Opisthotonus in dinosaurs

The other day I posted about a new paper (after spotting it on Grrlscientist’s blog, with another excellent follow-up by Bora) that aimed to describe why so many dinosaurs died with an arched back, appearing as if they wanted to touch their nose to their tail. This particular phenomenon is known as Opisthotonus, and is primarily discussed in medical circles as it is indicative of brain damage and often associated with disease (i.e. meningitis), and it can be observed in life or after death in many different organisms (from birds to cats to people). Could the fact that so many dinosaurs exhibit this pose show that they too died agonizing deaths as a result of disease or asphyxiation? The authors of the new article, Kevin Padian and Cynthia Marshall Faux, think so, but like so many ideas in paleontology we need to go backward in order to go forward.

From Moodie, 1918. A Plesiosaurus macrocephalus exhibited the arched-neck death posture. A cast of this fossil sits on my back door.

Dinosaurs were not the only fossil vertebrates to appear to have died agonizing deaths; plesiosaurs, crocodilians, mammals, and other groups all seemed to show the same pose in deposits where preservation was ideal (or at least allowed for articulation), and in 1918 Roy L. Moodie caught on. In a paper entitled “Studies in Paleopathology. III. Opisthotonus and Allied Phenomena Among Fossil Vertebrates,” Moodie describes an articulated Struthiomimus as follows;

The attitude is typically opisthotonos, the jaws exhibiting trismus, with the head thrown sharply back over the sacrum, the tail thrown sharply up; the toes strongly contracted, with the phalanges closely appressed. The whole attitude of the body strongly suggests some severe spastic distress. The animal may have been a plant feeder and its death and spastic distress due to feeding on some poisonous plant, which as to-day causes tetanic spasms in animals. It may have suffered death from a severe cerebrospinal infection. But whatever the cause of its death, the attitude of the animal strongly suggests the effect of disease, and in
discussing the history of disease among animals, the opistliotoilic position exhibited by fossil skeletons must be considered as indicating a possible diseased condition.

From Moodie, 1918. This specimen is still on display at the American Museum of Natural History in New York City.

To Moodie, the postures of the various vertebrates pointed to disease and brain damage prior to (if not the direct cause of) death, the many openings of archosaur skulls (in his view) allowing plenty of bacteria to get in and subsequently kill the animal. He is tentative in his explanation, however, saying that not all of these fossils directly point to disease, but a good number of them seem reminiscent of what has been recorded in medical studies up to that time. Indeed, while I have yet to read to new paper I imagine that it probably builds upon what Moodie interpreted at the beginning of the 20th century, but why haven’t we heard more about this hypothesis before? Paleopathology is giving us same interesting insights into animals that did appear to suffer from disease (see this 2002 paper by Hanna about the pathology of “Big Al,” a subadult Allosaurus fragilis that seemed to have lots of problems with infection), but does the presence of disease automatically explain the death poses we see in well-preserved dinosaurs?

Most everyone is familiar with the term term “rigor mortis,” even if they don’t fully understand what it means. Although usually associated with the limbs, rigor mortis is an effect seen after the death of an organism where a chemical change in the muscles causes them to become stiff or difficult to move. This usually happens within 72 hours of death (as far as we’ve observed, at least), when ATP is used up contracting the muscles in the body. The trouble is, however, that by time the muscles have stopped fully contracted most of the ATP has been used up, and the muscles do not have a source of ATP needed to relax them, “fixing” the position of the animal. It is at this point that we would do well to remember that dinosaurs (and many of the animals in which the head-tail bending is seen) would have had many muscles along the neck, back, and tail for support of flexibility, and if those powerful muscles all contracted (and couldn’t stop contracting) the neck would bend backwards and the tail forwards.

What I personally find more credible about the “rigor mortis hypothesis” is the surprisingly similar (even near-identifical) positions of many fossil dinosaurs. While I disagree with Alan Feduccia about the true identity of Sinosauropteryx’s feathers, his recent paper is worth another look for another reason; a side by side comparison of two Sinosauropteryx specimens. Both of these specimens exhibit opisthotonus, but if they had both died agonizing deaths due to suffocation, poisoning, or brain damage, I would have expected more differences in their final position. Maybe it’s just coincidence, but I find it more likely that their fossil postures (along with the postures of other well-preserved vertebrates) has more to do with rigor mortis than painful death throes. Once again, it could be argued that both likely died as a result of volcanic activity like ash falls (hence the wonderful preservation), but I think what we’re seeing is rigor mortis shaping these fossils rather than the dinosaurs just happening to fall down and be buried in nearly exactly the same pose.

Small, bird-like dinosaurs are not the only ones to be subject to such death poses; the massive Seismosaurus also appears to have arched its back in death. Size alone doesn’t exempt an animal from death by disease or suffocation, but the fact that so many different taxa all seem to undergo the same phenomenon points to some shared anatomy rather than the chance of death by disease; the muscles along the top of the neck, back, and tail were stronger than those along the underside, and so when rigor mortis set in following death, heads arched back and tails moved upwards/forwards. Could these animals have died from poisons or bacterial infections? I’m sure some of them dead, and perhaps some of them do indeed exhibit opisthotonus because of those causes of death, but there are far too many specimens and far too many kinds of specimens showing the same pattern for me to say that they all died from the same cause, regardless of depositional environment. As I said in my last paper, current taphonomy studies (especially involving large, flightless birds) may be a big help in helping us understand this problem; a dinosaur was not a horse, nor was it a bird of prey (two animals cited mentioned in the popular press articles about the new study), and we should not say that merely because modern animals exhibit this pose as a result of painful death that it must have been the case for dinosaurs, too. Connections should be drawn and modern analogs observed, surely, but we’d be loathe to forget that dinosaurs were very different from anything alive today.

In the end, I feel that the rigor mortis hypothesis better explains the preponderance of fossils with arched necks and tails that we find, but we would be foolish not to check these animals for signs of disease. Bacteria were active then just as now, and looking at skeletal clues to infectious disease (especially if related to migrations and continent shifts) could tell us much about Mesozoic life and (perhaps) extinction. Still, it perhaps is no coincidence that many of the arched-spine specimens come from deposits with exceptional preservation; catastrophic events like floods, ash falls, etc. killed many animals, but likely kept scavengers away or covered the bodies relatively quickly after death, allowing us to see what various animals really looked like (even if they are a little bent out of shape).

Still, writing this post showed me that I need to brush up on my musculo-skeletal anatomy and taphonomy a bit; it was extremely difficult finding resources for this one, and many the taphonomy papers I found relating to large flightless birds primarily dealt with partial skeletal remains rather than what happens to a bird from death onwards. As I often have to admit, there is simply too much to know, and so I will ever be a student of nature.