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Re: T. rex vs Edmontosaurus speed
Does anyone know of exceptions to the apparent rule-of-thumb as it
applies to tetrapods? Specifically, that within taxa, the fastest is
neither the largest or smallest? Perhaps within bounders like 'roos?
Well, I would imagine that for clades that are very small-bodied
overall, the largest members may be the fastest. Essentially (and this
is taking a >very< broad brush) increased size tends to improve stride
length but decrease stride rate. Up to a point, the tradeoff produces
higher top velocity, but loss in stride rate eventually leads to a loss
of top velocity at very large sizes, so top speed starts to drop off.
The thresholds for all of these factors are going to be very morphology
dependent (so "very large" is relative), but we can generally predict
that some clades will have no members above their local size threshold
for stride-rate related velocity loss, such that the largest taxa will
be the fastest. Roughly similar trends exist for many swimming/flying
groups, related to advance ratios (but are somewhat complicated by
certain gait changes and use of unpowered phases).
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181