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One more speculation
First, I`d like to thank everyone for allowing me to speculate on diapsid
evolution. There`s just one more part of my "unique" view that I must get
off my chest. (I figure that weekends seem to have fewer posts, so...might
as well do it now). Also, I`ll try not to "hog up" the forum after this.
(thanks for the indulgence!).
Other "alternate" theories have their proto-bird models. George Olshevsky
points towards a thecodont closely related to Cosesaurus, Megalancosaurus
and Longisquama. Tarsitano and Hect also point to such a thecodont ancestry.
I, on the other hand, prefer to think it was a very early, primative
pterosaur that gave rise to a sister group that would eventually become
aves. To me, they share too many similar features to have been the cause of
"convergence". (I.e. similar pectoral girdles with supracoracoideus
function,hollow bones, similar brain structure). Plus, they occur at just
the right time to be ancestral to birds. Plus, if they did exist just prior
to the fully feathered avian form, wouldn`t their very presence be a
competitive obstacle to formation of a new avian type "from scratch". (This
also is in opposition to a "ground-up" model of flight evolution).
As for the missing furcula, or even clavicles in pterosaurs, I attribute
this to most pterosaurs being "top-heavy" in form, i.e. large crania
relative to body size. Such top-heaviness would have made for fairly hard
landings upon their forelegs, involving stresses that would have broken any
clavicles (if in existance). Therefore, evolution would have eliminated them
in the true pterosaur form. But if there existed a smaller, more primative
form, with much smaller cranium (insectivior), more adapted to making
perched landings on its hind legs, it would have been able to retain the
furcula to aid its powered flight. Such a smaller form would be able to land
on, and negotiate the more intricate branches of the gymnosperms, and newly
evolving angiosperms, where larger pterosaurs couldn`t follow. These newer
trees could invade colder enviorns, where insects were sure to follow, and,
in turn, so would these smaller, insectivorous pterosaurs. In order to
adapt, these proto-birds had to develope feathers, at first just for
insulation. Then, hairy proto-feathers evolving into contour feathers (to
keep the body streamlined), finally into true flight feathers, to aid both
in lift and maneuverability. There would have been definite selectionary
pressure against a long wing digit in this colder enviorn,(Bergman`s Rule),
and hence its eventual loss. It could have been a gradual exchange, a
trade-off, loss of length of wing digit for increased length of flight
feathers until finally...a bird!
Anyway, in looking for possible negative evidence that might test this
hypothesis, I was once told by my old professor of evolution, (Max Hect),
that..."it`s all in the wrist!" Even Dr. Kevin Padian, who was kind enough
to review my hypothesis, told me..."it`s a long way from a pterosaur wrist
to a bird wrist." I`m not sure exactly what this means, (I`ve seen evidence
of the vertebrate structure doing some pretty spectacular "morphing"). What
might convince me otherwise is if there happen to be MORE basic carpal
elements in the bird wrist than in the basic, primative , pterosaur wrist.
But, I`m not sure if this type of evidence is readily discernable. Bird
wrists are to large extent made up of fused carpal elements, although
embryological study might reveal the individual elements. Pterosaur wrists
(i.e. Eudimorphodon-Wellnhofer) seems to be made up of fused elements as
well (and there is no embryological evidence here). Does anyone have an idea
if this can be proven? Or can suggest where I might look for more detailed
analysis of these wrists (especially those of the early pterosaur)?