Rex Fall part 1

[Jeffrey Martz <martz@holly.ColoState.EDU>]

 Farlow, J.O., Smith, M.B., and Robinson, J.M. 1995: Body Mass, Bone 
     "Strength Indicator", and Cursorial Potential of Tyrannosaurus rex.  
     Journal of Vertebrate Paleontology 15(4) pp 713-725
 
     The first important point I'd like to make is that the authors do 
not say T.rex held its head 20' off the ground.  They estimate about 400 
cm, or 12'. 

FEMUR STRENGTH
     The authors based thier calculations after McNeil Alexander's 
"Dynamics of Dinosaurs and other Extinct Giants", which I do not 
personally own a copy of, but supposedly the formulas for femur strength 
in living animals that Alexander calculated give a pretty good indication of 
running potential.  
     Femur stength was calculated using the "section modulus" (which is found 
using the diameter of the femur at its midpoint, and how much of that 
thickness is cortical bone), the length of the femur (important, because the 
longer you make the bone, the greater the stress exerted on the midpoint, 
which is also the narrowest and presumably most easily broken part of the 
bone), and the mass of the animal.
     The femur they used was crushed, so unfortunately the exact shape
was difficult to determine (although they could apparently get the
cortical thickness).  Playing around with femur shapes and different
people's mass estimates, they came up with a strength indicator
between 7.5 meters squared per giganewton (no, I don't know exactly
what a giganewton is) [10^9 kg m/sec^2 -- MR ] and 12 meters squared
per giganewton.  Thier own mass estimates for T.rex that were used in
the calculations leaned toward the lower end of the scale (7.5-9
meters squared/giganewton).
     Compare this with:
      Elephant: 7.0 meters squared/giganewton
      African buffalo: 22 meters squared/giganewton 
      White rhino: 26.0 meters squared/ giganewton
      Ostrich: 44.0 meters squared/giganewton
      The gist of these calculations is that T.rex's femur doesn't 
appear to be especially strong for an animal of its size (in fact, it 
is just a tad stronger than an African Elephant of the same mass), and not 
what you might expect for a really fast running animal.      

HIP SOCKET
     The proximal ends of theropod femurs (where it connects with the 
pelvis) tend to be cylindrical shaped, and presumably more 
easily dislocated, while mammals have a ball and socket which holds the 
femur a little more securely in the pelvis.  I don't know what sort of hip 
set up ostriches and other ratites have.

INCREASED FALLING DANGERS FOR T.REX

     I already mentioned this, but here it is again:
1)  Most big, fast moving animals are built low to the ground and don't 
have as far to fall.
2) Most big, fast moving animals are quadrapeds, and so have a second set 
of legs to keep the animal from falling if one leg trips.
     T.rex had a lot farther to fall, and no way to catch itself if 
it did trip: it was a biped with puny arms.  The faster you are moving, 
not only the more likely you are to trip, but the harder it is to correct 
before you fall.  Try it yourself:  The response time needed before going 
down is inevitible is cut, and even if you are able to get your leg back up, 
the movement is so awkward that you usually go down anyway, at least to 
your hands and knees.  The way T.rex was built, that is the same as 
just falling on your face.

Continued.....

LN Jeff
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