Giant Rodent Fossil Found In Uruguay. In this image released in London Wednesday, Jan. 16, 2008 by the Royal Society, an artist's illustration shows variations on the head of Josephoartigasia monesi, a newly discovered extinct species of rodent. Eeek! 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 4 million years ago, when the continent was not connected to North America.