The Evolutionary Anatomy, Biomechanics, and Diversification Dynamics of Repeatedly Evolved Masseter Muscles in Rodents

Explaining large scale transitions in discrete morphology is one of the most challenging problems in biology, requiring input from anatomy, biomechanics, ecology, paleontology, development, and genomics to appropriately contextualize evolutionary events. Rodents evolved discrete masseter muscle arrangements including novel muscle units that evolve homoplastically in clades of diverse ecological strategies and differing levels of species richness. These derived conditions, sciuromorphy, hystricomorphy, and myomorphy, differ from the primitive condition, protrogomorphy, in the anterior attachment of novel masseter muscles that have been thought represent specializations for gnawing or chewing mechanics. Here I test the hypotheses that these novel muscles function optimally at particular bite points and bias clade histories towards more gnawing or chewing intensive ecologies. I examine the functional ecomorphology of rodent dentition relative to masseter conditions, the biomechanics of individual muscles in different chewing positions, and the detailed anatomy of these muscles in key taxa with extreme cranial modifications, using a dataset comprised of over 200 rodent species. These studies reveal that sciuromorphy and hystricognathy are associated with gnawing and chewing specializations, respectively. In contrast, clades characterized by myomorphy demonstrate limited morphological and biomechanical evolution at lower evolutionary rates despite their high rates of taxonomic diversification and wide range of ecological habits compared to other groups. Strikingly, non-myomorph arid adapted rodents with large auditory bullae and reduced size of the temporalis muscle demonstrate major jaw muscle rearrangements, but arid-adapted myomorphs with temporalis reduction fail to demonstrate similar compensatory adaptations, implying that dual anterior attachments of derived masseter muscles are sufficient to conduct most major functional roles of the masticatory system. These data suggest that at least sciuromorphy and myomorphy conform to the typical hypotheses of muscle function and gnawing specialties, and are consistent with myomorphy serving as a key innovation in rodent dietary and functional evolution. These studies demonstrate that masseter conditions have profound impacted the functional and ecological evolution of rodents and baised clades toward differing histories that favor gnawing, chewing, or both strategies for ingestion. These studies demonstrate that masseter conditions have profoundly impacted the functional and ecological evolution of rodents and baised clades toward differing histories that favor gnawing, chewing, or both strategies for ingestion. Furthermore, this study has shown that detailed examination of masseter anatomy in rodents can still reveal previously undocumented morphologies, and that the functional flexibility of myomorphy significantly affects the dynamics of rodent macroevolution