Re: Gliders to Fliers? (Was Re: Ruben Strikes Back)

[Casey <tuckercj@muohio.edu>]

Sorry, but I have to delurk for a little while.

If I remember correctly, part of Gatesy's hypothesis concerning the
decoupling of the hind limbs of archaeopteryx was due to the fact that
it had a partial arboreal, and partial terrestrial lifestyle.  Or at
least this is where Archaeopteryx came out on those trivariant diagrams
that involved three different variables (lengths of the bones of the
leg--I don't have the paper in front of me right now so I abologize).
So that we see a grouping of terrestrial animals, like terrestrial
theropods and ratites, and a grouping of perching birds like warblers,
sparrows etc.  Archaeopteryx falls closely in between the two groupings
suggesting a partial arboreal/terrestrial lifestyle.  This would agree
with a trees-down hypothesis for the evolution of flight from a small
theropod.  If you look at pterosaurs & bats, they are not efficient
terrestrial locomotors, but archaeopteryx was partially.  If you look at
the evolution of many of the other groups of birds we almost see a
progression (& I'm hesitant to say this) of a more well-developed
perching foot.  Now, if I remember correctly, the development of the
perching foot would be correllated with the evolution of gymnosperm &
angiosperm trees in the Cretaceous, as opposed to the more frond-like
cycads & early gymnosperms of the Jurassic.  So, Archaeopteryx would
have had limited perching capabilities partly because of the nature of
the flora of the Jurassic, and later species of birds would have had
more branches, from early trees, which they could utilize for perching,
etc.  But, perhaps a partial arboreal lifestyle afforded Archaeopteryx
enough protection from larger predators, or provided it with a new
feeding niche on some of the insects & small animals that would have
evolved in conjunction with the cycads & gymnosperms.  Regardless of the
reason, Gatesy's data suggests a partial arboreal/terrestrial lifestyle.

Casey Tucker

********************************************************************

Matthew Bonnan wrote:

> Dinogeorge said:
> It is >much< more likely that flight evolved in small archosaurs that
> >adopted an arboreal lifestyle and stayed small until true flight
> evolved in
> >their clade, taking full advantage of light weight and gravity assist
> for
> >gliding.
>
> Okay, George, I've somehow misconstrued your hypotheses on dinosaurian
>
> evolution before, so if I do it again here please try not to take
> offense.
> =)
>
> My question for you, or anyone else on the list, is this: all the
> gliding
> animals I am familiar with incorporate both their forelimbs AND
> hindlimbs in
> the act of gliding.  Flying squirrels use a patagium which spans from
> fore
> to hindfeet.  Gliding lizards have a sheet of skin/scales that
> stretches
> between the fore and hindlimbs.
>
> Now, both bats and pterosaurs have a patagium-like structure which
> also
> stretches fore to aft.  I no very little about proposed mechanisms for
> bat
> or pterosaur evolution, but I can swear that in both groups it has
> been
> proposed that they developed flight from gliding organisms
> (corrections to
> me please if I have this mixed up).
>
> Pterosaurs would seem to be ideal candidates for a trees-down glider
> evolutionary sequence because they apparently incorporated both their
> fore
> and hind limbs in flight.  See where I'm going?  As Gatesy, Padian,
> and a
> number of other folks have pointed out, the forearms of birds are
> decoupled
> from their hindlimbs.
>
> If birds are descendants of small arboreal archosaurs (dinosaur,
> non-dinosaur) which were gliders, how do you think the decoupling of
> both
> the forelimbs and hindlimbs took place?  Am I missing something: are
> there
> gliders which do not use both limbs with a sheet in between?
>
> Just curious, and you know what they say about curiosity and that cat.
> =)
>
> Matt Bonnan
>
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