Guinea-zilla? World's largest rodent identified as ancient sibling to guinea pigs

American Association for the Advancement of Science

Roughly the size of a buffalo, a giant rodent that roamed the banks of an ancient Venezuelan river some 8 million years ago, dining on sea grass and dodging crocodiles, was an evolutionary sibling to modern-day guinea pigs.

The largest rodent that ever lived, Phoberomys pattersoni, weighed about 1,545 pounds (700 kilograms) - more than 10 times the size of today's rodent heavyweight, the 110-pound (50 kilograms) capybara.

"Imagine a weird guinea pig, but huge, with a long tail for balancing on its hind legs and continuously growing teeth," said Marcelo R. Sánchez-Villagra of Germany's University of Tübingen. "It was semi-aquatic, like the capybara, and probably foraged along a riverbank."

The ancient creature's fossilized remains -- described in the 19 September, 2003 issue of Science, published by AAAS, the science society -- offer rare, tantalizing new clues to the Upper Miocene period in northwestern Venezuela.

Discovered in a now-arid region 250 miles west of Caracas, in the town of Urumaco, the fossil and associated plant evidence suggest a lush, tropical landscape, rich with super-sized turtles, catfish and crocodiles. The Science paper thus seems to reinforce the theory that a massive river called the Paleo-Orinoco-Amazon once flowed parallel to the Andes mountain range through Urumaco, in the Falcon State, northeast to the Caribbean Sea.

"The northern region of Venezuela holds the key to many mysteries of paleontology and animal evolution," said Sánchez-Villagra. "Yet, we have known very little about this area because regions covered with vegetation are not the best place to look for fossils. Most of the fossil evidence has been found in southern South America. With this work, we are taking steps toward broadening our knowledge of South America as a whole."

Why don't buffalo-sized rats roam the Earth today? And, why did Phoberomys pattersoni reach such massive proportions?

R. McNeill Alexander of the University of Leeds, author of a related Perspectives essay in Science, noted the relationship between body posture and the size of various animals: Tiny mice, for instance, crouch on very bent legs, whereas elephants tend to keep their legs relatively straight. "The giant rodent fossil raises wonderful questions about the constraints of evolution on size," Alexander said.

The cause of the demise of Phoberomys remains a mystery. But, Alexander pointed out that small mammals such as rodents typically escape predators by burrowing into a refuge. "Large mammals, too big to burrow, can generally escape only by running," he explained. "Ungulates -- with their long legs, light hooves and long elastic tendons -- seem best for that. Would large rodents generally be too slow to be successful?"

Dubbed "Goya," the 90-percent-complete fossil of Phoberomys pattersoni was trapped within sedimentary layers of brown shales and coal, within the Urumaco Formation. It was discovered by a research team under the direction of Orangel Aguilera of Venezuela's Universidad Nacional Experimental Francisco de Miranda, co-author of the Science paper. The Science team also includes Inés Horovitz, now at the University of California in Los Angeles.

Researchers found the fossil in mid-May 2000, but never specifically classified it, until now. Scientists had speculated that it might be related to various other rodents -- either chinchillas, viscachas or pakaranas. By examining the Goya fossil, together with a second specimen offering more complete skull evidence, the authors were able to identify Phoberomys pattersoni as a sibling to the pakarana Dinomys -- a close relative of the guinea pig (Cavia porcella). Some 9 feet long (3 meters) and 4.2 feet tall (1.3 meters), Phoberomys pattersoni had long teeth revealing an abrasive diet, perhaps of grasses from brackish water. Its hind quarters and rear legs were much larger and more powerful than its smaller forelimbs, much like a guinea pig. Yet, today's guinea pigs weigh about 2.2 pounds (1 kilogram).

Both creatures belong to a diverse radiation of South American rodents called Caviomorpha. Today, this group of rodents ranges in size from 8 ounces, or one-half pound (200 grams) to 110 pounds (50 kilograms). Andrew Sugden, an evolutionary biology expert and Science International's Managing Editor, described the research as a milestone within the field: "At a stroke, this giant rodent more than doubles the size range of this remarkable family of animals and provides fascinating new insights into life some 8 million years ago," he said.

Until the emergence of a land-bridge (the Panamanian isthmus) connecting Central and South America some 3 million years ago, South America had been an island for tens of millions of years, Sugden explained. South American animals thus managed to evolve in relative isolation, and the continent became home to giant representatives of a number of mammalian groups, some of which survived until
the arrival of humans.

Research in Venezuela was partially supported by the National Geographic Society and the University of Tübingen. The Smithsonian Tropical Research Institute and the Universidad Nacional Experimental Francisco de Miranda (UNEFM) supported Aguilera's field and laboratory work.

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