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Re: Airbagged(was Dive!Dive!Dive!)



On Wed, 6 Nov 1996, Nick Longrich wrote:

> T. rex's arms were relatively small, although in absolute terms
> quite large and heavily muscled,

     I think "relative" is definitely the important standard here.  It 
matters if the arms could support 10,000lbs, not 400.    

> extremely large scapulae of T. rex were for exactly such a
> shoulder-landing? Anyone who knows much about sports (not that I'm such a
> person, but my brother was a wrestler) has probably learned that you have
> to learn to take your falls. ( there's a reason cats land on
> their feet!). 

      Again, its an absoltely big scapulae, but I am wondering about the 
stress limitations of bone as a material. 
      Correct me if I am wrong (or if what I am saying makes no sense at 
all), but I don't think that the relationship between bone strength w. 
thickness and the mass of an animal is linear.  In other 
words, if the strength of a bone related to is a certain proportion between 
the thickness of a bone and the size of the animal, I don't think keeping 
that proportion canstant as you increase the mass on the animal (by
increasing bone thickness) will mean the animal can still do the same 
things because the bone will be just as strong relative to the mass of 
the animal.
     In other words, if T.rex weighed as much as a person it might be 
able to roll around, and a scapula that large RELATIVE to body size 
might be usefull in breaking the fall, but for a six tonne animal, it is 
more likely to break the scaluplae.
     To put it another way:  You can build a minerature skyscraper out 
of toothpicks that have a certain relative thickness to the overall size 
of the skyscraper, but if you were to built an 80 story version of the 
same building, it wouldn't work to use humongous toothpicks that were to 
the same scale.  Wood as a building material has practical limitations 
as you increase size, and so I suspect does bone.    
      
> But the limb proportions are simply nothing like what you
> see in an elephant or sauropod. The thigh bone is much shorter, the
> metatarsals, much longer. This may still be due to fleet-footed ancestors,
> but it doesn't look like a graviportal animal. And don't forget the bears,
> which can run quite fast! I'd be interested in knowing how the
> calculations for those work out.

      Morphology also reflects hw HABITUALLY something is done.  Perhaps 
if elephants ran more frequently, they would have similar limb 
proportions.  
     I am also wondering what sort of femur strength bears 
have.  Bears may have pretty short metatarsals and be huge predators by 
today's standards, but masss-wise they are not even in the ball park as 
far as large theropods are concerned.  
      The organic materials that an animal has to work 
with when it is evolving have limits.  You can thicken and strengthen 
bone to a degree as an animal gets bigger, but eventually that material 
will reach its limit.  I doubt we will even find a big sauropod that 
could really run (although big strides alone can give you impressive 
speeds), just because bone can only be stressed so far before making it 
any thicker becomes unfeasable.  I think T.rex was an animal who was 
making neccessary comprimises between mass and speed due to the stress 
limitations of its biological raw materials.  
     Can anyone provide a little information about the relative increases 
in mass and bone strength?  I also can't answer questions about how much 
support the interior of a bone (as opposed to the cortical exterior) 
provides in different animals.  

LN Jeff
O-