Capybara (Hydrochoerus hydrochaeris) and Guinea Pig (Cavia porcellus) skulls

Note: This capybara is not particularly large. I have one that is larger and have seen live capybaras with likely larger skulls. Guinea pigs are also rather big-headed in terms of rodent body proportions. Regardless, the size disparity is still striking.

 

Here is the largest rodent in the world, the capybara, with it's close cousin, the guinea pig, a top it's head. Some taxonomists put the capybara in its own family, the Hydrochoeridae, while other lump it into Caviidae, the guinea pig family. Regardless, capybaras are more closely related to caviids than other rodents, and both belong the remarkable South American radiation of caviomorph rodents, hysticomorphous rodents (note the large infraorbital foramen) that accidentally made their way across the Atlantic in the Eocene while South America was still splendidly isolated from the rest of the continents. There were many caviomorphs larger than the capybara before modern times, including rhino sized dinomyids.

Done