An extreme, dorsally hyperextended posture of the spine (opisthotonus), characterized by the skull and neck recurved over the back, and with strong extension of the tail, is observed in many well-preserved, articulated amniote skeletons (birds and other dinosaurs, pterosaurs, and at least placental mammals). Postmortem water transport may explain some cases of spinal curvature in fossil tetrapods, but we show how these can be distinguished from causes of the opisthotonic posture, which is a biotic syndrome. Traditional biotic explanations nearly all involve postmortem causes, and have included rigor mortis, desiccation, and contraction of tendons and ligaments. However, examination of the process of rigor mortis and experimental observations of drying and salinity in carcasses of extant animals show that these explanations of the “dead bird” (opisthotonic) posture account for few or no cases. Differential contraction of cervical ligaments after death also does not produce the opisthotonic posture. It is not postmortem contraction but perimortem muscle spasms resulting from various afflictions of the central nervous system that cause these extreme postures. That is, the opisthotonic posture is the result of “death throes,” not postmortem processes, and individuals so afflicted assumed the posture before death, not afterward. The clinical literature has long recognized that such afflicted individuals perish from asphyxiation, lack of nourishment or essential nutrients, environmental toxins, or viral infections, among other causes. Accepting the actual causes of the opisthotonic posture as perimortem and not postmortem provides insights into the causes of death of fossilized specimens, and also revises interpretations of paleoenvironmental conditions of many fossil deposits. The opisthotonic posture tells us more about the circumstances surrounding death than about what happened after death. Finally, the opisthotonic posture appears to have a phylogenetic signal: it is so far reported entirely in ornithodiran archosaurs (dinosaurs and pterosaurs) and in crown-group placentals, though the distribution in mammals may expand with further study. It seems important that the opisthotonic posture has been observed extensively only in clades of animals that are known or thought to have high basal metabolic rates: hypoxia and related diseases would be most likely to affect animals with high oxygen use rates.