"Imagine a weird guinea pig, but huge, with a long tail for balancing on its hind legs and continuously growing teeth," said Marcelo R. Sanchez-Villagra of the University of Tubingen in Germany, the first author of a study appearing this week in Science.
The formal name of the rodent is Phoberomys pattersoni. The last term is in honor of Brian Patterson, a Harvard professor who led a fossil-collection expedition to Venezuela in the 1970s. Informally, the skeleton is called Goya.
Researchers found the fossils in a semidesert area of Venezuela, about 250 miles west of Caracas.
When Goya lived there, some 6 million to 8 million years ago, the area was a lush paradise for a large plant eater.
"At the time it was forested and swampy with a big river and a lot of vegetation," said Sanchez-Villagra.
The giant rodent grazed on grasses, which he must have eaten in large amounts to support his great size. Goya had fur, a smooth head with small ears and eyes, and a large tail that enabled it to balance on two hind legs to watch for predators, said Sanchez-Villagra.
And there were a lot predators to worry about, he said.
"We know that there were crocodiles in the same location where we found this animal," said Sanchez-Villagra. "They were some of the largest crocs ever — more than 10 meters (33 feet) long."
Goya also had to worry about a large carnivore called the marsupial cat, and huge flesh-eating birds called phorracoids, he said.
Phoberomys pattersoni lived during a time when South America was isolated from the rest of the world. The isthmus of Panama had not linked the two Americas, and the southern animals evolved independently of those on the other continents.
That changed about 3 million years ago. The shifting land masses became joined at what is now Panama and animals from the two Americas began to mix. That may have spelled the demise of Goya, although it remains a mystery exactly why the animal went extinct, Sanchez-Villagra said.
"Many animals from North America made it to South America and many from the south went north," he said. "When that happened, many of the animals from South America became extinct because of competition."
In an analysis of the Sanchez-Villagra study, R. McNeill Alexander of the University of Leeds, England, wrote in Science that the large rodent may have died out because it simply couldn't escape predators.
Alexander said most rodents are small enough to hide in the ground when threatened, but Phoberomys pattersoni was too large to burrow. As do most large animals, it would have to depend on running to escape a predator. Alexander said that suggests this question: "Would large rodents generally be too slow to be successful?"
Sanchez-Villagra said Goya's skeleton, particularly the leg bones, suggests that it walked differently from most modern rodents, such as its close cousin the guinea pig. Mice, rats and guinea pigs scamper along in a crouched position, with legs bent at the knee and elbow.
Because of Goya's mass, however, it had to stand straight, more like a sheep than a mouse.
As a result, Alexander wrote in Science, "Seen from a distance, it would have looked much more like a buffalo than like a scaled-up guinea pig."
An analysis of Goya's teeth show they were ideally adapted for eating grasses. Sanchez-Villagra said chewing tough grasses can eventually wear out the teeth. But in Goya, the teeth were constantly growing so they remained at the length needed to grind up grass.
Sanchez-Villagra said the closest living relative to Phoberomys pattersoni is probably the pacarana, a slow-moving rodent that can grow to 33 pounds and lives in the tropical forest of the western Amazon River basin. It is considered rare.
The largest living rodent is another South American animal, the capybara, which can weigh up to 110 pounds. The most common rodents are mice, which weigh one to two ounces, and rats, which can weigh up to 10 ounces or more. The rodent clan also includes squirrels, beavers and prairie dogs.