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

Re: Fw: Dinosaurs and birds



  Don’t know enough to comment on “1)”. In fact, I will cheerfully, if 
provisionally, take your word for it. Although I must admit, the path to 
synchronous-forelimb-while-alternating-hindlimb use is not really clear to me 
from the behavioral perspective. Many seem to feel it trivial...


 For “2)”, at least in birds, I prefer a more comprehensive (in sense of 
sequential synergistic advantages) scenario. Fortuitous structure => thermal 
advantage => thermal + camouflage advantage => thermal + camouflage + display 
advantage (optional; begin inserting brooding here) => thermal + camouflage + 
display + locomotive advantage. 



I like the time profile of thermal (24/7), and the double advantage of 
camouflage for predators (prey capture + predator avoidance), and the 
(previously mentioned) size increase factor of display. Relative to locomotion, 
lift enhancement and drag avoidance provide a ready mechanism for cleaning up 
the leading edge and enlarging/ stiffening the trailing edge of the incipient 
wing (as you imply). Never thought much about brooding, but don't see why it 
can't be tacked on, at least.



But! What about bats and pteros? I mean, pteros and bats? No handy feathers 
there. 



Note also, that in 'trees-down' none of this a problem, with the possible 
exception of flapping... certainly the zero to minimal lift step is easy. Or so 
it seems to me.

   
  Don
  

----- Original Message ----
From: David Marjanovic <david.marjanovic@gmx.at>
To: DML <dinosaur@usc.edu>
Sent: Sunday, April 8, 2007 1:29:17 PM
Subject: 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.