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*To*: dinosaur@usc.edu*Subject*: Rex Fall part 1*From*: Jeffrey Martz <martz@holly.ColoState.EDU>*Date*: Tue, 5 Nov 1996 12:10:24 -0500 (EST)*Reply-to*: martz@holly.ColoState.EDU*Sender*: owner-dinosaur@usc.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 0-

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