Characteristic 'Death Poses' in Theropods May Have Been the Result of a Cerebral Disorder Producing Muscle Spasms After Death

Opisthotonic postures, more colloquially known as "death poses" are a common occurrence in theropod and bird fossils. They are characterized by, in life, what would be a certainly impossible feat: the neck arching backward at an incredibly sharp angle, to the point of where the head is even touching the back; the tail arching forward in many ways the neck does, even to when the tail and the head are close to touching; and the hindlimbs appear rigid and stiff.

An explanation for such phenomena has not yet reached the surface, however, paleontologists do know that these are muscle spasms creating the "death pose", not the geological/geographical conditions of its site or the way it was buried or fossilized. Conversely, the exact cause of the spasms are unknown. Or are they? A recent paper in the journal Palaeobiodiversity and Palaeoenvironments may have found the answer. Mental disorders having to do with the cerebral hemisphere of the brain may have caused the spasms postmortem,* according to the article, which was written by paleontologists Achim G. Reisdorf and Michael Wuttke.

To test this theory, they analyzed two basal coelurosaurs, Compsognathus longipes (below) and Juravenator starki, of which are both known for their "death poses". By analyzing the biomechanical constraints of the two dinosaurs plus experiments concerning decomposition tested using deceased domestic fowls, it has been found that muscle spasms are postmortem phenomena.

* The cerebrum has many functions, of which one is the complete control of your physical body, including via muscles.

Compsognathus longipes in the opisthotonic
posture, also known as the "death pose".
Image taken by Wikipedia user Ballista, here.

Abstract - More or less complete and articulated skeletons of fossil air-breathing vertebrates with a long neck and tail often exhibit a body posture in which the head and neck are recurved over the back of the animal. Additionally, the tail is typically drawn over the body, while the limbs have a rigid appearance. In palaeontological literature, this “opisthotonic posture” of such fossils still requires a causal interpretation in an etiological context. According to this hypothesis, there is a presumption of a cerebral disorder generating perimortem muscle spasms that are preserved by rapid burial or other sequestration of a skeleton in the fossil record. We re-evaluate this “opisthotonic posture hypothesis” by analysing the non-avian theropods Compsognathus longipes and Juravenator starki from the famous South Franconian plattenkalks of the Upper Jurassic Solnhofen Archipelago. Decay experiments with the extant domestic fowl Gallus gallus L. and analysis of the theropods’ constructional morphological constraints reveal that the opisthotonic posture is not a peri- but a postmortem phenomenon. By analysing the timeline of decomposition, it is possible to recognise different stages of decay, depending on the varying decay resistance of soft tissues. Adipocere formation must have blocked further decay until embedding was completed by minimal sedimentation. Analyses of the palaeoenvironment of the basins of the Solnhofen Archipelago show that the conditions of deposition of individual basins cannot be considered to be similar, even inside the same time frame. Therefore, a generalised approach of looking at the depositional setting must be excluded. Assumptions by Faux and Padian (2007) that the accepted palaeoenvironmental reconstruction of the Solnhofen Fossillagerstätte has to be questioned in the light of the opisthotonic posture hypothesis enforce the need for a review of palaeoecological factors of the Franconian Plattenkalks from a taphonomic perspective.

Reference - Reisdorf, A.G., Wuttke, M. (2012). "Re-evaluating Moodie’s Opisthotonic-Posture Hypothesis in Fossil Vertebrates Part I: Reptiles—the taphonomy of the bipedal dinosaurs Compsognathus longipes and Juravenator starki from the Solnhofen Archipelago (Jurassic, Germany)". Palaeobiodiversity and Palaeoenvironments, in press. doi: 10.1007/s12549-011-0068-y.