Think the rodents you've seen in movies are scary? Scientists who've analyzed the fossilized remains of an extinct South American relative of guinea pigs say that the ancient bruisers were as large as bison. HIDE THE CHEESE. A bison-size rodent, Phoberomys pattersoni, grazed on aquatic grasses and roamed the riverbanks of ancient Venezuela about 8 million years ago.C.L. Cain/Science Researchers first described Phoberomys pattersoni in 1980 but until recently had only bone fragments and isolated teeth to study. Despite that limitation, scientists suspected that the animals were huge, says Marcelo R. Sánchez-Villagra, a paleontologist at the University of Tübingen in...