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Re: Fw: Dinosaurs and birds



----- Original Message ----- From: "don ohmes" <d_ohmes@yahoo.com>
Sent: Saturday, April 07, 2007 11:21 PM


Note: I first began thinking about the evolution of flight about 1965, and my ideas jelled about 1995, and have changed little since. I see 2 major problems specific to ground-up flight scenarios: 1) the establishment of a phenotype that allows synchronous flapping of the forelimbs while simultaneously using the hind limbs in alternating fashion. 2) crossing the threshold between zero and minimally beneficial forelimb aerodynamic locomotive assistance.

1) is not a problem when we're dealing with bipedal saurischians -- the forearms cannot rotate and are fixed in such a position that the palms face each other. This is still the condition in birds. The flapping motion (in the widest sense) is thus the normal state of affairs (the plesiomorphy) for saurischians (actually a larger group, Archosauria at least) and has not been modified on the way to birds; retentions do not need to be explained.


2) becomes easy, I'd say, if the animal in question has decent-sized wings (long wing feathers) before it starts thinking about using them in locomotion in any way, because if it already has wings, noticeable aerodynamic effects become easy to generate. So the problem shifts: how do you get a wing -- or half a wing -- as the result of selection for something unrelated to locomotion? I can think of at least two proposals.

One is the idea that wings are for brooding (Hopp & Orsen 1998 -- 2004). The larger the wing (the longer its feathers), the more eggs can be packed under it. Maybe the first wings grew only seasonally and were lost after the brooding season; maybe flight was at first just a part of courtship dances... these parts are probably not falsifiable, but it's easy to imagine how flight can evolve once wings are at least seasonally present.

The other is that wings evolved as display surfaces and similarly became used in courtship fights that culminated in flight (Cowen & Lipps in Cowen 1994): http://www.dinosauria.com/jdp/archie/feather.htm.

The former has an advantage over the latter: usually both sexes brood, but only one fights.