Why are dinosaur fossils' heads turned up and back?

OK, it's been about 20 years since I was last in vet school and I have fogotten most of the stuff I learned there. But I remember a few things.

I clearly remember the Pathology class (and especially the lab!) and the Five Signs (or stages) of Death: pallor mortis (paleness), algor mortis (cooling), rigor mortis (stiffening), livor mortis (blood settling/red patches) and decomposition (rotting). The linked Wikipedia articles are pitifully anthropocentric, though, and there is much more cool stuff to learn when comparing various animals.

The most interesting of the five signs of death is Rigor Mortis. If you go back to the very basic physiology of muscle contraction, you may remember that ATP is needed for the cross-bridges to be released (i.e., to separate actin from myosin). After death, ATP breaks down and the muscles remain stiff for a period of time until decay and decomposition start breaking down muscle proteins. Exactly when rigor mortis sets in, and when the muscles start softening up again depends on a number of factors, including species, body size, proportional muscle mass, physical condition, physical activity prior to the time of death, age, cause of death, environmental temperature and humidity.

I also remember the word Opisthotonus, a backward arching of the head and neck caused by injury of the cerebellum, meningitis, and some types of poisoning (e.g., strychnine). Opisthotonus also occurs after death as a result of rigor mortis.

Back in vet school, all I was interested in was equine medicine (so I studied other species only as much as needed to pass the class), so I spent some time studying that all-important Ligamentum nuchae in the horse. If you ride and train horses, that is one of the most important pieces of equine anatomy, the biggest and strongest ligament (actually a fused composite of hyndreds of smaller ligaments) in the horse's body, connecting the poll (top of the head, a ridge on the occipital bone), the top-line of the neck, withers, back, loins, rump and dock (the base of the tail).

I thought back then, that the contraction of the nuchal ligament was the cause of the occurence of opisthotonus after death. The ligament is so large and powerful, no groups of muscles are supposed to be able to counteract this movement. Particularly in later stages after death, as the muscles start decomposing, nothing would stop the ligament to pull the head and neck up.

Apparently, I was wrong:

Smith (1921) mentioned the function of the funicular ligamentum nuchae. He believed it assisted the muscles in keeping the head extended as, for example, when grazing. He also said that shortening of the ligament was responsible for the dorsiflexion (opisthotonus) of the head/neck after death. This is not the case since severing the ligament does not release such dorsiflexion; rigor mortis of the dorsal cervical muscles causes opisthotonus after death.

Now, Grrrl and Laelaps point to and discuss at length a new paper by a veterinarian, Cynthia Marshall Faux, and a famous dinosaur paleontologist Kevin Padian, who argue that the opishtotonus seen in many dinosaur fossils is not a result of rigor mortis, but a result of pre-death brain injury or poisoning. Contrary to the quote above, they did not observe opisthotonus in dead horses.

Apparently, Kevin Padian promised to come by Grrrl's blog and answer questions in the near future. I'll let you know when this happens. I am intrigued. Not persuaded yet, but open to changing my mind if their evidence is persuasive. Perhaps opisthotonus has different causes in different fossils, depending on the species and the individual case: some got poisoned or brain-injred, while others curved due to rigor mortis. After all, an Archaeopterix is not exactly built like a horse. What do you think?

Update: Kevin Padian responds.