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Re: book reviews, parsimony and some loose ends



From: Ronald Orenstein <ornstn@inforamp.net>
 > Stan Friesen wrote:
 > but I can't agree with your
 > classification.  First of all, it only fits mammals - I'm not sure how
 > arboreal reptiles would fit into this (snakes, for example)  just to
 > stick to tetrapods.

OK, so add "legless" as a third catagory.

 >  And what about sloths, just to name a mammalian oddball?

Hmm, offhand, they seem to me to be mostly a odd variant of an
arboreal quadruped.

 > >Now it is interesting that these long armed arboreal forms tend to
 > >use a bipedal walking stance when on the ground (invariably so
 > >for gibbons).
 > 
 > Chimps and gorillas are usually quadrupedal on the ground,

They are *knuckle-walkers*, which is NOT a normal quadrupedal gait.
And even chimps will often use a bipedal walking stance (the smaller
the individual the more likely it is to use bipedality as opposed to
knuckle-walking).  This size related factor also applies to oranutans,
where the males are knuckle-walkers and the females are more or less
bipedal.

 > though their long forelimbs give them a fairly upright stance.  As
 > for the Indri, it is really more of a vertical clinger and leaper
 > (cf Mittermeier et al, Lemurs of Madagascar (1994)) than a
 > brachiator.  It occasionally comes to the ground to eat earth, but I
 > do not know how it gets around when there.

Neither do I.

My main point in the classifications was to distinguish those forms in
which the forlimbs provide the main arboreal propulsion as opposed
to those with a more equal distribution of work.

 > I think this is flawed by the attempt to use a mammalian body plan to
 > arrive at a bird.  I would be astonished to hear of a reptilian brachiator;

But how about a "vertical clinger and leaper" like the indri?

This does not seem all that remote from Scleromochlus or even that
new madagascaran "maniraptor/bird". (In fact that beastie sounds
a  great deal like an indri with wings).

 > I suspect that bird ancestors, if they indeed were dinos, were bipedal
 > BEFORE becoming arboreal and used their hands as climbing assists rather as
 > the hoatzin chick does today.

I agree, this is quite possible.  The evidence right now is insufficient
to tell one way or the other. (I could even imagine Eoraptor climbing
trees to some degree, wo the timing will be very hard to check).

What I am suggesting is that via a switch to arm dominance in arboreal
locomotions it acquired the elongate arms which then became airfoils.
This is to deal with the supposed difficulty in going from a flying
squirrel type glider to a powered flapper, and not really with the
origin of .

 >  Certainly I find it hard to imagine a brachiator evolving towards
 > loss of grasping function in the hands!

Certainly not until it was fully a flyer.  But then as far as I can
see even Archaeopteryx had grasping capability in its hands.

In short, the loss of hand grasping was a very *late* avian change.

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.