“Mighty Mouse”or Perhaps more Accurately “Mighty Guinea Pig”

Hidden away in a dark corner of the storage vaults of the Natural History and Anthropology museum in Uruguay, lay the huge skull of a prehistoric mammal that scientists claim belonged to a rodent the size of a bull.

The fossil, consisting of the upper portions of a skull, was discovered 20 years ago in the River Plate estuary by a Uruguayan fossil collector.  It was donated to the museum by lay in storage until museum curator Andres Rinderknecht and researcher Ernesto Blanco decided to study it.

Their findings have just been published in the proceedings of the British Royal Society B: Biological Sciences and estimates for this animal give it an approximate length of 3 metres and perhaps a body weight close to 1,000 kilos.  Such estimates have to be treated with a degree of caution, as it can be difficult to determine body size from only partial remains, particularly if there are few extant genera to make a direct comparison with.

Large animals tend to have disproportionately smaller heads when compared to the body masses of more diminutive creatures so the estimates for this new animal – named Jospehoartigasia monesi are based on scientific deduction.  If other fossils of an adult can be found such as limb bones then perhaps a more accurate assessment can be made.

The Fossil Skull of Jospehoartigasia

Picture Credit: Royal Society B: Biological Sciences

The picture depicts the rodent skull compared with a typical rodent of today. The parts modelled in grey provide a reconstruction of the entire skull, as can be seen the lower jaw and the incisors are missing.

The fossil has been dated to the Pliocene and is estimated to be around 4 million years old.  The skull is typical of that of a rodent but the animal was of an exceptional size.  Perhaps wandering the grassy plains of South America eating roots, fruit and leaves and sharing the plains with Sloths and Glyptodonts whilst trying to avoid the predatory “Terror Birds” such as Phorusrhacus.

This fossil may well represent the largest rodent known to date, although the rodent Phoberomys pattersoni from the Miocene of Brazil and Venezuela may also have been around the same size but perhaps with a longer tail.

An Artist’s Impression of Jospehoartigasia monesi

Super-sized Rodent

Picture Credit: Royal Society B: Biological Sciences

The picture above shows Jospehoartigasia (A) compared to the largest rodent today, the semi-aquatic South American Capybara (B), which ranges over much of the continent today.  It is not known whether Jospehoartigasia was amphibious, but the fossil was found in an area rich in ancient waterways so this large rodent could have spent some time in the water, safe from land predators, grazing in peace on the lush vegetation.  The jaws may have been relatively weak for such a large animal, this may support the theory that these ancient rodents fed on soft water plants.

This new fossil find has been classified into the Dinomyidae family, effectively close relatives to Guinea Pigs and Capybaras as well as the extinct Phoberomys.

It certainly was a big animal, not the sort of rodent that you would expect to catch with a conventional mouse-trap, more of a “mighty Guinea Pig” rather than  “Mighty Mouse”!

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