Buffalo-size rodent fossils stir scientists / Relative of guinea pig gives researchers evolutionary pause
Eight million years ago, a giant relative of today's pet guinea pigs wallowed in the coastal marshes and lagoons of northwestern Venezuela -- a buffalo-size beast that scientists say is by far the biggest rodent that ever lived.
Now, the fossil remains of two of the mysterious creatures have been unearthed from what has become a dry and scrubby semidesert, and the discovery is fascinating scientists who puzzle over the evolutionary lineage of animals that were once so big and are now mostly so small.
The scientific name of the long-vanished rodent is Phoberomys pattersoni, but the scientists who found it call it Goya. A detailed report on the size and bone structure of the ancient beast is appearing today in the journal Science by an international team of paleontologists.
Until now, the largest rodent was believed to be South America's capybaras - - the San Francisco Zoo houses two capybaras -- which weigh in at little more than 100 pounds each and are about the size of a pig. Not so any more, for Goya was at least 10 times bigger.
Goya was about 9 feet long, with slender forelegs and thick, stumpy hind legs, said Ines Horovitz, an Argentine-born adjunct professor of paleontology at UCLA who analyzed the huge rodent's bones.
Its long tail, she said, must have enabled it to stand upright, but with some difficulty, on the soft ground of the riverbeds or lagoons where it preferred to graze. From its roughened teeth, she said, it probably grazed largely on sharp-edged sea grasses.
"There are still many uncertainties about just how it lived and how long its species survived," Horovitz said. Although the two sets of fossils have been dated at approximately 8 million years old, related members of its clan may have existed at any time during what is known as the Miocene epoch, which lasted from 25 million to 5 million years ago.
RUNNING FROM CROCODILES
Its principal predators in the tidal lagoons, Horovitz and her colleagues said, were most probably the giant crocodiles that existed at the time -- many of them more than 40 feet long and armed with fiercely sharp teeth -- as well as the swift-running, meat-eating marsupials that are distantly related to today's kangaroos.
Phoberomys, Horovitz said, belongs to the order of Rodentia that includes the largest number of mammals in the world -- everything from porcupines, beavers and muskrats to hamsters, gerbils and chinchillas. South America in particular is home to an extremely large variety of those rodents because for many millions of years, geologists say, it was an "island continent" where varied species could evolve in isolation.
Horovitz's colleagues who found the fossils were Marcelo Sanchez-Villagra of Tubingen University in Germany and Orangel Aguilera of the National Experimental Museum in the Venezuelan state of Falcon.
To Tom Turowski, who cares for the two capybara sisters at the San Francisco Zoo, the finding of their extinct relatives was amazing. "Why, those beasts were as big as our own big black rhinoceros!" he said. "Too bad they're not around any more."
At the University of Leeds in England, R. McNeill Alexander has long specialized in the evolution of movement in large extinct animals. In a commentary in Science on the discovery of Phoberomys, he said: "Seen from a distance, it would have looked much more like a buffalo than like a scaled-up guinea pig."
Speculating on how the giant rodents ultimately were doomed to extinction, Alexander noted that the overwhelming majority of their rodent relatives that have been tiny for millions of years -- the mice and rats and guinea pigs and squirrels, for example -- had successfully escaped predators by running swiftly and burrowing into small holes.
Any out-size, heavy-footed giant rodents like Phoberomys must have been slow-moving and thus more vulnerable to fast predators, Alexander reasoned. "Would large rodents generally be too slow to be successful?" he asked.