RE: Gliders to Fliers? (Was Re: Reuben Strikes Back)

["Larry Febo" <larryf@capital.net>]

Matt Bonnan wrote:


>Date: Thu, 23 Sep 1999 13:26:02 PDT
>From: "Matthew Bonnan" <mbonnan@hotmail.com>
>To: dinosaur@usc.edu
>Subject: Gliders to Fliers? (Was Re: Ruben Strikes Back)
>Message-ID: <19990923202603.76713.qmail@hotmail.com>


>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.

Padian states that the wings of pterosaurs (at least the earlier ones like
Dimorphodon) had membranes that were NOT batlike, in that they didn`t attach
to the lower extremeties of the hind limbs. In this case, I would think that
the forearms were decoupled from the hind (again for some of the earlier
pterosaurs).

>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?

Well,...(you did say anyone could answer, so I`ll give you MY
impression...8^). I suspect that pterosaurs evolved into birds at an early
stage in the mid-Triassic. A decoupling in both early pterosaurs and later
refined by birds, probably had to do with making precise landings on tree
branches. The forearms had to be employed in manipulating for a precise
landing, and the feet become more specialized for grasping that branch. (a
miss would cause great inconvenience, if not demise,  for  the creature in
question). The feet of Dimorphodon seem adapted to landing on and grasping
Cycad branches. Birds become more and more specialized for grasping  tiny
gymnosperm branches.

The fact that some of the earliest pterosaurs seem to have this decoupling,
as well as an avian-like upright posture (in this I agree with Padian),
seems to me just another "clue" pointing to an avian-pterosaur split.

.....Just my opinion anyway ( for more,....go to:
http://www.capital.net/~larryf/ )