Meeting Abstract
Mastication in rodents is highly derived, being composed of quasi-independent gnawing and chewing systems and a complicated array of adductor muscles. The masseters are arranged in four qualitatively different configurations, with two anterior masseter units evolving rostral attachments multiple times independently, and these define the three derived forms sciuromorphy, hystricomorphy, and myomorphy. Since the Eocene, masseter configurations have remained qualitatively static despite the massive diversifications and ecomorphological disparification. We sampled 3D linear measures of muscles attachments, lever arms, and dental measures from 140 species across all major rodent groups to analyze functional differentiation and versatility within and across qualitatively different muscle configurations. We found a trade-off between incisor gnawing proficiency and the occlusal area of the cheekteeth, suggesting few rodents are well specialized for both gnawing and chewing simultaneously. Additionally, our data demonstrate different roles for the temporalis in chewing and gnawing depending upon masseter configuration. The temporalis is correlated with cheekteeth size in sciuromorphs and myomorphs, but negatively correlated in hystricomorphs. Additionally, the temporalis is strongly positively correlated with incisor depth across both sciuromorphs and hystricomorphs, but shows no relationship whatsoever with incisor depth in myomorphs, which show reduced incisor disparity despite high muscular and ecological diversity. Our results indicate previously unknown functional trade-offs between gnawing and chewing adaptations across rodents, and imply that the qualitatively different muscle systems perform in fundamentally different ways, even for muscles like the temporalis, that lack qualitative differentiation in morphology.
