Guinea pigs are common pets around the world, with over one million of these furry, or sometimes naked, little rodents being cared for by people. They are relatively easy to look after, and pretty cute too. My daughter likes to watch guinea pigs on You Tube – she finds it relaxing. I was flabbergasted to see that some videos of guinea pigs just sitting or eating, have over 10 million views! People do like the cuteness of these relatively large rodents. Capybaras, the largest living rodent and close relative to the guinea pig, have the same cuteness appeal. Their sleepy looking eyes and docile nature, along with their large size win the hearts of whoever sees them.

Endemic to South America, capybara are very large rodents, about the size of a fairly large dog, only much rounder and shorter legs. But they are not the biggest rodents to have waddled on Earth.  This crown currently belongs to the enormous rodent Josephoartigasia monesi, which was about as large as a cow!

This enormous beast was found in rocks dating between 4 and 2 million years old, and has only relatively recently described in 2008. (Another example of the importance of museum collections, the skull was in the National Museum of Natural History in Uruguay, after being excavated in 1986.) This skull, missing the lower jaw, is a whopping half a meter long. Rodents are not this big! The capybara’s is less than a third this length. For something so large, how do we know it’s a rodent and not something else? It’s the teeth that give it away.

Teeth are very diagnostic in mammals. Different groups have their own unique shapes, which are a little modified within species. All the species belonging to the Order Carnivora, for example, have large conical canines, flat sharp incisors, large sharp premolars, and slicing molars. All groups of carnivores share these traits, although within the group, different species will have slightly modified versions depending on their dietary needs. (The European sabretooth cat, Homotherium, for example, has serrations on its incisors which is like a hot knife through butter. Only the butter is flesh.) Rodents on the other hand, belong to the Order Rodentia, and have four very large incisors (two on the top, two on the bottom), which grow continuously throughout their lives. Most species have no canine or premolars, with a gap between the incisors and the tough ridged, plated molars. The teeth of J. monesi are the teeth of a rodent.

So far, we only have the skull. But this can still give us a lot of information, including its size and who it is related to. The bone structures in the skull of J. monesi, along with the premolars and teeth, show it belongs to the Family of rodents called Dinomyidae. Although once a very diverse group, appearing around 30 million years ago, today there is only one living species, the pacarana, which is about the size of a cat. Various measurements and analysis of the skull put J. monesi in the range of 1200kg, which is about the same as a cow or bison.

The skull and teeth also provide clues about the animals feeding habits, and the environment it lived in. The bones which attached the cheek muscles are quite slender, and this hints that the muscles were relatively small for such a big animal. This along with the relatively small grinding teeth suggest that the diet was softer vegetation and possibly fruits. They may have eaten aquatic plants, as other fossils found with J. monesi are typical of a delta type environment with nearby forests, similar to the environment of the capybara today. The large incisors have been compared to the tusks of elephants, which may have been used for digging or scraping for food, and even fighting for defence or for females.

There is still a lot more to learn about this huge beast, and when more fossils of the skeleton are found, they will give even more information. The rocks the skull was found in age to between 4 and 2 million years ago, so more fossils will also give us a better time range for this species and give us clues to why it became extinct. The skull was found alongside other fossils, such as giant sloths and sabretooth cats. Sabre tooth cats only made their way to South America around 2.7 million years ago. Before then, J. monesi would have been adapted to the natural predators of South America, which was an isolated landmass. With North and South America joining around 2.7 million years ago by underwater volcanoes and sediment, this meant that animals from both landmasses could move between the two. New predators, like sabre tooth cats would have seen these giants as an easy meal. That’s one possibility. The climate was also becoming a little cooler around this time, and we don’t know if this had an impact on our cow-sized rodent.

It would be quite something to see this enormous creature wallowing in the shallow waters of a river in South America. With the rustling leaves of a nearby tree as a giant sloth slowly moves past. And perhaps a low rumbling of a sabretooth cat nearby, heard but not seen. The mammals of South America at the beginning of the Pleistocene were part of a truly unique landscape.

Written by Jan Freedman (@JanFreedman)

Further reading:

Blanco, R. E. 2008. The uncertainties of the largest fossil rodent. Proceedings of the Royal Society. 275. pp.1957–1958. [Full article]

Blanco, R. E., Rinderknecht, A., & Lecuona, G. 2012. The bite force of the largest fossil rodent (Hystricognathi, Caviomorpha, Dino[1]myidae). Lethaia 45. pp.157–163. [Full article]

Cox, P. G., Rinderknecht, A., and Blanco, R. E. 2015. Predicting bite force and cranial biomechanics in the largest fossil rodent using finite element analysis. Journal of Anatomy. 226. pp.215-223. [Abstract only]

Cox, P. G., Rinderknecht, A., and Blanco, R. E. 2015. Masticatory biomechanics of the largest fossil rodent. The FASEB Journal. https://faseb.onlinelibrary.wiley.com/doi/10.1096/fasebj.29.1_supplement.865.7

Fields W.R. 1957. Hystricomorph rodents from the Late Miocene of Colombia, South America. Univ. Calif. Publ. Geol. Sci. 32. pp.273–404. [Full article]

Millien, V. 2008. The largest among the smallest: the body mass of the giant rodent Josephoartigasia monesi. Proceedings of the Royal Society. 275 (1646). pp.1953-1955. [Full article]

Mones A. 2007. Josephoartigasia, Nuevo nombre para Artigasia Francis & Mones, 1966 (Rodentia, Dinomyidae), non Artigasia Christie, 1934 (Nematoda, Thelastomatidae) Comun. Paleontol. Mus. Hist. Nat. Montevideo. 36. pp.213–214.

Rinderknecht, A., & Blanco, R. E. 2008. The largest fossil rodent. Proceedings of the Royal Society. 275 (1637). pp.923-928. [Full article]