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'Dead bird’ posture of non-avialan dinosaurs



Ben Creisler
bcreisler@gmail.com

A new online paper:

A. P. Russell and A. D. Bentley (2015)
Opisthotonic head displacement in the domestic chicken and its bearing
on the ‘dead bird’ posture of non-avialan dinosaurs.
Journal of Zoology (advance online publication)
DOI: 10.1111/jzo.12287
http://onlinelibrary.wiley.com/doi/10.1111/jzo.12287/abstract


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.