Distinctly Big, if Extinct: The 1,500-Pound Rodent

Rodents, as well as scientists, stand on the shoulders of giants.

For the rodents, however, it is size and weight that make a giant, not farseeing intellect. One particular giant outweighs them all.

Until recently the creature was known by teeth alone, which made size estimates a bit shaky. But today paleontologists are reporting the discovery of a nearly complete skeleton of Phoberomys pattersoni, a distant relative of the guinea pig, that they can now confirm weighed about 1,500 pounds.

The eight-million-year-old skeleton was excavated in Venezuela three years ago.

The name can be translated as ''Patterson's fearful mouse,'' but Phoberomys cannot have been afraid of much. Nor did it look like a mouse. A drawing based on fossil evidence, being published today in the journal Science, looks for all the world like a cross between a beaver and a hippopotamus.

Upper and lower pairs of ever-growing incisors are what make Phoberomys a rodent, but it is, fortunately, nothing like the enormous rats sometimes reported in subways and other urban settings. Its nearest living relative is the pacarana, a 30-pound nocturnal herbivore that feeds on the slopes of the Andes.

The researchers reported the skeletal evidence for the weight estimate, and noted that the order Rodentia can now be said to have a greater size range -- from less than half an ounce to 1,500 pounds -- than that of any order of placental mammals. One order of marsupials beat out the rodents because of an extinct creature something like a giant kangaroo.

The Phoberomys fossils were excavated in arid land near Urumaco, a town about 250 miles west of Caracas, by a team of paleontologists, including Dr. Orangel Aguilera, of the Francisco de Miranda National Experimental University in Venezuela. He and Dr. Marcelo R. Sánchez-Villagra of the University of Tübingen in Germany and Dr. Inés Horovitz of the University of California, Los Angeles, analyzed the fossil bones.

Dr. Horovitz said Phoberomys was one more example of how successful rodents are. Of 4,600 mammal species, 2,600 are rodents. In terms of individuals, there are lots of them. ''Wherever there have been rodents,'' she said, ''they have been very abundant.''

Dr. Sánchez-Villagra, in a telephone interview after a news briefing in London, acknowledged that public interest would focus on size, but said the find was significant for study of the evolution of rodents. He added that because northern South America had not produced as many fossils as other parts of the continent, like Patagonia, there were gaps in evolutionary history. The Urumaco formation is an exception, and has produced a great diversity of fossils, including Phoberomys.

The area was apparently marshy, with lagoons, when Phoberomys lived there. If it was indeed fearful, it might have been fearful of crocodilians 30 feet or longer that lived in the same place, or of ferocious birds, nine feet tall, that raced around looking for animals to carve up with their powerful beaks.

Phoberomys is thought to have grazed, with no violent intent, on grasses. In short, it was not an H. G. Wells rodent, like the enormous predatory rats that killed and carried off a horse in ''The Food of the Gods.'' It was a C. S. Lewis rodent, like the large and friendly beavers of ''The Chronicles of Narnia.'' South America was also home to a giant beaver, now extinct, but Lewis notwithstanding, there is no evidence that it was a talking beaver.

Dr. Anne H. Walton, a visiting scientist at the Pratt Museum of Natural History at Amherst College in Massachusetts, who has studied fossil rodents of South America, said in an e-mail message that the importance of the paper was that it nailed down the size of Phoberomys.

''Before this paper,'' she wrote, ''the best we could do was hold up a bone next to other bones in a museum collection and say 'about the size of an ox' or 'roughly the size of a rhino.' ''

Now the facts are in. It was the size of a 1,500-pound chipmunk.