It's a common feature of archaeopteryx specimens, and shows up in the skeletons of dinosaurs with long necks (and IIRC I think also in cadavers of horses/giraffes) - a backward recurvature of the neck, known as "opisthotonus" (Gk "tightening behind").
I had always assumed it was because these creatures while alive have powerful gravity-opposing muscles in their neck that then undergo contraction during rigor mortis. Not quite. Apparently it's the ligaments that are responsible, as a New York Times article indicates:
Photo: Mike BeauregardMr. Reisdorf thought water might be the key. So he and a colleague, Dr. Michael Wuttke, decided to try some kitchen science. They bought fresh chicken necks from a butcher and plunged them into water buckets. Immediately, the necks bent backward by 90 degrees. After three months and significant rotting, they had twisted further backward — to 140 degrees...To test whether muscle contractions caused the spasms, both groups of researchers removed the skin and muscle from the birds, and got the same result. Only by cutting the ligaments between vertebrae could they prevent the necks from bending backward in water.