The giant skull of a newly discovered one-ton prehistoric rat—shown here next to a modern-day rat—was revealed on January 16, 2008.

Measuring 53 centimeters (21 inches) long, the skull was found in Uruguay by an amateur fossil hunter among fallen cliff rocks in the San José region. Analysis of the bizarre find by paleontologists suggests it belonged to a bull-size species, which has since been named Josephoartigasia monesi.

The previous holder of the title world's largest rodent was a 1,500-pound (700-kilogram) fossil creature (see pictures) from Venezuela, revealed by scientists in 2003.

South America's rodents evolved in isolation for more than 60 million years, resulting in stunning diversity. The "experiment" ended when the continent became attached to North America via the Panama land bridge some three million years ago, allowing new predators into the southern continent.

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