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Re: Sauropods vs. Gravity
"There seems to be a certain restriction on size these days, and I wonder why."
I suspect that if there were large continents with plenty of the same type of food, uninterupted for millions of years (by ice ages or inland seas and mountains) , we would have larger animals now (if man did not kill them).
"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? "
Breathing should not be much of a problem as there would be little air pressure differential, even if the head was held high. Moving ribs might be difficult though. A bigger question is - How did they overcome the large air space between the lungs and head? Probably with air sacs and a flow through lung, but possibly with other adaptations such as an alternate used air return route such as through the neck air sacs.
Remember that when scaling, joints still have to transfer loads and rotate, resulting in more of a linear contact, not an area contact. To get around this, joints on larger animals should have porous bone endings filled with flexible, load spreading pads along with wide, large diameter joints with reduced rotation. In addition, and perhaps more importantly, when you scale the area of a bone to match the weight, you do not scale the ability of the bone to resist bending. To do this you would need the same amount of bone in a large circle around a hollow opening. I suspect that large diameter, relatively thin wall bones would break easier on impacts with branches and such, as well as be a poor anchor points for the muscles. As such boned are often solid and an animal 2 times the height, is usually more than 8 times the weight.
Also, while the mass of the animal is cubed, the internal air volume is cubed also. Getting porportional oxygen would not be a problem. Furthermore, a larger leg would also not be able to rotate at the same frequency as a shorter one without a relatively greater energy expenditure. Hence, do not expect the larger animal to run 2 times as fast for the same energy expenditure.
"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..."
Most of the mass was near the body. Expect that the neck made maximum use of elastic materials and air spaces to reduce the weight.
"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.""
Do not forget that they probably did not need the same amount of oxygen for their brains that we do. There are possible alternatives to a massive heart (besides just the head held at heart level). An oxygen exchange tissue in the head on neck air sacs could provide the oxygen needed (but not other things that the blood brings).
"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? "
One possibility - When the head is down, blood flows and collects where needed most, near the head on the artery side of the capillaries. When the head raises, blood quits flowing in and tension cuts off most of the return vein blood flow. The blood, slowly draining from the head through the capillaries provides the nutrients and oxygen. Supplimental oxygen is also absorbed in the veins via neck and head air sacs. When the blood is gone, the neck lowers for a refill. Note, there might also be some sort of syphon effect as the blood returns. How, with flexible blood vessels, I do not know.