Re: Sauropods vs. Gravity

["Matthew Bonnan" <mbonnan@hotmail.com>]

Paul:

You asked:
"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."

This idea (gravity fluctuating) has been seriously proposed by a few 
researchers, and is mentioned in a book called "Newton Rules Biology" by 
Pennecuik (sp.?).  However, if earth's gravity did fluctuate, that would 
mean that it would have been physically smaller in the past and has become 
larger and more massive now.  Before the acceptance of plate tectonics as a 
mechanism for how continents may have changed position, there was a 
hypothesis which treated the plates as static pieces that cracked apart as 
the earth became larger in mass.  Tying these two ideas together, an 
argument was that during the Mesozoic, the mass of the earth was smaller 
(hence Pangea) and therefore gravity was lower.  At present, plate tectonics 
explains plate movement much better because of its strong support by 
geological and fossil data.  So far as we know, the earth's mass has not 
changed considerably over the past 4 billion years, so it loooks as if 
gravity would not have fluctuated much if at all.

"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.""

Exactly -- and if you've been following the posts and news on the T. rex 
"was not a runner" paper, this is, in part, why Hutchinson and Garcia feel 
that T. rex could not "run."  In order for T. rex to run, it would need to 
have extremely large muscles, something like 80% of its body weight, to 
exert the appropriate forces necessary for running.  As you can tell, this 
would not be a realistic expectation.

Sauropods, like other large so-called "graviportal" mammals, have skeletons 
that allow them to get around some of these "drawbacks" of skeletal muscle 
power.  For instance, they have columnar limbs that support the body weight 
more efficiently than if the limbs were angled or bent, and this allows 
living graviportal mammals and supposedly sauropods as well to use less 
muscle power and force to stand or move.  In their forelimb, sauropods have 
reduced the olecranon process on their ulna, which is the insertion point 
for the triceps muscle that extends the forearm.  This tells us that 
sauropods were not flexing their forearms as much as smaller animals, and 
thus they required less of a triceps muscle to extend the forearm.  Granted, 
I have dissected an elephant forelimb and have seen their triceps up close 
and personal, and it is huge by our standards, but it is not as large or 
powerful in a relative sense compared with our triceps or those of other, 
smaller mammals.  Sauropods have special processes on their dorsal vertebrae 
that locked their vertebral column into a relatively immobile strut, 
decreasing the need for large muscles here as well.  Overall, the skeletal 
plan of sauropods and other large terrestrial vertebrates is a design that 
allows them to make slow and deliberate motions without needing an 
extraordinary amount of muscle.  This is the "solution" to the problem -- of 
course, the devil is in the details, and this is a very oversimplified 
explanation of what is going on.  I'm only trying to show you that there do 
appear to be anatomical parameters that allow animals to become gigantic on 
land in "normal" gravity (or 1 G).

"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?"

This has been discussed many times on the list, but according to Mike 
Parrish and Kent Stevens who have a computer model of the diplodocids 
Apatosaurus and Diplodocus, the neck does arch downward and is held 
horizontally.  You can also see this if you look at Greg Paul's skeletal 
restorations of these two sauropods in the Scientific American book or his 
various articles -- the Complete Dinosaur book has much of Greg's skeletal 
drawings in the Sauropod chapter.  The proximal neck vertebrae of these two 
sauropods appear to have allowed the base of the neck to slope downward.

Your question is something I am very interested in, as are many other 
people.  We have many good hypotheses and ideas for how sauropods became as 
huge as they did, but we still don't know a lot and understand still less.  
Hope this helped,

Matt

Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155
mbonnan@hotmail.com
MF-Bonnan@wiu.edu

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