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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.