Opisthotonus in dinosaurs

11 06 2007

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.

Plesiosaurus macrocephalus
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.

Struthiomimus

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.

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9 responses

12 06 2007
utenzi

I like your using Rigor mortis as an explanaion for the fossil “poses”, Brian. Of course since Rigor only lasts about 24 hours there still needs to be an explanation of why the dying animal remained in that pose. Some external factor would have to be involved. Both volcanic ash deposits and quick sedimentation would certainly suffice in that regard. Of course both those explanations also open up the possibility of suffocation…

21 11 2010
yahday

rigor mortis is a post-mortem phenomenon that prevents cross bridges between actin and myosin from rupturing, resulting in stiffness (inability of the muscle to relax). This essentially ‘fixes’ a carcass in the position it is already in, rather than contracting the muscles. Muscle contraction is not possible in this situation, as there is a lack of ATP that would be necessary for such contraction (lack of ATP to release the crossbridges causes rigor in the first place). why is it then so prevalent for paleontologists to state that ‘contraction of muscles during rigor mortis’ causes opisthotonic posture?

12 06 2007
laelaps

Thanks for the input! As for rigor mortis, I’m not so sure that the dinosaur poses would return to “normal” after 24 hours or so; most references I’ve been able to find deal with rigor in humans and mammals, focusing on rigor in the limbs. As I explained in the post, rigor is caused by muscle contractions fueled by ATP, and muscle relaxation also requires ATP (but there isn’t much, if any, left because it was used up in the contractions) so I think that dinosaurs could have very well stayed in an arch-backed pose well after the muscles stopped actively contracting.

While some of the arched-backed fossils we find are likely to be the result of brain damage/poison/suffocation, there are other (notably larger) fossils may not have died from suffocation but still show the pose (as in the Seismosaurus writings I cited, as well as larger theropods like Tyrannosaurus and Allosaurus). Like I mentioned in the post above, I also find it curious that fossils from similar environments show almost identical death poses, which to me suggests a taphonomic factor rather than an agonizing death (I would expect to see more differences between remains).

Again, I think the ideas of Padian and Faux merit further discussion and interest, but I still think the primary factor for arched-back poses in fossil vertebrates (especially archosaurs) is rigor mortis. Hopefully I’ll get to actually see the new paper soon, but until then I’m going to keep looking up information on rigor mortis to see exactly what muscles are involved and how it differs across vertebrate groups.

13 06 2007
Utenzi

I’m looking forward to hearing what you think about the paper once you get a chance to read it, Brian. I had taken their claims at face value and your objections are quite interesting. Rigor “relaxes” in mammals due to pH changes–I have no idea if that happens in other types of existing animals and can’t even imagine what would happen biochemically after death in dinosaurs.

13 06 2007
laelaps

Thanks for the info on pH; I’ve been trying to find resources about what happens to crocodiles, birds, and lizards after death, but it is difficult. What I think might be a key factor here is the construction of the neck, spine, and tail in dinosaurs, especially in terms of muscle mass and attachment. I would imagine that dinosaurs would have more powerful muscles along their vertebrae just by virtue of the weight they’d be carrying along their spine, which could explain the especially strong contractions.

This is a great opportunity to learn more about pathology and taphonomy, however, and hopefully I’ll be able to write more on this topic in the near future.
PS> If anyone has a copy of the paper, I would appreciate it if you could forward that to me at evogeekAT gmail DOT com. Otherwise I have to wait until my university subscription catches up to issue the paper is in.

29 01 2008
Ken Turnbull

I am a very amateur paleontologist and have raised ostriches since 1992. Used to have 125 birds. 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.

The 10- percent that do not assume this position are from quick deaths such as heart attack, injury, head caught in fence, or bullets. In these cases the body is on either side with limbs and neck where ever they happen to fall. (But never in the opisthotonos position!)

After death rigor mortis sets in within 12 to 24 hours (this depends on ambient temperature – they freeze in just a few hours). There is no further tightening/curling due to “drying” of ligaments after death, before or after rigor mortis.

This is so common and easily observed, I am puzzled why “academics” don’t just and watch the process. I have contacted several museums over the years and have always been ignored (no credentials, closed-club?). Cynthia’s paper is a welcome relief for me to see the facts finally be reported to academia.

I have photos if you send me an email address to send them too. If you want to observe ostriches I live just south of Denver.

I hope these comments are helpful.

ken@kenturnbull.com

14 10 2010
Why are dinosaur fossils’ heads turned up and back? (repost) | A Blog Around The Clock

[...] in the near future. I’ll let you know when this happens. Update: Kevin Padian responds and Brian has an update – see the comment by the ostrich breeder there as well stating that these birds assume the [...]

2 04 2012
dan miller

Speaking as a veterinarian, it isn’t at all uncommon to see animals dying to assume an opisthotonus position at the moment of death.

3 06 2013
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