Limb bone robusticity is coupled with mass distribution in terrestrial tetrapods

The vertebrate body is a highly modular system within which evolutionary adaptation is expected to occur synchronously at a variety of hierarchical scales, from single tissue to whole organism. For example, the evolution of different body shapes, associated with disparate locomotor ecologies, will affect the loading regimes experienced by limbs, and may therefore be coupled with adaptations to limb bone morphology. However, such a relationship between body shape, limb loading and bone morphology has not been tested. Here, we find significant positive relationships between whole-body relative anteroposterior centre of mass and the robusticity of the humeral shaft relative to the femoral shaft across a disparate sample of tetrapods. As centre of mass shifts towards the shoulder, the humerus becomes proportionally more robust. However, the magnitude of this increased robusticity and the anatomical planes across which it occurs vary between tetrapod clades, reflecting the different limb loading regimes imposed by postural differences. These relationships illuminate the osteological adaptations associated with variation in mass distribution and limb posture, and provide a framework within which centres of mass in fossil tetrapods such as dinosaurs can be predicted, opening the door to large-scale studies of tetrapod centre of mass and body plan macroevolution.