By Jonathan Amos
BBC News Online science staff
The fossil remains of a gigantic rodent that looked something like a monster guinea pig have been identified by scientists in Venezuela.
The 700-kilogram beast - about the size of a buffalo - lived among the reeds and grasses of an ancient river system that threaded its way into the Caribbean Sea eight million years ago.
Researchers think the creature, which was 10 times as big as today's largest rodents, could have run in huge packs.
Evidence suggests it also had to dodge the constant attentions of super-sized crocodiles and carnivorous birds, which stood three metres tall.
The discovery of "Guinea-zilla", as some have already dubbed it, is reported in the journal Science.
The remains were pulled out of brown shale and coal beds at the town of Urumaco, 400 kilometres west of Caracas.
Researchers from Venezuela, the US and Germany were involved in the discovery. The lead author on the Science paper is Dr Marcelo Sanchez-Villagra of Germany's University of Tubingen.
"Urumaco was a place of giants eight million years ago," he said. "The world's largest turtle - three metres long - was found there. It had some of the largest crocs ever seen and there are undescribed fish that were also three metres long."
The new rodent has been given the scientific name Phoberomys pattersoni, after a pioneering palaeontologist who worked in the region in the 1970s, Professor Brian Patterson.
The weight of P. pattersoni substantially exceeds that of today's biggest rodent, the 50-kg capybara.
P. pattersoni would have been about three metres long and just over metre tall.
Dr Sanchez-Villagra said it had long ever-growing teeth - its incisors would have been about 20 centimetres from root to tip - housed in a skull that was twice as long as a capybara's.
A great river system once ran through Urumaco
Its grinding teeth probably helped the animal dine on sea grasses growing amongst the brackish lagoons and inlets of its wetland habitat.
"It was probably semi-aquatic, spending some time in the water and some of the time on land - just like the capybara," Dr Sanchez-Villagra said.
"And because it spent so much time in the water, its eyes were probably more dorsal - higher on the head than say a rat. Its jaw was also deeper than a rat's which is quite pointed."
Although it may have been distantly related to today's guinea pig (Cavia porcellus), with hind quarters and rear legs larger and more powerful than its smaller forelimbs, the beast would have dwarfed its modern cousin which normally weighs about a kilo.
"And of course the guinea pig doesn't have the robust tail we know this creature had," Dr Sanchez-Villagra said.
The 90%-complete Guinea-zilla remains were actually discovered in 2000. But scientists have held back from a formal classification of the animal until now because they wanted to compare the fossil with a second specimen with more extensive skull features.
Scientists believe a once mighty river system - the Paleo-Oronoco-Amazon - moved through the Urumaco area carrying water from the interior of the Southern American continent north-east to the ocean.
Other fossils found at Urumaco support this idea and illustrate what a lush landscape this now arid part of Venezuela must have been towards the end of the Miocene Epoch.
Nowhere to run
But that rich biodiversity may have brought the rodent trouble in the form of other, spectacularly big creatures.
These animals, such as lion-sized marsupial cats, could have played a part in the eventual demise of P. pattersoni, believes Professor Neill Alexander, of the University of Leeds, UK.
"Predation would certainly have been a factor," said the expert in the mechanics animal movement.
"Being so big it would not have been able to run into a burrow and hide. And the problem with being a really big rodent is that you are slower than the competition; you are vulnerable unless you are somewhere where the predators aren't too fast either."
Commenting on the discovery, Science journal's International's Managing Editor, Dr Andrew Sugden, said P. pattersoni would have a major impact on our understanding of evolution in South America.
"The first rodents appeared about 40 to 50 million years ago," he said. "Rodents are now the most diverse group of mammals with more than 2,000 species; 40% of all mammalian species.
"And they also have a huge size range - the capybara may be 10,000 times larger than the pygmy gerboa. But at a stroke this new fossil stretches this size range by more than another order of magnitude and reshapes our view of the evolution of these animals."
South America intrigues scientists because until the emergence of a land-bridge (the Panamanian isthmus) connecting it to Central and North America about three million years ago, the landmass had been isolated for tens of millions of years.
The fossils were found in 2000
Its plants and animals developed in isolation to the rest of the world's flora and fauna. And although palaeontologists have made many amazing discoveries recently in countries like Argentina, the northern fringes of South America have not been so well studied.
"This fossil fauna from Urumaco in north-western Venezuela opens a new chapter in the history of biodiversity for that region," Dr Sugden said.