Opisthotonus or a typical “death pose” of many dinosaurs and early birds fossils
A lot of dinosaurs and early birds fossils were found preserving in an awkward “death pose”. The head and neck are thrown back over the body, sometimes almost touching the spine. This pose is known as opisthotonus. And since 1920s there were three theories to explain the nature of this phenomenon.
Opisthotonus or opisthotonos, from Greek roots, ὄπισθεν, opisthen meaning “behind” and τόνος tonos meaning “tension”, is a state of severe hyperextension and spasticity in which an individual’s head, neck and spinal column enter into a complete “bridging” or “arching” position. Source: Wikipedia.
There are two schools of thought on the subject. Some researchers have proposed that the contortion — technically called the opisthotonic posture — is caused at the time of death by poisoning, lack of oxygen to the brain, or similar circumstances that cause neck and tail to spasm into weird angles. Other paleontologists have suggested that the pose happens after death, with immersion in water or decay tensing muscles and ligaments that pull the head back and the tail up. Source: http://phenomena.nationalgeographic.com
Another theory was proposed in 2012 by paleontologist Michael Wuttke and Achim Reisdorf, a doctoral student at the University of Basel.
Reisdorf and Wuttke suggested that strong neck ligaments that were “preloaded” were essential for all long-necked dinosaurs with a long tail, to provide the appropriate balance. However, following death and immercion in water, the stored energy was powerful enough to arch the spine back, a phenomenon that increased with decay… Archaeopteryx and many of the other feathered dinosaurs are animals with long bony tails — Sinosauropteryx has the largest number of tail vertebra of any dinosaur. Perhaps the death pose is a consequence of the neck ligaments necessary to counterbalance a long tail. Source: John Pickrell. Flying Dinosaurs.
In 2015 this phenomenon was also tested by biologists Anthony Russell and A.D. Bentley in Opisthotonic head displacement in the domestic chikcen and its bearing on the ‘dead bird’ posture of non-avialan dinosaurs. Russell and Bently did series of X-rays of ten thawed, plucked chickens.
Russell and Bentley did place the birds after thrawing in different “death poses” slowly moving head and neck towards the spine. X-rays showed how neck vertebrae angles were changing with each position.
It actually didn’t take all that much for the birds to get to the dinosaur death pose. The posture, Russell and Bentley write, “can, in chickens at least, be facilitated simply through the limpness associated with death combined with the imposition of a relatively modest displacing force.” Getting the neck to arc downwards was something different altogether. The chickens’ necks locked when they were angled down and required significant force to keep them that way. The natural thing for a dinosaur neck to do, then, is to arc backwards.
The greatest changes happened in the middle of the neck. While the base and the very front of the chicken necks didn’t move much, Russell and Bentley found that two neck joints in the middle changed their orientations significantly and contributed the most to the pose. The flexibility of the skull helped, too. The spot where skull met neck stayed flexible in every position, and this undoubtedly helped some dinosaur skeletons achieve the posture where snout touches hip. This might also explain why many fossil dinosaur skeletons are found decapitated. Perhaps the anatomy that gives the skull a wide range of motion also allows it to easily be lost as soft tissues decay, letting heads roll as the rest of the skeleton is pulled towards becoming an osteological circle.
So while there’s probably an array of immediate causes for the dinosaur death pose, the ability for the saurians to take up the posture at all is because of flexible necks that can more easily be retracted back than pressed downwards. That’s the past of least resistance, literally, at or after the time of death, and why today’s dead chickens and emus look like they’re doing impressions of their fossilized predecessors. Source: http://phenomena.nationalgeographic.com/2015/12/30/flexible-necks-made-the-classic-dinosaur-death-pose/
