RE: (sigh)sauropod necks again--long! -Reply

[Toby White <augwhite@neosoft.com>]

I suppose this thread is long dead, but I had a couple of brief reactions:

1) Can we really assume that sauropods stood up on a routine basis?  I'm not 
arguing that they didn't.  I'm just not comfortable in assuming they did.  With 
anything that big and graviportal, you have to wonder whether it simply stayed 
on four legs all the time, or at least other than in exceptional circumstances.

2) I've been wondering whether the problem might be solved by an extension of 
the croc circulatory design.  The croc design approaches a 3-compartment model, 
rather than the usual body-lung 2-compartment architecture.  As I understand 
it, the croc circulatory system effectively treats the head and 
posterior+viscera as two separate compartments.  If the sauropod case is not 
too different, the sauropod might even be able to maintain higher pressure in 
an anterior+cranial compartment without placing undue strain on the rest of the 
system.

  --Toby White

On Tuesday, July 13, 1999 8:41 PM, tons [SMTP:tons@logicsouth.com] wrote:
> I won't comment on this part of the thread, but I would like to add some
> comments on some of the physiology of defying gravity.  I haven't been
> keeping up much lately so if I go over old ground please forgive me.
>
> First, I'm assuming part of the thread is that sauropods are deprived of
> blood and therefore O2 when the head is raised substanially over the heart.
>
> I will make an assumption that sauropods had enough blood and therefore O2
> delivered to the brain when arising.  If they were able to "stand up" then
> we can probably assume there was a neurocardioregulatory mechanism of some
> sort.  Getting up would place a much greater demand on this system than
> raising the head because the volume of blood pooling would be far greater
> with both the body and the head/neck involved than the neck itself.
>
> I do not know the neurocardioregulatory mechanism in reptiles and suspect
> that it is not as sophisticated in today's reptiles as it was in some
> dinosaurs because no modern reptile stands up very high.  I'm just guessing
> this though.
>
> In erect mammals there is a very sophisticated mechanism for maintaining
> blood to the brain on arising.  To simplify, there are baroreptors in the
> neck that sense diminished flow/volume and send messages to the heart to
> speed up its rate and to beat harder therefore ejecting more blood per
> stroke.  This increase in stroke volume (SV) and heart rate (HR) increases
> blood pressure (BP) and cardiac output (CO).  The autonomic nervous system
> also cause vasoconstriction to prevent pooling of blood and decreased return
> to the heart as much as possible.  In some animals the system does not work
> properly.  If the HR, SV, and CO do not match needs the animal feels bad on
> arising or actually passes out.  Sometimes the system overreacts and CO goes
> to high.  Neuroreceptors and cardiac receptors decide there is too much CO
> and it drops.  Sometimes it drops too far and syncope occurs.  Lower and
> higher centers in the brain can also alter CO.
>
> Since sauropods were so successful for so long, we have to assume they could
> arise from the ground, drink, and probably raise there heads to feed.  I'm
> certain there was some sort of neurocardioregulatory mechanism as well as
> anatomical considerations to allow them to do that.
>