An Approach to the Biomechanics of the Masticatory Apparatus of Early Miocene (Santacrucian Age) South American Ungulates (Astrapotheria, Litopterna, and Notoungulata): Moment Arm Estimation Based on 3D Landmarks

Abstract

Notoungulates, litopterns, and astrapotheres are among the most representative mammals of the early Miocene Santacrucian Age. They comprise a diversity of biological types and sizes, from small forms, comparable to rodents, to giants with no analogues in modern faunas. Traditionally, all of them have been considered herbivores; this diversity is reflected in different morphologies of the masticatory apparatus, suggesting a variety of feeding habits. The application of biomechanics to the study of fossil mammals is a good approach to test functional hypotheses. Jaws act as a lever system, with the pivot at the temporomandibular joint, with masticatory muscles providing the input force, whereas the output force is produced by the teeth on food. The moment arms of the lines of action of the muscles can be estimated to analyze relationships between bite force and bite velocity. A morphogeometric approach inspired by Vizcaino et al. (1998) is applied to estimate muscle moment arms in a static 3D bite model based on three-dimensional landmarks and semilandmarks on crania with mandibles in occlusion. This new 3D geometric method to evaluate jaw mechanics demonstrated its reliability when applied to a control sample of extant mammals that included carnivores, herbivores, and omnivores. Our results indicate that, except for Pachyrukhos, in no Santacrucian ungulate does the masseter muscle have greater mechanical advantage than the temporalis. Among them, notoungulates have a better configuration to develop force on the molar tooth row than litopterns. This indicates a diet richer in tough plant materials for Santacrucian notoungulates (e.g., grass or even bark) than for litopterns (e.g., dicots). This is consistent with recent ecomorphological approaches applied to this fauna. Finally, the approach proposed here proves to be useful for comparing masticatory performance and it is a powerful tool to validate ecomorphological dietary hypotheses in fossil taxa.