SRA

Understanding the number of times a trait has evolved is a necessary foundation for comprehending the selective regimes, developmental constraints, and evolutionary successes of clades. Rodents make up over 40% of mammalian species and their success has been partially attributed to the increase in biting efficiency which results 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 tested. We estimated an ultrametric phylogeny using ultraconserved elements, the first to include all rodent families, and examined support for evolutionary relationships among the five rodent suborders. We then incorporated relevant fossils, fit models of character evolution, and used stochastic character mapping to determine that a portion of the masseter muscle has moved forward onto the rostrum at least seven times, with one reversal during the ca. 70 MY history of rodents. The repeated evolution of this key innovation, its increasing prevalence through time, and the species diversity of clades with this character underscore the adaptive value of improved biting efficiency.