[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Campbell's even crazier than a MANIAC? (archeopteryx climbing)

-- On Sun, 9/28/08, Jaime A. Headden <qilongia@yahoo.com> wrote:

>   I am not sure this concensus was reached at all. What is
> your source? I understand it's been argued here on the
> list, 

OK, then you know where I got that impression. Like I said, I am sure the 
consensus isn't 100%. I am also sure it could go away in a caffeinated 
heartbeat, if it exists.

> and I also understand I have even argued for something
> akin to it, 

Well, heck, maybe I got it from you.

> but you should not take it for granted that such
> a condition must be the basis of the models by which gliding
> leads to flapping in other taxa, 

I appreciate the advice. It seems unnecessary, as I do not take such a position 
at all, but thanks. 

> especially since bird
> ancestors appear to LACK arboreal traits, 

Yes, they do. Although they do not totally lack the physical capacity to climb. 
I contend that it is therefore incorrect to claim that said lack of arboreal 
traits precludes exploitation of gravity-powered flight by bird ancestors.

> which puts the lie
> to the statement that arboreality precedes gliding, 

That does not follow, imo. In fact, the claim that birds never went through a 
gliding phase is being advanced by some. Could be true. Or not.

> and says
> nothing for gliding leading to flapping, since we have no
> clue how bats developed flight.

Get right down to it, we have no clue how anything developed flight.
> <I will dispute the conclusion(s) that *therefore*, a)
> passive gliders cannot transition to powered flight, and b)
> gravity-driven flight origin in birds is falsified -- if and
> when such claims are made. I frankly do not remember if such
> claims WERE made. I THINK they were at least implied by
> someone, but am not sure at this point. My contention that
> there are places in the glide sequence that selection can
> act upon other than midflight to create an incipient
> flapping module was made as counter-point, and stands as
> such, imo.>
> Your argument has been to conceive of a glider that
> can flap its limbs to produce control and power in the glide
> path, and with enough lift, retard and negate the glide
> path. 

Not quite. I argue that tetrapod gliders use movements of the forelimbs to 
exert control. Basic components of flapping are embedded in these motions, and 
they are subject to selection. 

> However, the above animals are for the most part
> incapable of flapping.

Yes, but the "least" part is still available for modification. The fact that 
the "above animals" are not being modified in such a way as to produce powered 
flight (as far as we can tell) does NOT imply that gliding cannot lead to 

Let me put it this way; if all the vertebrates EXCEPT flying squirrels (for 
example) were removed from the planet, do you think that carnivorous squirrels 
would eventually evolve? How about powered flyers? If your answer to either 
question is "no" we have VERY different ideas of what evolution is.

>   1. Membraneous four-limbed gliders such as all of the
> above mammals employ a unique system whereby the entire
> arrangement of limbs stretches a gliding membrane between
> them. At least the rodents and colugos have special osseous
> modifications of the elbow or wrist that anchor this
> membrane with special tendons that are then linked to the
> leg. These intimately inhibit independant action of the
> limbs.

Very interesting information. Thanks.
>   2. What birds use to control flight controls in the
> wings, including pitch and yaw, are used in these mammals by
> twisting about the body axis and the tail, because they
> cannot drop their membranes without decreasing the drag they
> need to keep their glide-path smooth. To do otherwise is to
> drop. Thus, if one of these animals started tucking their
> arms down for any reason, they might die.

Or experience a change in trajectory that avoids predation.

>   3. For a glider to begin using its arms to develop lift,
> they must be wholly separate from a structure linked to the
> body, so as to allow free movement. The tendons of flying
> squirrels and the rest of the gliding mammals contradicts
> this. Moreover, while bats possess a membraneous connection,
> they lack the tendinous link between fore and hind-limb,
> which would otherwise skew the membrane's shape. As it
> is, bats use their legs and hips much as birds and flying
> squirrels use their tails, to moderate pitch and yaw. 

Again, very interesting.

> This
> disconnect between the limbs allows this, and are
> neccessary, I would argue, for a powered flier.

I do not dispute it.

Again: if all the vertebrates EXCEPT flying squirrels (for example) were 
suddenly removed from the planet, do you think that their current state is such 
that they would have to become terrestrial bipeds before they could achieve 
powered flight, or that the round-about route would be quicker?

I do not think so, and I do not think the evidence points in that direction.