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Re: was: Pterosaur.net now: pteroid

On Jan 15, 2010, at 8:27 AM, David Peters wrote:

What tells you the pteroid can be depressed? Which muscle depresses the pteroid? What bone anchors that muscle? There's nothing out at the tip to pull it up and down. If you're attempting to manipulate the pteroid near its articulation, then you've got a very short lever to work with. And finally, are you articulating the pteroid principally on the radiale?

Mark and Jim have already taken care of these issues, so I will not belabor the point.

On Jan 15, 2010, at 10:07 AM, David Peters wrote:

So you advocate the depression of the pteroid even though it creates an anterior V in the leading edge of the wing?

Even though the leading edge of the propatagium is maintained by tension?

Jim already indicated why this V shape does not occur, but I would like to note that you actually answered this question with your own second question: part of what prohibits the "kink" you imagined is the tension in the leading edge. That tension does not prohibit pteroid depression, but it means that the pteroid takes most of the propatagium with it during depression. Again, remember that this is a very small about of depression. It does not take much.

Remember the rest of the wing is creating drag that is trying to overextend the elbow. That is a constant force while flying. The propatagium is the only thing that prevents that from happening, as in birds and bats. It's a passive organ.

While the propatagium helps, birds and bats also have perfectly reasonable elbow flexors which can engage to resist elbow extension. Also, their propatagia are not passive organs, either (albeit probably not as finely controlled as the pterosaur propatagium).

1. That fragile tip of the pteroid ain't gonna hold up if you put lateral stresses on it. It can only survive longitudinal stresses.

I presume you mean bending, when you say lateral? It is indeed weaker in bending than in tension and compression, but Palmer and Dyke (2009) already did the calculations on this, and the bending strength is sufficient for a minor depression.

2. It's extremely difficult to put a kink in the middle of an object being pulled from both ends. It keeps rebounding to a straight line. That kink would be right at the tip of the pteroid.

See above. Tension prevents "kinking", but it does not prevent depression.

3. There's no muscle strong enough to pull the pteroid ventrally with such a short lever arm originating at the articulation no matter what bone you'd like to articulate it on.

You have your lever arms backwards - the short base of the pteroid is the out-lever, not the in-lever. Most muscles work at a serious force disadvantage because they work on a long bone element - a long out- lever. The short one at the base of the pteroid actually reduces the disadvantage. In other words, less force is required. Same reason that digging animals have short, stout humeri - less excursion, more force output.



Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
(443) 280-0181