It was 9ft long, it weighed 700kg or 1,500lb and it grazed on the lush banks of a South American river 8 million years ago. Phoberomys pattersoni, alias Goya the giant guinea pig, is the biggest rodent ever found, according to the journal Science today.
In May 2000 researchers began recovering the fossilised bones from a layer of shale in what is now a hot desert region of Venezuela called Urumaco, 250 miles west of Caracas. Only when they found the jaw did they realise that they had found a rodent 10 times bigger than any on earth today. And only when they had taken a closer look at the almost complete skeleton did they realise they had a caviomorph, cousin to the modern cavy or guinea pig. The bones were found at Tio Gregorio – and the Spanish diminutive for Gregorio is Goya.
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“Imagine a weird guinea pig, with a long tail for balancing on its hind legs and continuously growing teeth,” said Marcelo Sanchez-Villagra of the University of Tübingen in Germany. “It was semi-aquatic, like the capybara, and probably foraged along a riverbank.”
There are around 5,000 species of mammal on the planet today. Around 2,000 of them are rodents. The smallest weighs a few grams, the largest, the capybara, a few tens of kilograms. The unearthing of a species at least 10 times bigger than all the rest raises huge questions about the pace and direction of evolution – and economies of scale.
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“This really is a stunning animal,” said McNeill Alexander of the University of Leeds, an expert on the mechanics of animals. “If you saw this animal at a distance on a misty day it would look much more like a cow or a buffalo than like a guinea pig. Supporting its weight is no problem. How about feeding itself? That is going to be easier to satisfy its appetite because it is big. It has teeth which are obvious used for grinding abrasive food – grass.”
Like a cow, the giant guinea pig would have depended upon micro-organisms in its gut to turn grass into nourishment. The bigger an animal’s gut, the faster the supply of energy from the fermenting grass. But also, the bigger the animal, the more it conserved energy and the less it needed to eat.
“It is ever so much cheaper to feed a five-tonne elephant than it is to keep five tonnes of mice,” Prof Alexander said. “That means the bigger you are, the easier it is going to be to satisfy your appetite.”
Other rodents have not grown to this sort of size because they tend to run down boltholes to get away from predators, which is why it helps to be small. They cannot outrun their enemies in the open.
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The implication is that Phoberomys had relatively few enemies at the time. The Panamanian landbridge formed only 3 million years ago. Until then, South America was separated from the northern continent, and Phoberomys lived in a very different world.
It would probably have been at home on dry land and in the water, and it lived alongside catfish, turtles with carapace 6ft long, and predators that included a giant flightless, carnivorous bird, a marsupial cat the size of a lion and crocodiles more than 30ft long.
“This Urumaco was a place of giants 8 million years ago,” Dr Sanchez-Villagra said. “The northern region of Venezuela holds the key to many mysteries of palaeontology and animal evolution.
“Yet, we have known very little about this area because regions covered with vegetation are not the best places to look for fossils.”
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