A. P. Russell & A. D. Bentley
The research undertaken in this study, published in Journal of Zoology, seeks to examine the intervertebral movement that occurs in dead chickens when the head is drawn back into the “dead bird” (opisthotonic) posture seen in many dinosaur fossils. This posture is highly evocative because it suggests that the animals suffered death throes, thus potentially having implications for how the dinosaur died.
Our work was initiated by my co-author, Adam Bentley, who, at the time, was an undergraduate student seeking a suitable final year research project. He was fascinated by a recent publication that attempted to evaluate all of the theories that had been advanced to explain this phenomenon in dinosaurs, and to arrive at the most plausible of these. That publication favoured the clinical condition of opisthotonus, resulting from neuromuscular trauma at the time of death. In discussing with Adam the various publications that had considered the opisthotonic posture (and its causes) in dinosaurs, we realized that nobody had systematically documented the vertebral movements associated with this. Given that birds are living dinosaurs, and that chickens are readily available in grocery stores (Asian ones being particularly good sources of chickens with the head and neck still attached), we set out to design a project that would yield statistically robust data and provide us with solid anatomical information.
The key to being able to conduct the project successfully was the presence in my lab of a state-of-the-art digital radiography system that allowed us to pose and image the chicken carcasses in repeatable positions that mimicked the head retraction process. We began by determining the carriage of the head “at rest” (when the chicken is standing), and then established five equally-spaced stages of displacement to the fully retracted position with the top of the head resting on the hip girdle. We were able to document 15 chickens in this way (ten of which constituted our test group), and we recorded the angles of 11 neck joints for all of the stations along the cervical retraction pathway. We then repeated these procedures for protraction of the neck (a movement we dubbed protonic displacement), to determine the relative mobility of the joints when the head was forced forward and downward. The patterns here turned out to be quite different from those seen in neck retraction. Overall, for our sample of 10 chickens we were able to record displacement data for 11 joints in 14 positions, yielding 1540 angles for analysis.
This work was time consuming and challenging, but the outcome was very revealing. We found that certain neck joints are highly flexible whereas others are highly restricted in their movements. Although our research cannot answer questions as to why, in any given instance, a particular dinosaur fossil may have assumed the opisthotonic posture, our findings provide us with information about how to explore the anatomy of the neck vertebrae of dinosaurs (and other fossils) to see if such displacement was physically possible (because it has been reported that it is confined to certain groups of amniote vertebrates). It also raises questions about scale. Chickens are relatively small, and their head and neck relatively easily displaced by modest external forces. At larger sizes (as is typical of dinosaur fossils and even large birds, such as ostriches) the head and neck are bulkier and offer greater resistance to displacement (especially in terrestrial as opposed to aquatic situations). Thus, circumstantial evidence relating to the conditions of preservation and burial of each specimen remains important for determining why that particular animal expresses the opisthotonic posture. The form of the neck facilitates such displacement, but whether perimortem physiological symptoms, or postmortem decay phenomena, were responsible must be determined by examining “the scene of the crime”.