Abstract
Several extinct chinchilloid rodents in the clades Dinomyidae and Neoepiblemidae grew to sizes much larger than any living rodent species. However, the exact size of these rodents is a matter of controversy, with authors disagreeing due to issues over extrapolation and model selection. Prior estimates for the two largest extinct rodents, Phoberomys pattersoni and Josephoartigasia monesi, range from 230 to 700 kg for P. pattersoni and 350 to 2600 kg for J. monesi. Here, I estimate body mass in large, extinct rodents using occipital condyle width (OCW), a strong predictor of body size in mammals, using a dataset that circumvents many of the issues faced by previous studies of species. Body masses under shape-corrected OCW are much lower than previous studies: 108-200 kg for P. pattersoni and 480 kg for J. monesi. Mass estimates for other rodent taxa (Neoepiblema, Telicomys, Dinomys) agree with previous studies. Estimates using skull length, corrected condyle width and head-body length are similar, suggesting estimates of 150 kg for Phoberomys and 480 kg for Josephoartigasia, and that larger estimates of 700 and 1200 kg are unlikely. High estimates in previous studies appear to be due to the unrecognized, nonlinear relationship between certain skeletal measurements (skull size) and body mass.