tags: researchblogging.org, archaeopteryx, dinosaurs, mammals, fossils
Archaeopteryx fossil showing the distinctive head-back death pose of many articulated fossilized birds, dinosaurs and early mammals. Archaeopteryx is an ancient feathered dinosaur. This specimen is at the Humboldt Museum, Berlin. The skull is approximately two inches long.
If you've looked at the articulated 150-million-year-old Archaeopteryx fossils, you probably have noticed that they all have a weirdly similar pose; their heads are thrown over their backs, mouths open and tail curved upwards. Scientists have been puzzled for years by what caused this distinctive pose, but now two paleontologists propose an explanation: this characteristic posture was the result of agonized death throes triggered by brain damage and suffocation.
This position is not unusual for intact fossils; many dinosaurs and their relatives, as well as many early mammals, exhibit this classic head-back pose. The usual explanation provided by paleontologists is either that the animals died in water and the currents drifted the bones into that position, or that rigor mortis or drying muscles, tendons and ligaments contorted the limbs.
"I'm reading this in the literature and thinking, "This doesn't make any sense to me as a veterinarian,'" said Cynthia Marshall Faux (pronounced "fox"), lead author on a paper recently published in the quarterly journal Paleobiology. Faux is a veterinarian-turned-paleontologist who also is a curatorial affiliate with Yale University's Peabody Museum.
"Paleontologists aren't around sick and dying animals the way a veterinarian is, where you see this posture all the time in disease processes, in strychnine cases, in animals hit by a car or in some sort of extremis."
Instead, Faux and her co-author, Kevin Padian who is professor of integrative biology and curator in the Museum of Paleontology at the University of California, Berkeley, argue that dinosaurs and birds died with this posture because of damage to their central nervous systems.
This is because the head-back death pose is well known as opisthotonus and is the result of damage to the cerebellum. In humans and animals, cerebellar damage can result from suffocation, meningitis, tetanus or poisoning, and often accompanies a long, slow death. Basically, the cerebellum is a brain region that controls fine muscle movement, which includes the body's antigravity muscles that keep the head upright. When the cerebellum ceases to function, the antigravity muscles pull at full force, tipping the head and tail back, contracting the limbs and opening the mouth.
It is likely that at least some animals fossilized in this posture suffocated in an ash fall during a volcanic eruption -- consistent with the fact that many fossils are found in ash deposits, Faux and Padian noted. But other sources of brain damage also exist, including disease, brain trauma, severe bleeding, thiamine deficiency or poisoning.
Tested this hypothesis, Faux watched the progression of body movements made by dead animals.
"In horses and smaller animals, rigor mortis sets in within a couple of hours, so I just looked to see if they were moving or not," Faux noted. "And they weren't moving. They were staying in whatever position I'd left them in. I thought, 'If birds aren't doing it, and I'd never observed a horse doing it, then why would dinosaurs be doing it?'"
To further test whether muscles or tendons contracted asymmetrically and give rise to this head-back position, Faux conducted an experiment with two euthanized red-tailed hawks, which she dried for two months in styrofoam peanuts. She found that most joints have counterbalancing muscles that dry in the opposite direction, so there was no reason to expect that muscles could pull at a joint asymmetrically during drying. She also tested tendon drying by pinning out beef tendons as they dried. She found that even though they shrank somewhat, they did not shrink enough to dislodge the pins.
Further, all opisthotonic dinosaurs are very well preserved, so their bodies did not sit out in the open for long, exposed to scavengers that would have quickly scattered their bones. In that case, Padian wondered, how could they have been exposed long enough to dry out?
An ostrich-like dinosaur, Struthiomimus; in the classic posture indicative of brain damage and asphyxiation at death. Drawn from a specimen at American Museum of Natural History. The skull is approximately a foot long.
The authors also mentioned a particular fossil Allosaurus that suffered bone lesions suggestive of a bacterial infection that also can cause meningitis, a disease that can produce opithotonus. The authors point out that their explanation of the opisthotonic posture in dinosaurs and other animals provides a way to assess the role played by microbes in evolution, whether through disease or through other processes such as algal blooms - so-called "red tides" - that can suffocate aquatic animals.
These observations "suggest that reevaluation may be in order for an untold number of paleoenvironments whose story has been at least partly explained on the basis of the death positions of many of their fossil vertebrates," the authors concluded in their Paleobiology paper.
NOTE: If you have questions or comments, feel free to leave them here because one of the authors will be stopping by in the next day or two to respond to you.
Sources
Faux, C.M., Padian, K. (2007). The opisthotonic posture of vertebrate skeletons: postmortem contraction or death throes?. Paleobiology, 33(2), 201-226.
