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Re: Bipedalism
At 05:13 PM 02/04/98 -0600, Rob Meyerson wrote:
>I went with the idea that bipedalism has it's advantages for
cursorial critters and can be seen as a cursorial adaptation; I then went
on to suggest that theropod bipedalism evolved on the ground, before they
moved into the trees.
I think this is only partly true, and the reason for this is the nature
of the spinal column. This is where we run into one of the big
differences between mammals and dinosaurs/birds. If you have a
spinal column capable of a considerable degree of dorsoventral flexion
and extension, bipedality is certainly NOT going to be an advantage for
cursoriality. Instead, you would be best as a quadruped like a
cheetah, which can arch its spine to bring its hind legs well forward
during running and thereby increase stride length far beyond what would
be possible for a similar-sized running biped. This may be why
there are so few bipedal terrestrial mammals, and those that do exist -
with ourselves as the only exception I can think of - hop rather than run
(kangaroos, jerboas, the springhaas). And, of course, we are almost
certainly bipedal as a result of having evolved from arboreal
brachiators, a most unusual development (perhaps the extinct ground
sloths, and some of the subfossil Malagasy lemurs, shared the same
pathway). [This, BTW, is one reason I find it hard to swallow the sci-fi
premise that the humanoid body shape is the likeliest one for intelligent
aliens!]
In birds, however, we have an almost inflexible spinal column, and for
them bipedality has certainly turned out some pretty fast runners - an
ostrich is one of the fastest land animals going. But whether
bipedality evolved as a cursorial adaptation is another matter - it could
have evolved because it was advantageous to have the hands free to
perform other roles, or for rearing to reach higher vegetation.
About all I would care to say is that bipedality was not evolutionarily
possible until the spinal column lost much of the LATERAL flexibility
seen in early tetrapods, so that it related to a shift in posture and
gait - but these would have to have evolved first, making bipedality a
structural option whose initial advantage may have had nothing to do with
locomotion (or, to give a nod to George, with terrestrial
locomotion).
>To take a nod to BCF, I then suggested the following model:
Bipedalism evolves on the ground; >these "prototheropods"
discover an advantage to life in the trees, and begin to adapt to that
>lifestyle; a part of that adaptation is longer arms, to help grasp
the branches; these longer arms >are now preadapted to become wings.
These guys, then, are the ones that evolve into true >theropods.
I doubt that there is any way to establish whether bipedality evolved
first on the ground or as a climbing adaptation, for the reason suggested
above - namely, that it may have evolved for reasons unconnected with
locomotion, reasons that could apply either on the ground or in trees (eg
prey handling). One of my big problems with BCF (which has its
attractions as a concept) is that it assumes that you can make this sort
of distinction. I haven't the faintest idea how you would tell from
structure alone, assuming you had the fossil, if the first bipedal
archosaur lived in trees, low bushes, rock piles, open country or all of
the above, and from a purely phylogenetic standpoint of course it doesn't
matter.
>I acknowledge that bipeds can adequately function in trees, with some
modification of their form. >Having claws or wings certianly helps
things along. I merely suggest that the bipedalism *trait* is >most
likely to develop on the ground.
I would point out that any biped that does NOT have claws or wings is
going to be a highly derived form indeed! In fact I cannot think of
a single bipedal tetrapod (leaving aside certain burrowing lizards and
pythons, which hardly qualify as bipeds) that has entirely lost all
pectoral skeletal elements while retaining the pelvic ones.
In fact, unless you believe that bipedality evolved at (as it were) a
single bound, the first archosaurs to assume bipedal postures and gaits
must have had well-developed, fully-functional forelimbs. These
could well have been used for climbing quite early in the evolution of
bipedality - in fact you could argue (though again I know no way of
proving this) that the more advanced a biped becomes in terms of
reduction or modification of the forelimb elements, the less well-adapted
it might be for hauling itself up into a tree. I can see bipedality
evolving either on the ground or in trees, with early generalized bipeds
having considerable potential to shift either into or out of trees and
back again. Thus I don't see bipedality as an argument either for
or against an arboreal habit in its first possessors.
>>Then why do monkeys and apes retain a normal walking ability?
Well I
>>suppose hopping/leaping will only become obligatory if the animal
returns to
>>the ground only very rarely, and then it may not happen (only
some of
>>lorris's, potto's etc leap/hop, and many apes are too big to risk
it).
>
>Firstly, I would point out that most of the above are
"temporarily bipedal." When chimps or gorillas travel in a
casual walk, they are quadrepeds, using all four limbs for motion. These
critters stand only occasionally, and only for brief periods of time.
Basically, their skeleton is not designed for constant bipedalism.
But gibbons ARE obligate bipeds on the ground - I have never seen one
walking quadrupedally. Bear in mind that one of the biggest
anatomical shifts in primate locomotion was not coming out of the trees
(which quadrupedal primates like baboons, patas monkeys and ring-tailed
lemurs have managed quite nicely) but shifting from running/leaping to
brachiation. Brachiation has produced major modifications in the
structure and proportion of the limb elements, and I would speculate that
in primates at least (not in sloths, for example) it involved loss of
quadrupedality at an early stage. It strikes me that the larger
apes are secondarily quadrupedal on the ground as a function of increased
size; the fact that they walk on their knuckles, a most unusual type of
gait, suggests to me that their ancestors lost the ability to move
quadrupedally on the ground and that this had to be re-evolved.
--
Ronald I.
Orenstein
Phone: (905) 820-7886
International Wildlife
Coalition
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