BODY MASS ESTIMATION OF RODENTS: WAS THERE A RODENT AS BIG AS A BUFFALO?

Body mass estimation is an important aspect of reconstructing the paleoecological role of fossil animals. A variety of skeletal proxies have been used by paleontologists for reconstructing body mass in fossil mammals. The most commonly used proxy is M1 area, but postcranial dimensions, such as the length or diameter of limb bones, are also used. Body mass estimation can be particularly problematic in cases which extrapolate beyond the range of masses of extant relatives. Exceptionally large fossil rodents, in particular the giant beaver Castoroides and the large caviamorph Phoberomys, have been the subject of recent studies refining estimates of their size, using primarily skeletal proxies; Phoberomys has been suggested to have weighed 741 kg, the size of a water buffalo, while estimates of the mass of Castoroides were recently revised downward from “Volkswagen-sized” to only about 60-100 kg.

I use toothrow dimensions to generate estimates for rodent body mass, in the hopes that the constancy of energetic scaling across orders of magnitude in body size will yield a robust relationship. I use the dimensions of the entire toothrow, rather than M1 alone, as that value is generally problematic in rodent taxa in which the M1 is either enlarged (as in muroids) or reduced and lost (as in the extinct mylagaulids). Two alternative proxies, toothrow length and toothrow area, are shown to be very tightly correlated to body mass, and hence good predictors of body mass among species of rodents. Regression within families and subfamilies of the Rodentia provides an even tighter fit. Applying this proxy to the toothrows of several fossil rodents provides an estimate of their masses. Comparison to body sizes of extinct rodents previously estimated from postcrania finds that body masses for Castoroides and Palaeocastor estimated from femur length are consistent with those from dental proxies, but toothrow-based estimates for Phoberomys, the largest known fossil rodent, are discordant with the mass estimated from limb bone diameter. The estimate from toothrow length is only 170-330 kg, about the size of a pygmy hippo. This difference may be explained by both allometric scaling of limbs and locomotor differences between Phoberomys and the caviamorph rodents on which the limb bone diameter regression was based.