Largest rodent ever

First reported in 2008, Josephoartigasia monesi inhabited what is modern-day Uruguay around 2.6 million years ago. Based on its 53-cm-long (1-ft 8.9-in) skull, an estimated body mass of 1–2 tonnes (2,200–4,400 lb) has been proposed.

Rodents are an extremely diverse group that comprise around 40% of all living mammal species, and body size distribution among living rodents span four orders of magnitude. This is testified by the caviomorph rodents endemic to South America, including the familiar guinea pigs as well as the largest rodent alive today, the capybara (Hydrochoerus hydrochaeris). For comparison, the skull of a typical capybara reaches around 24 cm (9.4 in) in length with adults obtaining an average body mass of 50–60 kg (110 lb 3 oz–132 lb 4 oz); females tend to grow slightly bigger than males and occasionally can top 100 kg (220 lb).

For a long time, the title of the largest rodent ever was held by Phoberomys, a gigantic caviomorph which inhabited Venezuela around 8 million years ago. Based on a partial fossil skeleton, palaeontologists estimated that Phoberomys weighed around 436–741 kg (961–1,634 lb).

Dr Virginie Millien (McGill University, Montreal, Canada) considered the aforementioned body size estimations based on skeletal proportions to be somewhat overoptimistic. By analysing and comparing predicted body mass derived from 11 different skeletal measurements from a variety of modern rodent species, Millien derived a more modest estimation of 220–340 kg (485–750 lb) for Phoberomys, around the size of large bull kudu antelope. Lower-bound estimates for J. monesi provided by Millien fall in the 651–715 kg (1,435–1,576 lb) range, but her calculations did not dismiss a weight of over 1 tonne (2,200 lb) to be definitively implausible, and still places J. monesi far ahead of all other contenders for the title of biggest rodent ever

Like elephants, the tooth row of J. monesi is relatively short compared to the enormously constructed skull. It has thus been suspected that similar to how the elephant’s tusks and cheek teeth are responsible for very different functions in the food acquisition-processing work flow, J. monesi could have relied on its massive gnawing incisors to obtain a wide range of soft and tough vegetation, which are then thoroughly chewed up by the cheek teeth comprising of multiple longitudinally aligned enamel ridges, a feature J. monesi shares with today’s capybara, and incidentally also a feature that evolved by convergence in the cheek teeth of elephants.