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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. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu