Re: Agonized death in dinos - my thoughts.

[Jura <pristichampsus@yahoo.com>]

> >>>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.

++++++++++++++++++++++++++++++++++++++++++

True enough (though the authours also mention
ankylosaurs, which would have been the most limited of
all dinosaurs, in terms of dorsoflexion).

There is a good reason to think that the dorsal
osteoderms in crocs would hamper dorsal movement. The
role of croc dorsal osteoderms (well, one of the
roles) is to support the epaxial muscles and allow the
vertebral column to arch high enough to keep the belly
off the ground. If these osteoderms are keeping the
vertebral column from bowing down, then one would
expect that they aren't going to allow for much dorsal
arching at all.

Unfortunately the only picture I can think of, that
illustrates my point, is from the Ross "Crocodiles and
Alligators" book (1989, pg 212). It shows a little kid
holding a baby croc by the snout and tail base. There
is very little bend in the back. I thought I had a
scan of that pic somewhere too. If I find it, I'll
upload it.

----------------------------------

 
> >>>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.  


++++++++++++++++++++++++++++++++++++

I'm not so sure about that. Faux mentioned the
following in the paper:

"In cases of pleurothotonus, as in opisthotonus, the
successive vertebral segments are flexed to more or
less the same degree, so the curvature of the spine is
regular, not discontinuous or kinked (Fig. 2A), and so
it also can be distinguished from the actions of water
currents, scavenging, or other disturbance."

Though the actual cause is up for debate (still seems
to be related to CNS problems), it can be
distinguished from post-mortem movement just as easily
as opisthotony. 

As for why one side out-contracts another side, I'm
not sure. It is possible that "sidedness" was present
in these species. It wouldn't be unheard of. Sidedness
is alleged to be present in pleurodire turtles (which
favor placing their heads along one side of their
shells, vs another). The result might just be a random
case of one side outcontracting another. 

Also, despite the authours statements that opisthotony
often results from the expaxial muscles outcontracting
the hypaxial muscles, they admit that this is not
always the case. Note the _Gorgosaurus_ in fig 1H. The
neck is arched dorsally, while the tail is arched
ventrally.

--------------------------------------

> Don't get me wrong, I agree that it should be
> studied more (I'm 
> interested in any references you have on induced
> pleurotony)

+++++++++++++++++++++++++++++++++++++

All I've found so far is Moodie's 1918 paper. If I
come across something else, I'll pass it on to you.

--------------------------------------

 
> >>>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).

+++++++++++++++++++++++++++++++++++++++++

It is, though. Look back at the paper again. They
compared early mammals, and eutherian mammals. The
results came up ambiguous in both cases. Some
eutherian mammals showed opisthotonic positions, while
many others did not.

------------------------------------------
 
> >>>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.

++++++++++++++++++++++++++++++++++++++++++++++++

Pterosaurs that showed the opisthotonic postion, were
preserved on their sides. The two questionable
examples that the authours used in the paper, had
pterosaurs preserved in a dorsoventral position. 

As for birds, I wish the authours included more in the
paper. The only example they illustrated was of
_Archaeopteryx_ (which was still laterally
compressed).   I would be interested in seeing how
many of the 13 bird species reported to have shown
opisthotony in the Messel Oil Shales, were preserved
on their sides, rather than on their stomachs/backs.

For the record, I'm not saying that I don't think
opisthotony can occur in dorsoventrally compressed
animals. I'm only saying that it is less likely to be
preserved in these animals. Especially if they have
splayed limbs, which makes it very hard to lie on
one's side.

So, yes, people do exhibit opisthotony (one need only
do a google image search to prove that one), but how
many people are preserved in the fossil record that
way?

Jason

"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer


      
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