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Re: Agonized death in dinos - my thoughts.

Which brings me to my real bugaboo with this paper. Under
discussion there is a section that comments on
an apparent phylogenetic signal related to opisthotony. First off, this has little to do with a phylogenetic signal. <<<

It's both. It is empirically a phylogenetic signal (dinosaurs, pterosaurs, mammals), and via inference a thermophysiology signal (more below).

The argument (as Scott had mentioned in a previous post) is that the
CNS in
creatures with high basal metabolisms, will be more susceptible to the effects of hypoxia, and result in opisthotonic postures.<<<

Sort of: The arguement is that cerebellar shut-down prior to death allows muscle contraction to occur uninhibitted, causing the opisthotonic posture. How quickly the cerebellum shuts down during CNS disfunction (hypoxia, poisoning, etc) would delimit the amount of time that uncontrolled muscles spasms occur (or whether they happen at all). Since ectotherms utilize far less oxygen per unit time, their cerebellums (cerebelli?) are not going to be effected as quickly by reduced oxygen flow caused by CNS disfunction. There can be little question here; organisms with lower oxygen consumption per unit time are going to have tissues (and therefore organs) that are less suceptible to reduced oxygen availability than organisms with high oxygen consumption rates. Unless you challenge the entire causal mechanism postulated for opisthotony, it's hard to see how this is much of a leap. And again, there is strong empirical support based on the distribution of opisthotony in fossil organisms (see below) .

The authours mention that there are crocodyliformes that show
opisthotonic positions. They discounted 3 out of 4 mentioned by Lortet (1892), because the back and tail showed little arching. Did they not consider the possible anatomical
limitation imposed by the dorsal osteoderms? This was taken into account when attempting to explain why opisthotony is so rarely seen among ornithischians.<<<

Is there any reason to think dermal armor would effect crocodilian dorsoflexion? At a totally annecdotal level, crocs picked up by television "naturalists" don't seem to have any problems with their tails curving dorsally when picked up by them (although they don't seem pleased, either...). They authors reasonable cite ossified tendons in derived ornithiscians as one limiting cause (although primitive ornithiscians that lack ossified tendons do exhibit opisthotony), and limit the roll of ornithiscian armor to stegosaurs, whose plates surely would have done more to hamper movement that crocodillian armor.

Pleurothotony is the exact same thing as opisthotony, only the neck
and tail arch to the side, instead of
over the back.<<<

But are they the same? The consistent opisthotonic pose results from stronger dorsal musculature "out contracting" ventral musculature; what explanation is there for lateral motion? Opisthotonic death poses are the specific result of tissue contraction, but pleurotonic postures can just as easily result from water motion or other physical disturbance. Don't get me wrong, I agree that it should be studied more (I'm interested in any references you have on induced pleurotony), but pleurotony has a whole range of potential causes that don't requre (althogh could include) spasmatic muscle contraction, while opisthotony apparently has a consistent cause, so is not as succeptible to confounding environmental signals.

The argument that opisthotony only occurs in automatic endotherms,
hits another snag when they tested it in
mammals. Many of their cases were ambiguous. There were clear cut examples of placental mammals showing
this trait, but basal mammals were harder to come by.<<<

It isn't ambiguous if it just means that basal mammals consumed less oxygen. Even amongst extant mammals non-placentals have lower oxygen consumption rates (on average) than placentals (on average).

Finally, the biggest problem I noted was that there was never a
mention of the different body design of
mammals and dinosaurs, compared to most reptiles.Mammals and dinosaurs are laterally compressed animals. When they die they fall to their sides. Most reptiles are dorsoventrally compressed. When they die, they die on their stomachs.<<<

Except pterosaurs are dorsal-ventrally compressed, yet still demonstrate opisthotonic posture. Extant birds cannot really be described as "laterally compressed", yet show opisthotony. And notably, people are extremely dorsoventrally compressed, yet apparently also exhibit opisthotony.

If this were presented as an acid-test for thermal physiological signal in the face of counter evidence I would agree with Jason's skpeticism, but the empirical distribution is consistent with other lines of evidence, and is in fact seen across groups with different body shapes. Additional research is (always!) a good idea, but the conclusions seem in line with their data, as well as independent lines of data about dinosaur and pterosaur physiology, and I feel the paper is better for having included it.

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333


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