a ton and is the size of a bull. Uruguayan scientists say they have uncovered fossil evidence of the biggest species of rodent ever found, one which roamed South America about 4 million years ago, when the continent was still an island. A herbivore, the beast may have been a contemporary - and possibly prey - of saber-toothed cats in a prehistoric version of Tom and Jerry.