One of the most interesting evolutionary patterns is an increase in the disparity of sizes in a group, small representatives persisting and changing even as some lineages get larger (I’ll address this issue a bit more in a separate piece of Cope’s Rule, if such a thing even truly exists). A new Pleistocene fossil rodent from Uruguay called Josephoartigasia monesi further elucidates this trend, being the largest fossil rodent yet discovered with an estimated body mass of about 1000 kg (one tonne). Contrary to some misunderstandings in the popular media and elsewhere, this animal was not a giant rat but belongs to the family Dinomyidae, the only living representative of the group being Count Branicki’s terrible mouse (or pacarana [Dinomys branickii]), the Dinomyidae also being closely allied with the Cavidae (which includes capybaras, guinea pigs, maras, etc.). Josephoartigasia is larger than the largest known living and extinct rodents, though, (see image below) and the mostly-complete skull of the animal gives us a few clues as to how it might have lived.
From the evidence from the rocks in which J. monesi was found, it seems that this large rodent was living in an estuary or delta-type habitat with surrounding forest, its relatively small teeth suggesting that it was eating softer food than its relatives. Fruit and perhaps aquatic plants likely made a larger part of the diet of J. monesi than harder foods, although why J. monesi attained such a large size is more difficult to ascertain. Still, the authors note that aside from diprotodontian marsupials Rodentia displays the largest size range among mammalian orders, J. monesi pushing the upper limit.
[Hat-tip to John Wilkins]
Rinderknecht, A.; Blanco, R.E. (2008) “The largest fossil rodent.” Proceedings of the Royal Society B, Online Date: Tuesday, January 15, 2008