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Re: T. rex

        Alexander's got a book out called "Dynamics of Dinosaurs &  
other extinct giants" (1989: Columbia Univ Press). His estimates for  
the "athletic ability" of Triceratops place it in the range of a  
modern African buffalo, tops. Tyrannosaurus ranks much worse, in the  
range of an African elephant. The implication is that T. rex surely  
wasn't zipping across the terrain like a monstrous ostrich, and  
meanwhile Triceratops probably could get up to a stately gallop.  
HOWEVER, Alexander also offers estimates for actual running speeds  
based on trackways, as follows:

large theropod  4.5 - 4.9 mph WALKING
ornithopods     9.6 - 10.7 mph RUNNING

        He goes on to say that he has no direct estimates for running  
large theropods, BUT 1) the "dimensionless speed" of the 4.5 mph  
theropod was about 0.4, and 2) the minimal dimensionless speed that  
defines running is 0.7. Because elephants can run (they reach  
dimensionless speeds of 1.0: see his fig. 3.10) and dimensionless  
speed is a linear function of actual speed, it seems safe to say that  
a running large theropod could reach at least 0.7/0.4 * 4.5 = 7.9  
mph, and perhaps 1.0/0.4 * 4.9 = 12.3 mph. This brackets the range  
for ornithopods (no ceratopsian data are given), and I conclude there  
is no plausibility problem with regard to having large theropods  
actively hunting ornithschians.
        There is, however, a major plausibilty problem with large  
theropods being pure scavengers, namely, that there is no large  
vertebrate known to follow this strategy in the modern world.  
Hyaenas, for example, are very effective predators and their  
"scavenging" largely involves chasing off other large predators  
(lions, leopards, etc.) from kills! Of course dinosaurs and mammals  
are very different, but I think we should hold off from attributing  
any completely novel ecological/behavioral strategies to dinosaurs  
unless we absolutely have to.