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That is a really cool article. But JTOR does not have articles from Paleobiology after 2002, I think. I got mine from BioOne.
Correction: JSTOR.
The skull of Archaeopteryx is about 2 inches long, not 6.
Interesting. I wonder if carbon monoxide poisoning, IE from volcanic or other tectonic-related processes, might also contribute to this?
That is a really cool piece of science, thanks for the post!
Following from Hermagoras, the abstract of the paper is here.
Bob
is it possible they were doing that thing michael hutchinson did?
Anyone know what proportion of dino-skeletons are contorted in this way?
Volcanoes have been known to release clouds of hydrogen sulfide. One good whiff of that and the death throes would begin.
I suspect Faux is not the first person to have recognized opisthotonus in fossils and helpfully pointed out what it was, but this may be the first time the phenomenon is being widely disseminated (thanks to this blog) and thus may end up in a textbook where it can inform students.
Hermagoras: i am pleased to say that kevin padian sent me the PDF yesterday, and i can share it of you are interested. further, i believe that kevin will be popping in to answer questions and to respond to comments from my readers.
Lee: YIKES! thanks, i should have known that, having seen the fossil myself. thanks for the gentle reminder.
Lofi: i am confused as to what you are asking. please elaborate.
I'd like a PDF as well, if it's OK with you...
I am looking forward to Kevin's appearance here and the discussion. I have always thought that the cause is the contraction of ligamentum nuchae which, at that point, has no muscles (or at least not in rigor mortis) to oppose the movement. I am looking forward to the explanation to why that is not the case.
The UCB press release says "Also, because the posture has been seen only in dinosaurs, pterosaurs and mammals, which are known or suspected to have had high metabolic rates, it appears to be a good indicator that the animal was warm blooded. Animals with lower metabolic rates, such as crocodiles and lizards, use less oxygen and so might have been less traumatically affected by hypoxia during death throes, Padian said."
That opisthotonus posture is impossible in reptiles, amphibians, and fish because their spinal flexure is lateral, not dorso-ventral as it is in dinosaurs, pterosaurs, mammals, and birds.
California and Rough-skinned Newts do adopt a head-back defensive posture, so dorsoventral neck flexure is not automatically ruled out in cold-blooded vertebrates.
Most of Audubon's illustrations show the exact same pose. His models were all specimens sent from the frontier by naturalists. I read a claim in one Audubon book that this is caused by slow post-rigor-mortis tightening of the muscles along the back of the neck which are huge because they hold up the neck.
No idea if this is true.
While I think the authors did a good job refuting the mainstream hypothesis, I don't believe their alternative is any better. I particularly have problems with them using archaeopteryx as an example. All the archaeopteryx fossils have been recovered from the Solnhofen limestone. That's a marine sediment, representing a shallow, fine-grained muddy sea bottom. The imprints show some evidence of decomposition prior to becoming embedded in the mud. This would mean they had floated out to sea for some time, going through the bloat process before sinking and beginning the fossilization adventure.
In other words, these skeletons can NOT be considered to be in their death posture. Whether the muscle shrinkage hypothesis is correct or not, the "death throes" argument is even less correct.
Not all dinosaur skeletons are found in sea bottom sediments, of course, but ALL of those skeletons that are well preserved involve preservation by water-lain sediment. If it wasn't a sea bottom, it was a river bed, or a mud pit. To expect the posture of the skeleton to reflect the posture of the animal immediately after death is unreasonable. Fossilization requires rapid burial, and rapid burial is a high-energy process. There's not much chance that the context of the death is going to be well preserved.
(While some fossilized creatures were indeed entombed by ash, the examples that the authors specifically refer to were not, which kind of negates that argument as a general idea.)
Note 2: The example of the Allosaurus with meningitis is used here in a misleading context. While the bone lesions may be properly interpreted, it should not be used as an example juxtaposed onto a discussion of skeletal postures, since there's no indication that the allosaur bone in question was actually found in that same posture. I think that might be an improper conflation of two facts that aren't necessarily related to each other.
Kevin Padian says;
Lofi's comment was in reference to the accidental death of Michael Hutchence, lead singer of INXS, as the result of a bit of autoerotic asphyxiation gone wrong (at least, that's how it was reported in the media).
(why yes, I do have a head full of completely useless random trivia, why do you ask?) ;)
Haven't you heard? The dinosaurs all died that way because they didn't get on Noah's Ark and they died craning their necks back as the floodwaters rose, trying to keep their heads above water for as long as possible.
True story!