[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Rearing up on hind legs (was Re: Parrish's neck work ...)




On Monday, May 03, 1999 9:14 PM, Stanley Friesen [SMTP:sarima@ix.netcom.com] 
wrote:
> At 05:16 PM 5/3/99 -0400, Danvarner@aol.com wrote:
> >  Don't forget that whales (and mosasaurs!) are bouyant and nearly 
weightless
> >
> >in their native environment, although pressure gradients must be taken into
> >consideration. Sauropods were the heaviest animals ever, bar none.
>
> Except *possibly for the very largest sauropods, none match the mass of the
> blue whale, not even for those few species that are as *long* as the blue
> whale.

Slightly testy response:

I've been waiting for Dr. Teuton or someone else who knows whereof they speak 
to say something, but we seem to have lost everyone with a medical or 
physiology background.  Hence these admittedly ill-informed remarks.

The problem is not mass and is not gravity per se.  The problem is the blood 
pressure necessary to keep a reasonably constant flow of oxygen to the brain in 
particular and to the cranium in general.  The pressure the heart needs to 
generate to achieve this feat is a function of (a) the difference in height 
between the head and heart and (b) frictional factors which are largely 
determined by (i) the distance between heart and head (regardless of 
orientation) and (ii) the diameter of the vessels through which the blood 
travels.

With these factors in mind, you will note that whales have nothing to do with, 
and nothing resembling, the sauropod problem.  By comparison with a sauropod, a 
whale is, for all practical purposes, spherical.  That is, there is normally 
little difference in the elevation of heart and head, the distances involved 
are significantly smaller in relation to mass and, because the body is compact, 
there is little constraint on the diameter of the vessels in between.

Not only is the sauropod neck potentially at a very different elevation from 
the heart, it supports a head at the very end of a thin, extended, and rather 
crowded neck structure.  Thus the blood pressure problem is acute.  Worse, the 
sauropod had, perhaps, a very large vertical range of motion.  Thus, the 
problem is not only blood pressure, but radical *changes* in blood pressure to 
avoid either passing out from lack of oxygen or suffering an aneurysm from too 
much pressure.

Obviously these problems got solved somehow -- maybe in part by restricting the 
height to which the head could be raised.  The point is that this not at all a 
trivial problem and may have significant consequences for our concept of 
sauropod behavior and physiology.

Finally, there has been some casual mention of accessory blood pumps.  Is 
anyone aware of any vertebrate that has such organs?  Hagfish (the sister clade 
to Vertebrata) have them, but nothing more closely related that I've heard of. 
 Off-hand, I'd think they'd raise hell with any number of vital functions, but 
maybe not.

  --Toby White (itinerant curmudgeon)