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Re: Dinosaurs vs. mammals: a hypothetical scenario



2011/4/8 Raptorial Talon <raptorialtalon@gmail.com>:

>
> Huh. Every source I've ever read on the subject has claimed that it's
> the silica content *in* the grass that not only drove the evolution of
> derived grazing-adapted molars, but also that it was part of the
> defensive adaptations of grass and thus part of what allowed it to be
> so successful. Can you recommend a source? If this is new information
> that overrides all the other stuff I've seen, I need to know about it
> . . .
>
Well, the teeth batteries of ceratopsians and hadrosaurs, by renewing
continuously, would have an advantage over the hypsodont, not
continuously growing dentition of most ungulates. This may be also an
advantage in other dinosaurs, that even if not having batteries, renew
their teeth continuously. Now, I am not sure if their enamel was
harder than those of ungulates.

>>Is there a source stating this? Because I would rather think stride
> lenght to be variable among both bipeds and quadrupeds, and can mind
> long-legged bipeds having a stride advantage over many quadrupeds with
> proportionally shorter legs. In addition, moving two legs instead of
> four while running looks to me like more energy-efficient also in
> running (because of the same reason you mention, expending energy in
> moving two extremities instead of four).
>
> I can't pin down a source right now, but consider: at the same limb
> lengths and the same range of flexion, a quadruped will always have a
> slight stride advantage purely because one pair of limbs is situated a
> distance in front of/behind the other set. A biped, with only one pair
> of limbs, has to rely entirely on the reach of those limbs; it cannot
> use a pair of forelimbs to reach beyond the flexion of the hindlimbs.
> As an extreme example, imagine a regular cheetah and a "bipedal
> cheetah" - all the additional stride distance the regular cheetah
> gains through extra flexing of the spine and forelimb girdles could
> not be matched by the bipedal analogue, which lacks those "tools."
>
Here I see that the greater difference giving quadrupeds the edge is
spinal flexion, at equal effective limb lenght. It adds a segment,
right. But, this advantage can actually be nullified with relatively
longer limbs in an animal with similar mass. So, the edge for
quadrupeds would count for similarly elongated and designed limbs, not
necessarily for equal mass. After all, not only cheetahs make use of
spinal flexion; also do many other mammals which are no faster than
ostriches, including greyhounds. There is also the addition to stride
lenght which gives "jumping" at the suspended phase, but this counts
both for quadrupeds and bipeds.