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Sauropods vs. Gravity



Sauropods vs. Gravity

Hello all! I have a certain topic that I would like to address. One thing I 
have been interested in for a while is the problem large terrestrial animals 
have in our present gravity on Earth. There seems to be a certain restriction 
on size these days, and I wonder why. This apparently affects flying animals 
too in their relation to, well, flying, and getting off the ground. I wonder 
where along the timeline the Earth's gravitational pull weakened, if it infact 
did that. Let me explain.

I have been doing some research concerning the body mass of sauropods and what 
it would suppossedly take for them to simply exist in our habitat today. I have 
to say the formulas I have come across are a little strange and overwhelming. 
But basically this is what I have read. 

I took this paragraph from Knut Nielson's "Scaling, Why is Animal Size So 
Important" (1984):

"It appears that the maximum force or stress that can be exerted by any muscle 
is inherent in the structure of the muscle filaments. The maximum force is 
roughly 4 to 4kgf/cm2 cross section of muscle (300 - 400 kN/m2). This force is 
body-size independent and is the same for mouse and elephant muscle. The reason 
for this uniformity is that the dimensions of the thick and thin muscle 
filaments, and also the number of cross-bridges between them are the same. In 
fact the structure of mouse muscle and elephant muscle is so similar that a 
microscopist would have difficulty identifying them except for a larger number 
of microchondria in the smaller animal. This uniformity in maximum force holds 
not only for higher vertebrates, but for many other organisms, including at 
least some, but not all invertebrates."

If you compare sauropod muscle filaments to that of a human's, you would find 
(according to the above reading) that they are pretty much the same. But as an 
animal gets larger, the weight goes up in proportion to the cube of the 
increase in dimension, since weight is proportionate to volume, of course. My 
question is how in the world would a huge sauropod even be able to breath, let 
alone function in a world with our gravity? 

Take the issue of their necks. Sauropods like the giant brachiosaurs (i.e. 
ultrasaurus) and seismosaurus had necks 40 to 60 feet long weighing 25,000 to 
40,000 lbs. According to torque physics, that means that if they held their 
necks out horizontally (like Mark Hallet's rendering), 400,000 to a million 
foot pounds of torque would have to be applied just to hold up the neck. One 
might say that the sauropod's suspension-like skeleton worked in a way that 
would support this. But hanging a 30,000 lbs neck 40 feet into open space seems 
a bit stretching it (pardon the pun). If you look at an elephant skeleton, you 
will see the spine is built a bit like a roman arch, with the legs acting like 
columns to support the weight. Fine. But a seismosaur neck weighed may times 
that of an entire elephant. Wouldn't it actually arch DOWNWARD if held out 
horizontally?

According to Peter Dodson (Lifestyles of the Huge and Famous), "...the vetebral 
spines at the base of the sauropod neck were weak and low and did not provide 
leverage for the muscles required to pump blood to the brain, thirty or more 
feet in the air, would have placed extraordinary demands on the heart and would 
have seemingly placed the animal at severe risk of a stroke, an aneurysm, or 
some other circulatory disaster." 

Well, what about brachiosaurs? Didn't they hold their necks at a more upright 
angle, above the 20 foot mark? How could they function at all? In our gravity 
system, it doesn't seem to work. Anyone have some ideas?