Opisthotonos and the allied phenomena, pleurothotonos and emprosthotonos, are quite frequently seen among fossil vertebrates. It has been suggested elsewhere that these attitudes represent possible cerebrospinal infections or other neurotoxic conditions, and they must be considered in connection with the study of disease among fossil animals. The skeleton of the small dinosaur, Struthiomimus altus, described by Osborn, shows a very well-developed condition of opisthotonos, with the head thrown sharply back, the tail strongly flexed, and the toes contracted and appressed. The whole attitude strongly suggests a spastic distress, possibly brought on by some form of poisoning of the central nervous system, from infection or the deglutition of some poisonous substance.The fossil Moodie referred to, a nearly complete specimen from Alberta described by Henry Fairfield Osborn of the American Museum of Natural History.
Moodie's idea, while not embraced by many scientists, was still around in 1968, when W.H. McMenemy referred to "the wretched Struthiomimus altus that perished in opisthotonos in the humid epoch of the giant saurians" in his review of the book Diseases in Antiquity. The more widely accepted explanation for what caused the skeleton of this dinosaur to constrict like this this was taphonomy, not pathology: it was believed that rigor mortis caused the muscles of the neck and back to tighten, pulling head and tail together.
Then, in 2007, paleontologists Cynthia Marshall Faux and Kevin Padian published a paper supporting Moodie's view. After studying rigor mortis in extant creatures, including horses and red-tailed hawks, they saw no evidence that it would have caused the opisthotonic position in so many dinosaurs. Instead, they proposed that the posture might occur at the time of death, the result of poisoning, perhaps by volcanic gas released at the time of an eruption. Then, a fast burial preserved the animals' last moments of agony. If you want to kill a bunch of things and bury them quickly, volcanoes are a great way to make it happen.
There hasn't been much done on this since the paper was published three years ago. A page at the university of Bristol refers to it in an overview of the exquisitely preserved Jehol Biota, bringing up the good point that Faux and Padian's explanation requires rapid burial, and that isn't necessarily the case with some dinosaurs found in the opisthotonic position -- notably, the famed Archaeopteryx specimens of Solnhofen which lived near a shallow lagoon in Germany and evidently did not receive a quick post mortem interment.
I'm also intrigued by what Kenneth Turnbull, a self-described amateur paleontologist who also has owned an ostrich ranch, had to say on the subject. He had an interesting comment on the expansive Laelaps post on this topic. He describes a phenomenon that is neither due to a problem with the nervous system or rigor mortis.
When ostriches die, the opisthotonos position is “normal” in about 90+ percent of natural deaths – disease, illness, impaction, starvation, etc. It is not an “agonizing death”, it is a very peaceful death. The birds first go into their normal “sleeping” position; and as they get closer to death the neck goes into the extreme “curled” position while they are still alive. They drop into unconsciousness many hours (12 to 24?) before death; but retaining this position...Considering how similar in form Struthiomimus was to ostriches, this seems like an interesting place to look - they're separated by a wide gulf of time and came upon their similar body plans independently, but it's not totally implausible that this could effect both of them. If it hasn't happened yet, hopefully someone will take him up on his offer to provide access to ostriches for this purpose.
Can't get enough paleopathology? Read about an interesting study from last year that linked lesions found on a number of Tyrannosaurus rex mandibles to a disease similar to Trichimonas, which infects modern birds, primarily raptors.