Experimental modification of morphology reveals the effects of the zygosphene-zygantrum joint on the range of motion of snake vertebrae

Variation in joint shape and soft tissue can alter range of motion (ROM)
and create trade-offs between stability and flexibility. The shape of the
distinctive zygosphene–zygantrum joint of snake vertebrae has been
hypothesized to prevent axial torsion (twisting), but its function has
never been tested experimentally. We used experimental manipulation of
morphology to determine the role of the zygosphene–zygantrum articulation
by micro-computed tomography (μCT) scanning and 3D printing two mid-body
vertebrae with unaltered shape and with the zygosphene digitally removed
for four species of phylogenetically diverse snakes. We recorded the
angular ROM while manipulating the models in yaw (lateral bending), pitch
(dorsoventral bending) and roll (axial torsion). Removing the zygosphene
typically increased yaw and dorsal pitch ROM. In the normal vertebrae,
roll was <2.5 deg for all combinations of pitch and yaw. Roll
increased in altered vertebrae but only for combinations of high yaw and
ventral pitch that were near or beyond the limits of normal vertebra ROM.
In the prairie rattlesnake and brown tree snake, roll in the altered
vertebrae was always limited by bony processes other than the zygosphene,
whereas in the altered vertebrae of the corn snake and boa constrictor,
roll ROM was unconstrained when the pre- and post-zygapophyses no longer
overlapped. The zygosphene acts as a bony limit for yaw and dorsal pitch,
indirectly preventing roll by precluding most pitch and yaw combinations
where roll could occur and potentially allowing greater forces to be
applied across the vertebral column than would be possible with only
soft-tissue constraints.