A phylogenomic rodent tree reveals the repeated evolution of masseter architectures

Understanding the number of times a trait has evolved is a necessary foundation for comprehending its potential relationships with selective regimes, developmental constraints, and evolutionary diversification. Rodents make up over 40% extant of mammalian species and their ecological and evolutionary success has been partially attributed to the increase in biting efficiency that resulted from a forward shift of one or two portions of the masseter muscle from the zygomatic arch onto the rostrum. This forward shift has occurred in three discrete ways, but the number of times it has occurred has never been explicitly quantified. We estimated an ultrametric phylogeny, the first to include all rodent families, using thousands of ultraconserved elements. We examined support for evolutionary relationships among the five rodent suborders and then incorporated relevant fossils, fit models of character evolution, and used stochastic character mapping to determine that a portion of the masseter mu...