The sight of small rodents unnerves a lot of people. Now imagine one weighing upwards of 1,500 pounds and standing over four feet tall. That's the picture scientists are painting of Phoberomys pattersoni, judged to be the world's largest extinct rodent based on newly classified fossils. The ancient creature, described in a report published today in the journal Science, lived eight million years ago and is an extinct cousin of today's guinea pig.

Researchers led by Orangel Aguilera of Venezuela's Universidad Nacional Experimental Francisco de Miranda discovered a nearly complete fossil of P. pattersoni, dubbed Goya, in May of 2000. The remains were trapped within sedimentary layers of brown shales and coal in an area of northwestern Venezuela 250 miles west of Caracas known as the Urumaco Formation. Analysis of the skeleton led to estimates of its immense size: 4.2 feet tall and 9 feet long. "Imagine a weird guinea pig, but huge, with a long tail for balancing on its hind legs and continuously growing teeth," says study co-author Marcel R. Sánchez-Villagra of the University of Tübingen in Germany. "It was semi-aquatic, like the capybara [the largest living rodent] and probably foraged along a riverbank." R. McNeill Alexander of the University of Leeds observes in an accompanying commentary, that "seen from a distance, it would have looked much more like a buffalo than like a scaled-up guinea pig."

What caused the demise of P. pattersoni remains unknown. In comparison to the behemoth, today's rodent heavyweight, the capybara, is a slight 110 pounds. "The question that puzzles me is not how Phoberomys could have been so large, but why the overwhelming majority of rodents are so small," Alexander writes. P. pattersoni's girth would most likely have made burrowing--the common escape route for rodents--difficult, he notes, and perhaps large rodents were too slow to outrun their predators.