It is very common to see whole dinosaur skeletons with their heads thrown back. This is referred to as the Opisthotonic pose.
How does this happen?
So the concensus is that damage to the central nervous system lead to this position, and that the bones did not sit out for long after this happened.
Faux and Padian argue in Paleobiology that the dinosaurs died in this posture as a result of damage to the central nervous system. In fact, the posture is well known to neurologists as opisthotonus and is due to damage to the brain’s cerebellum. In humans and animals, cerebellar damage can result from suffocation, meningitis, tetanus or poisoning, and typically accompanies a long, slow death.
Some animals found in this posture may have suffocated in an ash fall during a volcanic eruption, consistent with the fact that many fossils are found in ash deposits, Faux and Padian said. But many other possibilities exist, including disease, brain trauma, severe bleeding, thiamine deficiency or poisoning.
Padian pointed out, too, that all opisthotonic dinosaurs are very well preserved, meaning they evidently did not sit out in the open for long, or scavengers would have quickly scattered the bones.
Strangely enough, scientists experimented with plucked chickens to test a hypothesis of theirs:
So without central nervous system damage (the chickens were already dead) they can also make this pose when submerged in water. The above article went on to say that it happened instantly, and there was not much difference when the chickens were left submerged for months.
Cutler placed plucked chickens – both fresh and frozen – on a bed of sand for three months to see if desiccation would lead to muscle contractions that pulled the neck upwards – a previously suggested explanation for the death pose. The chickens decayed without contorting. When seven other chickens were placed into cool, fresh water, however, their necks arched and their heads were thrown back within seconds.
But wait, theres MORE!
When it comes to explaning why so many full skeletons of dinosaurs and early mammals in this pose are so prevailant, evidence is strongly in favor of a worldwide flood.
"Virtually all articulated specimens of Archaeopteryx are in this posture, exhibiting a classic pose of head thrown back, jaws open, back and tail reflexed backward and limbs contracted," said Kevin Padian, professor of integrative biology and curator in the Museum of Paleontology at the University of California, Berkeley. He and Cynthia Marshall Faux of the Museum of the Rockies published their findings in the March issue of the quarterly journal Paleobiology, which appeared this week.
Dinosaurs and their relatives, ranging from the flying pterosaurs to Tyrannosaurus rex, as well as many early mammals, have been found exhibiting this posture. The explanation usually given by paleontologists is that the dinosaurs died in water and the currents drifted the bones into that position, or that rigor mortis or drying muscles, tendons and ligaments contorted the limbs.