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Re: "The X Digit"...a Pteroid bone??



Sat, Jan23, 1999 Jeff Martz wrote:


     < A close relationship also means that two different groups evolving =
a similar adaptation are likely to use the same parts, since they have =
the same parts at their disposal.  In order to fly, both pterosaurs and =
birds needed to develop some kind of paired aerodynamic structures.  The =
only real coincidence that one might argue represents homology is that =
they decided to use their forearms, instead of their ribs (as in =
_Icarosaurus_ and _Draco_) or legs (as in _Sharovpteryx_). =20
     Since both "decided" to use their arms, they would have been taking =
advantage of the same arm bones, pectoral girdle bones, and muscles.  =
Flapping was aerodynamically advantageous for developing true flight.  =
Since the archosaur arm already has a nice joint between the humerus and =
shoulder, it isn't surprising that both groups chose to use it; and not =
surprisingly, the muscles used to move the arm in the proper flapping =
motion, being homologous and attaching to roughly the same place on the =
humerus and pectoral girdle, developed similarly.=20 >


How would you account for the "pneumatic foramina", the hollow bones that
are virtually identical in both birds and pterosaurs? And the braincase
being
"avian" in character? I suppose you think that these also were the result of
convergence? That birds hit on the same idea,... to hollow out the bones and
make them part of the respiratory system? ( to me...also a somewhat complex
development). The more points you claim are convergent, the less likely I am
to believe it.

    < Also note that the convergence is far from complete; feathered wing =
of the bird and the membranous wing of the pterosaur represent a pretty =
fundamental difference, and one that must have begun pretty early in the =
evolution of flight in both these groups.  Otherwise, one group must =
have radically changed the wing structure at some point; if this is the =
case, what was the common ancestral wing like? >

I believe the common ancestor was small, insectivorous (small cranium), had
furcula as
well as acrocoracoid process (enabaling rapid wingbeat allowing for
controlled perched
landing), typical pterosaurian flight membrane along with (of course)
integumentary fibres.
>From here, pterosaurs sought "meatier" prey, and developed large crania, and
general robustness
in size, leading to four point landings,eliminating fragile furcula, and
restricting their
arboreal habitat to the cycads, where they could safely land on the springy
compound leaves.
In a different direction went the "birds", seeking insects in colder
(perhaps upland) enviorns,
developing feathers from "fibrous integument" for better insulation, these
fibres becoming streamlined
along the flight surfaces.Wing digit tends to shrink (Bergmans Rule in a
colder enviorn) feathers
off the trailing edge of the membrane grow to compensate for loss of flight
surface, etc, etc, until
no wing digit, but instead flight feathers.


 < Presumably, the switchover was to develop a more =
aerodynamically efficient wing, which seems a little difficult to =
imagine happening since the intermediates with their combinations of =
fundamentally different wing designs would likely have been relatively =
clumsy.>


Not for aerodynamics sake, but just invasion of a new niche.

My previous statement:
<< I do believe that endothermy to the point of egg incubation evolved =
only once, and most likely began in the early Anapsid forms whence it =
either was improved upon by later mammals and birds and relinquished by =
other forms that assumed a condition of secondary ectothermy, such as =
squamates, turtles, crocodiles.>>


    < By relinquished you mean "re-evolved convergently".>

That`s making it too complex.


 < Why is that =
easier to believe in then endothermy evolving convergently?>

Endothermy is complex and requires large energy input. If a niche can more
advantageously
be occupied in a "cold-blooded" state, then natural selection would tend to
choose it.


  <The =
cellular basic for endothermy is pretty simple; cell membranes are =
"leaky", allowing Na+ and K+ ions to get across more easily (this forces =
the Na+-K+ pump to work a lot harder to fight the ionic gradient, =
consuming more oxygen and producing heat in the process).  Endothermy =
actually amounts to a simple cellular defect which produces an =
advantageous side effect when it is compensated for.  For more detailed =>
information, see:>


It certainly sounds too "simple". Guess I`ll have to check out your refs to
find out more.

Larry Febo.