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Re: book reviews, parsimony and some loose ends



Stan Friesen wrote:

> >  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.

Actually I am more inclined to think of them as odd brachiators - the front
limbs seem to have a lot more to do with propulsion than the hind in the
ones I have watched.

> > 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?

Well, in a way some lizards (and, of course, frogs) are pretty good at that
today (watch an anolis sometime); the ultimate in that category in my
experience are the flying dragons (Draco).

>What I am suggesting is that via a switch to arm dominance in arboreal
>locomotions it acquired the elongate arms which then became airfoils.

Except that birds do not have really elongate arms.  Even in bats and
pterosaurs it is the fingers that are elongate.  The length of bird wings
is contributed by the feathers, not the bony structure of the arm.  The
arms of Archaeopteryx seem to me (besides their function as wings) to be
more adapted for grasping prey or scrabbling about than brachiation, a
quite extreme adaptation requiring a very long arm and hooklike hand.

>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 .

I still find it hard to imagine such a transition myself, but I am more
inclined to see prey-grasping as an intermediate function.  A good many
birds get around in trees quite adequately by leaping from branch to branch
on the hind legs alone, without flight which may be used primarily to
volplane to the base of the next tree.  I have seen this in many species;
birds of paradise and the New Zealand kokako are Archie-sized birds very
good at it (and the kokako is a poor flyer).  I see no reason why a small
bipedal dino could not have done the same thing, with the forelimbs being
used for the occasional scrabble or to assist in getting up to branch level
in the first place.