Skip to comments.Guinea-zilla? World's largest rodent sibling to guinea pigs - Roughly the size of a buffalo
Posted on 09/18/2003 11:33:19 AM PDT by bedolido
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.
New information on the immense wombat-like Diprotodon optatum indicates it reached more than two and a half tonnes on average - nearly double some previous estimates.
If that was the case, researchers say, it gives the lie to a popular theory that weak vegetation growing on Australia's poor soils during the last ice age stunted the continent's big beasts.
With its gigantic bulk, D. optatum would also have been a mighty handful to hunt, suggesting humans were probably not the main cause behind its extinction more than 30,000 years ago.
The new assessment of this pouched behemoth is provided by Dr Stephen Wroe, from the University of Sydney, and colleagues.
They did their calculation by comparing the beast's fossil remains with the known bone dimensions and body masses of several living marsupials and other mammals.
"The equation we used to predict the weight included animals of the size of a mouse up to a big bull elephant," Dr Wroe told BBC News Online.
"This is considered to be among the most reliable methods. It's great if you have the whole skeleton - as we have D. optatum. It doesn't work with the many species for which we only have teeth."
The analysis pointed to the average D. optatum tipping the scales at almost 2,800 kilograms.
This would have made the creature heavier than the largest rhinoceros. Only elephants among living terrestrial mammals can claim to be more massive.
Australia had a remarkable and distinctive collection of ice age beasts.
Its megafauna, as researchers like to call them, included a tree-climbing crocodile (Trilophosuchus rackhami) and a marsupial lion (Thylacoleo carnifex).
D. optatum nurtured young in a pouch like modern-day wombats and kangaroos
But the nagging suspicion in scientific circles is that the Australian beasts were never quite in the same heavyweight league as the creatures found elsewhere in the world during this period in Earth history.
It has been suggested that poor soils on the continent produced weak vegetation; the herbivores were smaller and so too were the carnivores who preyed on them.
"Our research flies in the face of this argument," Dr Wroe said. "Go to Africa and you will find the biggest elephants are in Nambia, in the desert area, which has extremely low productivity. The smallest elephants are in the forest.
"This idea that if you throw more food into the system the animals will get bigger is over-simplistic in my view."
Whatever the truth about this size argument, what is certain is that right across the globe all of the great ice age animals went extinct very quickly.
By 11,000 years ago, the end of the Pleistocene Epoch, virtually all the beasts had been wiped from the face of the Earth.
We - the human species - are one of the chief culprits for this mass extinction. As our kind spread around the globe, it is thought we systematically hunted out many of the big game.
Long gone: Woolly rhino, mammoth and giant elk.
But Dr Wroe doubts this can fully explain the demise of D. optatum and some of the other Australian megafauna species.
He said the weapons technology required to bring down these big animals efficiently was not evident on the continent until 6,000 years ago, long after the beasts had perished.
"I wouldn't say early aboriginals couldn't kill them but they would have been less efficient hunters of this species than say the Clovis Indians, with their stone spear-points, were of mammoths in North America.
"This data doesn't take humans off the hook but it does suggest the reasons behind the extinction of this species are likely to be more complicated than some have said."
Dr Wroe said that major climatic changes taking place in Australia at that time had to be a consideration.
The D. optatum research is published in Biology Letters, a journal of the Royal Society, the UK's academy of science.
Nice mousie. Want some cheese?
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Note: this topic is from 9/18/2003.
and I thought the Javalina was big