Meeting Abstract
The axial skeleton serves as the chassis for both ventilation and locomotion in tetrapods. Questions remain, however, about the role of the axial skeleton in generating locomotor work. In lepidosaurs, lateral bending of the axial skeleton may serve to increase stride length by placing each limb further forward than it would land without bending. The axial skeleton may also directly generate locomotor work if the lateral bending pushes the animal’s center of mass forward relative to limb support. The relative motions of the vertebral column and ribs during locomotion, which could help to examine these roles, have not yet been measured in a tetrapod during locomotion. One null hypothesis for these movements could be that each vertebra rotates relative to adjacent vertebrae around a dorsoventral axis, with the ribs moving relative to each other but not moving relative to the vertebrae at the costovertebral joints. To test this null model we measured the relative motions of vertebrae, ribs, and the sternum during treadmill locomotion in the savannah monitor using marker-based and markerless X-ray reconstruction of moving morphology (XROMM). During a typical stride, as predicted, the vertebrae rotated towards each other around a dorsoventral axis, producing concave spinal curvature ipsilateral to front limb support. In contrast to the null model, however, costovertebral joint rotations included positive caliper motion, with the ribs lifted dorsally towards the vertebral column, and positive and pump handle rotation, such that the ipsilateral ribs elevated and folded caudally during the propulsive phase of the ipsilateral front limb. Continued study of axial kinematics during locomotion are critical to understanding the evolutionary pressures on the design of the vertebrate chassis.
