Meeting Abstract
Rodents are the most species-rich order within Mammalia and they have evolved a wide variety of morphologies to accommodate numerous locomotor niches, ranging from gliding squirrels to fossorial pocket gophers. They provide an excellent opportunity to understand how locomotor innovation can drive speciation. To evaluate the connection between the evolutionary success of rodents and the diversity of rodent locomotor ecologies, we used humerus and femur CT scans from 76 species across Myodonta and the Heteromyidae to examine internal and external limb shape. Internal morphology was quantified using cross-sectional geometric parameters and external morphology was quantified using 768 pseudo-landmarks applied to bone surfaces. Based on these data, only fossorial rodents displayed a major reworking of their proximal limb elements in both external and internal morphology with other locomotor modes plotting closely with terrestrial rodents. Fossorial rodents were also the only locomotor mode to consistently show increased rates of morphological evolution in both internal and external shape. There was no correlation between internal and external shape of the proximal limb elements, suggesting that internal and external morphology are decoupled. These results together suggest that only extreme locomotor shifts (i.e., fossoriality) require a substantial reorganization of proximal limb elements away from the terrestrial rodent bauplan. Additionally, there appears to be a decoupling of rates of speciation and rates of proximal limb morphological evolution. We propose that rodents have been so successful at evolving into new niches and colonizing new areas due to high locomotor plasticity, which allowed them to modify their locomotor mode without requiring major bauplan modifications.
