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Re: Sloping terrain Re: Woman against Abelisaur
On Jul 25, 2011, at 10:40 AM, Don Ohmes wrote:
> The assumption that the disadvantage of weight overwhelms the advantages
> of absolute power, power relative to surface area, and leg length is not
> supported by observation of extant animals. Not supported by the physics
Actually, the physics (as best we can apply them with several unknowns) predict
that sauropods would do poorly in extremely deep mud. I feel that you are
exaggerating the role of swamps and compliant terrain in the ecology of
Elephants, as well - yes, they can and do move through deep mud, and some of
them enjoy swampy habitats, but there is little evidence that they particularly
escape predation in this manner or that deep soft sediments are their primary
habitat. The videos you supplied are very cool, and they show that elephants
can move over soft terrain with heavy loads if required (and trained).
However, even taking the concept that elephants are "mud adapted", this is a
case where the elephant to sauropod analogy is weak on the point of functional
morphology (see below).
Theoretically, during walking, quadrupedal animals can indeed be more stable
than bipedal ones (both quads and bipeds are passively unstable during running,
of course). However, the accommodation of compliant substrates is not merely a
matter of stability. Propulsive efficiency is also of great importance, and
that depends (in part) on how much the substrate deforms. A biped with wide
feet can have much greater propulsive efficiency on compliant substrates than a
quadruped with narrow feet, for example.
Basic physics/mechanical modeling implies that sauropods would have had low
propulsive efficiency on extremely compliant substrates, and some of them would
have been prone to miring on account of having very heavily loaded foot
sections. Despite having passing similarities to elephants, the substrate
stress per foot in most sauropods was many times greater. Elephants actually
have relatively broad feet, while sauropods did not. Elephants also have more
potential flexion in their limbs (even though they usually walk with columnar
limbs). Despite their bipedality, large theropods would be expected to move
more effectively on compliant substrates and sink into them less.
That trend holds (mechanically speaking) until we move to a substrate that is
so compliant that all of the animals in question sink in it deeply, regardless
of foot area (i.e. theropods sink fully, as well). At that point, the most
effective way to move is be able to touch harder sediment below, and that means
simple height becomes the best determinate of propulsive ability - giving an
advantage to tall animals like sauropods as long as they can, indeed, reach a
less compliant medium below the "quick sand", as it were. As the sediment
becomes even more compliant, it effectively becomes a full fluid, at which
point propulsion is swimming, and theropods again become more efficient, though
I doubt they hunted while swimming very often.
I can describe that all in mathematical terms, of course, but I'm not sure
anyone here wants to read that...
Assistant Professor of Biology
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