Fig. 3.

ACS and external marker locations. We animated the body plane from at least five beads attached to the outside of the body (see Fig. 2) and parented the motion of the ACS to the body plane, with the blue Z-axis pointing dorsally, the green Y-axis pointing laterally to the left, and the red X-axis pointing rostrally. Depression of the urohyal and cleithrum was measured as negative translation of their associated markers along the Z-axis, and retraction was measured as negative translation along the X-axis. SH muscle strain was measured as the change in distance between the urohyal marker and the marker at the caudal end of SH muscle (near the tip of postcleithrum), and epaxial strain from the most caudal neurocranium marker to the most cranial epaxial marker. Mesh models of the urohyal, cleithrum, and postcleithrum are shown for reference. These bones were not animated; their motions were measured from translations of their attached external markers.