Masticatory Biomechanics of the Largest Fossil Rodent

Abstract

The largest known fossil rodent is Josephoartigasia monesi, from the Pliocene of Uruguay, with an estimated body mass of around 1000 kg. Previous work has revealed that the incisors would be able to withstand much greater forces than are estimated to have been produced by the masticatory muscles. Here, finite element analysis was used predict stress distributions across the cranium and maximum bite force at the incisors and the cheek teeth. A sensitivity study was conducted in which the muscle forces and orientations were sequentially altered, to account for the uncertainty in the estimated muscle forces. J. monesi was predicted to have a bite force of 1389 N at the incisors, rising to 4165 N at the third molar. The sensitivity analyses showed that varying muscle force magnitudes by 20% and muscle orientations by 10° led to a maximum change in bite force estimate of 30-33% at all teeth. Predicted stress patterns during biting were only minimally affected by changes to muscle forces and orientations, but revealed a considerable safety factor in cranial strength. These results, combined with previous work, suggest that J. monesi may have been using its incisors activities other than feeding, such as digging or defence, much as an elephant uses its tusks.