Re: T. rex vs Edmontosaurus speed

[Michael Habib <mhabib5@jhmi.edu>]

> 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?
>
> Don

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).

Cheers,

--Mike


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