Assessing Head and spine Alignment in Chickens Using X-ray Imaging

The opisthotonic (‘dead bird’) posture encountered in non-avialan dinosaurs and other fossilized archosaurs is characterized by extreme dorsal neck retraction and tail protraction. Extant birds

have been used to study this and attention has focused on the neck (because modern avians lack a long, freely articulated series of caudal vertebrae). The opisthotonic posture has been largely discussed in relation to prevailing circumstances that may bring it about. Less attention has been paid to the actual patterns of excursion between vertebrae along the cervical series, and how this may influence the postures that are able to be adopted after death. We studied cervical intervertebral displacement throughout the full range of sagittal travel of the head, employing radiographic imagery. Our findings show that the head remains freely mobile on the cervical column in all positions of displacement, and the cervical vertebrae can be grouped into three functional clusters (posterior, intermediate and anterior), based on their patterns of intervertebral excursion along the sagittal displacement arc. The intervertebral joints at the junctions between these three regions, as well as the joint between the skull and the first cervical vertebra, exhibit the greatest ranges of excursion. Other joints in the cervical series exhibit relatively small levels of excursion. The mobility of the cervical joints in combination makes the neck prone to opisthotonic displacement, but there is no potential for an equivalent pattern of ventral movement and instead much of the cervical series becomes essentially locked when moved in this way. The S-shaped curvature of the neck in the resting position, with its characteristic intervertebral angles, is contributory to the ability to assume the opisthotonic posture. Circumstances that lead to the adoption of this posture are likely varied.