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Re: Phylogeny of Maniraptora



On Thu, Jun 05, 2003 at 03:04:45PM -0700, James R. Cunningham scripsit:
> Re the gliding vs. flapping thing, when gliding you become more
> efficient by substantially increasing the aspect ratio to reduce
> induced drag.  Which also increases the moment of inertia about the
> shoulder (and wrist).  When doing incipient flapping you find that it
> is much more difficult when you also have to deal with increased
> moment of inertia. Which implies that incipient flapping is easier for
> animals that have relatively short wings and aren't good gliders. In a
> way, it seems to me to be more likely that you'd see an early, short
> winged bird evolve into Diomedes exulans than that you would see Dex
> evolve into a short winged bird that does rapid flapping.  As an
> aside, Archaeopteryx seems to be a fair to middlin' powered flapper as
> far as flight performance is concerned.  It also seems to be a truly
> crappy glider.  So which way would it be more likely to spend its
> time?

Are there any non-volant organisms that really glide, rather than
parachuting?  Anything among the flying snakes, flying squirrels, draco
lizards, etc. that can, frex, get energy out of a headwind to land
higher than they started?  Hell, is there a bat that can do that without
flapping?

I'm trying to get at a distinction between using aerodynamic forces to
reduce sink rate (parachuting) and turning velocity into actual lift
(gliding).

I think the whole glide/flap for power thing gets tangled in this; an
organism doing attitude control and fall-retarding things *isn't*
gliding, it's parachuting, and it'll do whatever it takes to get its
attitude on impact right, because it has to get it right.  You get
something like flapping from the predatory stroke if you push a basal
maniraptoran out of a tree; it may not do any good, but that's what its
arms _do_ when trying to move rapidly, they're not the peculiar
wide-range-of-motion things people have got.  If it's a maniraptoran, it
can flap.  (maybe not *well*, but it can flap.)

The trick is not identifying when lift comes in; 'assisted vertical
running' makes it clear that there *is* a nice shallow ramp for the
utility of lift for small maniraptors, one that can't be discounted from
fossil evidence even if it never took place in pre-volant species.

Somewhere, though, there's a biomechanical analysis that indicates the
sustained load bearing capacity of the arms; it's when that passes some
multiple of body weight that you've got a real flyer.  I'd *expect* that
number to be a bit over two, but that particular wild ass guess isn't
even sophisticated.

-- 
oak@uniserve.com | Uton we hycgan    hwaer we ham agen,
                 | ond thonne gedhencan    he we thider cumen.
                 |   -- The Seafarer, ll. 117-118.