[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)

Don Ohms wrote:

<That: a) bats are arboreal in origin, and b) probably went from gliding to 
flapping. Don't blame me, I didn't know that consensus was reached either. I am 
sure it is not 100%, anyway.>

  I am not sure this concensus was reached at all. What is your source? I 
understand it's been argued here on the list, and I also understand I have even 
argued for something akin to it, 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, especially since bird ancestors appear to LACK arboreal 
traits, which puts the lie to the statement that arboreality precedes gliding, 
and says nothing for gliding leading to flapping, since we have no clue how 
bats 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.>

  It is not that I feel I should re-read the thread, which I have been reading 
all-along, but that I have some attention on this issue for the sake of the 
model being proposed as it relates to flying squirrels, *Petaurus* and colugos. 
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. However, the above animals are for the most part 
incapable of flapping.

  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.

  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.

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


Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)