For many years, the record holder for largest rodent of all time was the giant beaver Castoroides ohioensis, like the specimen shown above on exhibit in the Science Museum of Minnesota. Castoroides was common in North America (including Virginia) during the Ice Age. At an estimated mass of up to 200 kg (440 lbs), it dwarfed the largest living rodent, the 130-pound capybara (Hydrochoerus hydrochaeris) from South America (below, from the National Zoo).
In 2003 Marcelo R. Sánchez-Villagra, Orangel Aguilera, and Inés Horovitz reported new material from the rodent Phoberomys pattersoni from the Miocene of Venezuela that indicated its mass at around 700 kg (about 1500 lbs), easily surpassing Castoroides for the title of world’s largest rodent (see their abstract here).
This week Andrés Rinderknecht from the Museo Nacional de Historia Natural y Antropología and R. Ernesto Blanco from the Instituto de Física described a new rodent from Uruguay in the Proceedings of the Royal Society B. Josephoartigasia monesi is based on a nearly complete skull, which in itself is unusual (most fossil rodent remains are isolated teeth.)
Josephoartigasia monesi is a huge rodent, with a skull more than twice the length of a capybara’s. Rinderknecht and Blanco calculate that J. monesi had a mass of around 1400 kg (over 3000 lbs), twice the size of Phoberomys pattersoni.
Rinderknecht and Blanco’s complete paper is available here (pdf format).