Modelling marsupial mastication: The biomechanical bite model of the Linnaeus's mouse opossum <i>Marmosa murina</i> (Marsupialia, Didelphidae)

Marsupials have evolved alongside other mammals on many continents, mainly in the southern hemisphere, developing their own traits and adaptations. Although the relationships between morphology, bite force, and diet have been well studied in many vertebrate groups, this has rarely been the case for marsupials until recently. Present-day American marsupials' diet and their feeding capacities, considered generalists, remain poorly understood. A better understanding of current American marsupials will lead to more accurate inference models for extinct metatherians. Here, we study and describe for the first time the masticatory apparatus of the Linnaeus' mouse opossum Marmosa murina, along with its performance. Bite forces data were collected for different marsupial species during a field mission in French Guiana in 2017. A 3D bite reconstruction model has been established through dissections and using the lever arm method, based on the static equilibrium of the muscular vectors in the jaw. The optimal gape angle and the contribution of each masticatory muscle to the closing of the mouth were determined. We identify and individualized the different fascicles of the masseter, zygomaticomandibular, temporal, and pterygoid muscles, together with their respective origin and insertion areas. The optimal gape is around 6°, supporting the use of the last molar to get the strongest bite forces. The M. masseter superficialis, the M. temporalis superficialis, and the M. temporalis profundus medialis are the muscles having the greatest impact on the maximum bite force. Our biomechanical model allows a correct approximation of the biting force. However, the muscle stress value has to be increased from 30 N.cm^-2 to 44.360 N.cm^-2 and 54.209 N.cm^-2 to match the in vivo bite forces on the last molar (m4) for Marmosa murina. These high values are rather surprising, suggesting that our model, with the use of standardized constants for all mammals, underestimates true bite forces.